Original : http://www.unlambda.com/cadr/memo528.html
Some mistakes are fixed by S.Takeoka

A.I. Memo 528


May 1979


Thomas F. Knight, Jr.
David Moon
Jack Holloway
and Guy L. Steele, Jr.


The CADR machine, a revised version of the CONS machine, is a general-purpose, 32-bit microprogrammable processor which is the basis of the Lisp-machine system, a new computer system being developed by the Laboratory as a high-performance, economical implementation of Lisp. This paper describes the CADR processor and some of the associated hardware and low-level software.

This report describes research done at the Artificial Intelligence Laboratory of the Massachusetts Institute of Technology. Support for the laboratory's artificial intelligence research is provided in part by the Advanced Research Projects Agency of the Department of Defense under Office of Naval Research contract N00014-75-C-00643.


The CADR microprocessor is a general purpose processor designed for convenient emulation of complex order codes, particularly those involving stacks and pointer manipulation. It is the central processor in the LISP machine project, where it interprets the bit-efficient 16-bit order code produced by the LISP machine compiler. (The terms "Lisp machine" and "CADR machine" are sometimes confused. In this document, the CADR machine is a particular design of the microprocessor, while the LISP machine is the CADR machine plus the microcode which interprets the LISP machine order code.)

The data paths of the CADR machine are 32 bits wide. Each 48-bit-wide microcode instruction specifies two 32-bit data sources from a variety of internal scratchpad registers; the two data-manipulation instructions can also specify a destination address. The internal scratchpads include a 1K pointer-addressable RAM intended for storing the top of the emulated stack, in a manner similar to a cache. Since in the LISP machine a large percentage of main memory references will be to the stack, this materially speeds up the machine.

The CADR machine has a 14-bit microprogram counter, which behaves much like that of a traditional processor, allowing up to 16K of writable microprogram memory. Also included is a 32-location microcode subroutine stack.

Memory is accessed through a two-level virtual paging system, which maps 24-bit virtual addresses into 22-bit physical addresses.

There are four classes of micro-instructions. Each specifies two sources (A and M); the ALU and BYTE operations also specify a destination (A, or M plus functional). The A bus supplies data from the 1024-word A scratchpad memory, while the M bus supplies data from either the 32-word M scratchpad memory (a copy of the first 32 locations of the A scratchpad) or a variety of other internal registers. The four classes of microinstruction are:

The destination receives the result of a boolean or arithmetic operation performed on the two sources.

The destination receives the result of a byte extraction, byte deposit, or selective field substitution from one source to the other. The byte so manipulated can be of any non-zero width.

A transfer of control occurs, conditional on the value of any bit accessible to the M bus, or on a variety of ALU and other internal conditions such as pending interrupts and page faults.

A transfer of control occurs to a location determined by a word from the dispatch memory selected by a byte of up to seven bits extracted from the M bus.

There are several sources and destinations whose loading and use invoke special action my the microprocessor. These include the memory address and memory data registers, whose use initiates main memory cycles.

Some of the ALU operations are conditional, depending upon the low order bit in the Q register and the sign of A source. These operations are used for multiply and divide steps.

The main features of this machine which make it suitable for interpreting the LISP machine order code are its dynamically writable microcode, its very flexible dispatching and subroutining, its excellent byte manipulation abilities, and its internal stack storage. While the design of CADR was strongly influenced by the requirements of the LISP machine design, a conscious attempt was made to avoid features that are extremely special purpose. The goal is a machine that happens to be good the particular order code of the LISP machine, bit which is general enough to at interpreting others almost as well. In particular, no critical parts of the LISP machine design (such as LISP machine instruction formats) are "wired in"; thus any changes to the LISP machine design can be easily accommodated by the CADR. However, there are several "efficiency hacks" in the hardware, designed to speed up certain common operations of the LISP machine microcode, which might not be useful for other microcodes. These are described in later sections of this document.

Notational Conventions

All numbers used to describe bit positions, field widths, memory sizes, etc. are decimal. Octal is used only (and exclusively) to describe the values of fields. Bits within a word are consistently numbered from right to left, the least significant bit being bit <0>. Fields are described by the numbers of their most and least significant bits (e.g. "bits <22-10>").

Whenever a particular field value is described as "illegal", it does not mean that specifying that value will screw up the operation of the machine. It merely indicates a value which happens to have a certain function, not because it is considered directly useful, but because the internal workings of the machine may force certain selectors to that value for other reasons, and the user can select this value too even though it is not normally useful. These illegal values are described for the benefit of someone who may wish to fathom these inner workings.

A field value described as "unused" is reserved for possible design expansion and should not be used in programs. Bit fields described as "unused" should be zero in programs, for the sake of future compatibility.

Since the use of the term "micro" in referring to registers and instructions becomes redundant, its use will be dropped from here on in this part of the document. All instructions discussed are microinstructions.

The following bits are treated the same in every instruction. They will not be repeated in the individual instruction descriptions.

IR<48>=Odd parity bit
IR<46>=Statistics (see the description of the Statistics Counter) This can be used to count how many times specified areas of the microcode are executed, to implement microcode breakpoints, or to stop the machine at a certain "time".
IR<45>=ILONG (1 means slow clock)
IR<44-43>=Opcode (0 ALU, 1 JUMP, 2 DISPATCH, 3 BYTE)
IR<42>=POPJ transfer. Causes a return from a micro subroutine, after executing one additional instruction.
IR<11-10>= Miscellaneous Functions
1Not used
2Write dispatch memory, if opcode is DISPATCH.
3Enable modification of the M-ROTATE field by the location counter (LC). See the description of the instruction-stream hardware.

Data Paths

The data paths of the machine consist of two source busses A and M, which provide data to the ALU and byte extractor, and an output bus OB, which is selected from the ALU (optionally shifted left or right) or the output of the byte extractor, and whose data can be routed various destinations. We first describe the specification of the source busses, which are identically specified for all instructions; then the destination specifiers which control where the results are stored; and finally the two instructions for controlling the ALU and the byte extractor.


All instructions specify sources in the same way. There are two source busses in the machine, the A bus and the M bus. The A bus is driven only from the A scratchpad memory of 1024 32-bit words. The M bus is driven from the M scratchpad of 32 32-bit words and a variety of other sources, including main memory data and control registers, the PC stack (for restoring the state of the processor after traps), the internal stack buffer and its pointer registers, the macrocode location counter, and the Q register. Addresses for the A and M scratchpads are taken directly from the instruction. The alternate sources of data for the M source are specified with an additional bit in the M source field.

IR<41-32> = A source address
IR<31-26> = M source address
         If IR<31> = 0,
                IR<30-26> = M scratchpad address
         If IR<31> = 1,
                IR<30-26> = M "functional" source    
                          0 Dispatch constant (see below)
                          1 SPC pointer <28-24>, SPC data <18-0>
                          2 PDL pointer <9-0>
                          3 PDL index <9-0>
                          5 PDL Buffer (addressed by index)
                          6 OPC registers (see below) <13-0>
                          7 Q register
                         10 VMA register (memory address)
                         11 MAP[MD]
                         12 MD register (memory data)
                         13 LC (location counter)
                         14 SPC pointer and data, pop
                         24 PDL buffer, addressed by Pointer, pop
                         25 PDL buffer, addressed by Pointer

Functional sources not listed above should not be used and may have side effects. Sources 15, 16, and 17 are reserved for future expansion. Source 4 is the PDL buffer, indexed by the PDL index, and the PDL pointer is decremented, presumably a useless operation.

Programming hint: it is often convenient to reserve one A memory word and one M memory word and fill them with constant zeros, to provide a zero source for each source bus. It is also convenient to have an M memory word containing all ones. These are particularly useful for byte extraction, masking, bit setting, and bit clearing operations. The CONSLP assembler in fact assumes that A memory location 2 and M memory location 2 are sources of zeros. The UCONS microcode stores all ones in location 3.

The M scratchpad normally contains a duplicate copy of the first 32 locations of the A scratchpad. The effect is as if there were a single scratchpad memory, the first 32 locations of which were dual-ported. This makes programming more convenient, since these locations are accessible to both sides of the ALU and shifter.


The 12-bit destination field in the BYTE and ALU instructions specifies where the result of the instruction is deposited. It is in one of two forms, depending upon the high-order bit. If the high-order bit is 1, then the low 10 bits are the address of an A memory location, and the remaining bit is unused. If the high order bit is 0, the low 10 bits are divided into a 5-bit "functional destination" field, and a 5-bit M scratchpad address, and both of the places specified by these fields get written into. The next-to-highest bit in the destination field is not used.

IR<25-14> = Destination
        If IR<25> = 1,
                IR<23-14> = A scratchpad address
        If IR<25> = 0,
                IR<23-19> = Functional destination write address
                         0  None
                         1  LC (Location Counter)
                         2  Interrupt Control <29-26>
                                Bit 26 = Sequence-Break request
                                Bit 27 = Interrupt-Enable
                                Bit 28 = Bus-Reset
                                Bit 29 = LC Byte-mode
                        10  PDL (addressed by Pointer)
                        11  PDL (addressed by Pointer), push
                        12  PDL (addressed by Index)
                        13  PDL Index
                        14  PDL Pointer
                        15  SPC data, push
                        16  Next instruction modifier
                                ("OA register"), bits <25-0>
                        17  Next instruction modifier
                                ("OA register"), bits <47-26>
                        20  VMA register (memory address)
                        21  VMA register, start main memory read
                        22  VMA register, start main memory write
                        23  VMA register, write map.  The map is
                                addressed from MD and written from
                                VMA.  VMA<26>=1 writes the level 1
                                map from VMA<31-27>. VMA<25>=1 writes
                                the level 2 map from VMA<23-0>.
                        30  MD register (memory data)
                        31  MD register, start main memory read
                        32  MD register, start main memory write
                        33  MD register, write map like 23
                IR<18-14> = M scratchpad write address

Functional destinations not listed may have strange results. Destinations 3-7 are reserved for expansion.

Note: If you write into the M-memory, the machine will also write into the corresponding A-memory address. Therefore you should never write into A-memory locations 0-37; this way the first 40 (octal) locations of the A-memory "map into" the M-memory.

The full details of the more complicated functional destinations are described in later sections below. The Q register is loaded by using the Q-control field of the ALU instruction, not by using a functional destination. In addition, it loads from the ALU outputs, not the output bus. This means that the left and right shift operations are ineffective for data being loaded into Q.

Programming hint: if a functional destination is specified, an M scratchpad location must also be specified. It is convenient to reserve one location of the M scratchpad for "garbage"; this location can be specified when it is desired to write into a functional destination but not into any other M scratchpad location. Since the CONSLP assembler defaults the M write address to zero, it is best to let location 0 be the garbage location. Location 0 of the A scratchpad will also be written and is also reserved as a garbage location.

The ALU Instruction

The ALU operation performs most of the arithmetic in the machine. It specifies two sources of 32-bit numbers, and an operation to be performed by the ALU. The operation can be any of the 16 boolean functions on two variables, two's complement addition or subtraction, left shift, and several less useful operations. The carry into the ALU can be forced to be 0 or 1. The output of the ALU is optionally shifted one place, and then written into the specified destinations via the output bus. Additionally, the ALU instruction specified one of four operations upon the Q register. These are do nothing, shift left, shift right, and load from the ALU outputs. An additional bit in the ALU operation field is decoded to indicate conditional operations; this is how the "multiply step" and "divide step" operations are specified. (Multiplication and division are explained in greater detail in another section.)

IR<44-43> = 0 (ALU opcode)
IR<41-32> = A source
IR<31-26> = M source
IR<25-14> = Destination
IR<13-12> = Output bus control
                0  Byte extractor output (illegal)
                1  ALU output
                2  ALU output shifted right one, with the correct
                   sign shifted in, regardless of overflow.
                3  ALU output shifted left one, shifting in Q<31>
                   from the right.
IR<9>     = not used
IR<8-4>   = ALU operation
                If IR<8> = 0,
                        IR<7-3> = ALU op code (see table)
                If IR<8> = 1,
                        IR<7-3> = Conditional ALU op code
                                 0  Multiply step
                                 1  Divide step
                                 5  Remainder correction
                                11  Initial divide step
IR<2>     = Carry into low end of ALU
IR<1-0>   = Q control
                0  Do nothing
                1  Shift Q left, shifting in the inverse
                       of the sign of the ALU output (ALU<31>)
                2  Shift Q right, shifting in the low bit
                       of the ALU output (ALU<0>)
                3  Load Q from ALU output

ALU operation codes (from Table 1 of 74181 specifications). All arithmetic operations are two's complement. Note that the bits are permuted in such a way as to make the logical operations come out with the same opcodes as used by the Lisp BOOLE function. Names in square brackets are the CONSLP mnemonics for the operations.

        Boolean (IR<7>=1)               Arithmetic (IR<7>=0)
IR<6-3>                       Carry in = 0            Carry in = 1
 0      ZEROS   [SETZ]        -1                      0
 1      M&A     [AND]         (M&A)-1                 M&A
 2      M&~A    [ANDCA]       (M&~A)-1                (M&~A)
 3      M       [SETM]        M-1                     M
 4      ~M&A    [ANDCM]       M|~A                    (M|~A)+1
 5      A       [SETA]        (M|~A)+(M&A)            (M|~A)+(M&A)+1
 6      M*A     [XOR]         M-A-1        [M-A-1]    M-A            [SUB]
 7      M|A     [IOR]         (M|~A)+M                (M|~A)+M+1
10      ~A&~M   [ANDCB]       M|A                     (M|A)+1
11      M=A     [EQV]         M+A          [ADD]      M+A+1          [M+A+1]
12      ~A      [SETCA]       (M|A)+(M&~A)            (M|A)+(M&~A)+1
13      M|~A    [ORCA]        (M|A)+M                 (M|A)+M+1
14      ~M      [SETCM]       M                       M+1            [M+1]
15      ~M|A    [ORCM]        M+(M&A)                 M+(M&A)+1
16      ~M|~A   [ORCB]        M+(M|~A)                M+(M|~A)+1
17      ONES    [SETO]        M+M          [M+M]      M+M+1          [M+M+1]

The BYTE Instruction

The BYTE instruction specifies two sources and a destination in the same way as the ALU instruction, but the operation performed is one of selective insertion of a byte field from the M source into an equal length field of the word from the A source. The rotation of the M source is specified by the SR bit as either zero or equal to the contents of the ROTATE field. The rotation of the mask used to select the bits replaced is specified by the MR bit as either zero or equal to the contents of the ROTATE field. The length of the mask field used for replacement is specified in the LENGTH MINUS 1 field. The four states of the SR and MR bits yield the following operations:

MR=0 SR=1 Not useful (This is a subset of other modes.)
PDP-10 LDB instruction (except the unmasked bits are from the A source). A byte of arbitrary position from the M source is right-justified in the output.

The masked field from the M source is used to replace the same length and position byte in the word from the A source.

PDP-10 DPB instruction. A right-justified byte from the M source is used to replace a byte of arbitrary position in the word from the A source.
The BYTE instruction automatically makes the output of the byte extractor available by forcing the output bus select code to 0 (byte extractor output).
IR<44-43> = 3 (BYTE operation)
IR<41-32> = A source
IR<31-26> = M source
IR<25-14> = Destination
IR<13>    = MR = Mask Rotate (see above)
IR<12>    = SR = Source Rotate (see above)
IR<9-5>   = Length of byte minus 1 (0 means byte of length 1, etc.)
IR<4-0>   = Rotation count (to the left) of mask and/or M source

The byte operation rotates the M source by 0 (if SR=0) or by the rotation count (if SR=1), producing a result called R. It also uses the MR bit, the rotation count, and the length minus 1 field to produce a selector mask (see description below). This mask is used to merge the A source with R, bit by bit, selecting a bit from A if the mask is 0 and from R if the mask is 1. This result is then written into the specified destination(s).

Output of mask memories:
        Right mask memory is indexed by 0 (MR=0) or by rotation count (MR=1).
        Left mask memory is indexed by (the index into the right mask memory) plus
                 (the length minus 1 field), mod 32.

octal   LEFT MASK MEMORY contents            RIGHT MASK MEMORY contents
   0    00000000000000000000000000000001     11111111111111111111111111111111
   1    00000000000000000000000000000011     11111111111111111111111111111110
   2    00000000000000000000000000000111     11111111111111111111111111111100
   3    00000000000000000000000000001111     11111111111111111111111111111000
   4    00000000000000000000000000011111     11111111111111111111111111110000
   5    00000000000000000000000000111111     11111111111111111111111111100000
   6    00000000000000000000000001111111     11111111111111111111111111000000
   7    00000000000000000000000011111111     11111111111111111111111110000000
  10    00000000000000000000000111111111     11111111111111111111111100000000
  11    00000000000000000000001111111111     11111111111111111111111000000000
  12    00000000000000000000011111111111     11111111111111111111110000000000
  13    00000000000000000000111111111111     11111111111111111111100000000000
  14    00000000000000000001111111111111     11111111111111111111000000000000
  15    00000000000000000011111111111111     11111111111111111110000000000000
  16    00000000000000000111111111111111     11111111111111111100000000000000
  17    00000000000000001111111111111111     11111111111111111000000000000000
  20    00000000000000011111111111111111     11111111111111110000000000000000
  21    00000000000000111111111111111111     11111111111111100000000000000000
  22    00000000000001111111111111111111     11111111111111000000000000000000
  23    00000000000011111111111111111111     11111111111110000000000000000000
  24    00000000000111111111111111111111     11111111111100000000000000000000
  25    00000000001111111111111111111111     11111111111000000000000000000000
  26    00000000011111111111111111111111     11111111110000000000000000000000
  27    00000000111111111111111111111111     11111111100000000000000000000000
  30    00000001111111111111111111111111     11111111000000000000000000000000
  31    00000011111111111111111111111111     11111110000000000000000000000000
  32    00000111111111111111111111111111     11111100000000000000000000000000
  33    00001111111111111111111111111111     11111000000000000000000000000000
  34    00011111111111111111111111111111     11110000000000000000000000000000
  35    00111111111111111111111111111111     11100000000000000000000000000000
  36    01111111111111111111111111111111     11000000000000000000000000000000
  37    11111111111111111111111111111111     10000000000000000000000000000000

After the two masks are selected, they are AND'ed together to get the final mask. This mask is all zeros, except for a field of contiguous ones defining the byte.

As an example, if MR=1, rotation count=5, and length minus 1 = 7, then the right mask index is 5 and the left mask index is 14 (octal). This results in a final mask as follows:

        Right mask 5         11111111111111111111111111100000
        Left mask 14         00000000000000000001111111111111
AND them together            --------------------------------
        Final mask           00000000000000000001111111100000
The byte is 8 bits wide, 5 positions from the right.

Programming hint: if the byte is "too large" (i.e. its position and size specification cause it to hang over the left-hand edge of a word), then the masker does not truncate the byte at the left edge. Instead, it produces a zero mask, selecting no byte at all; thus, the output of the byte operation equals the A source. The reason for this is that an overflow occurs in calculating the index into the left mask memory, and so the final mask is zero. For example, if MR=1, rotation count=20 (octal), and length minus 1=27 (octal), then the right mask index is 20 and the left mask index is 477 (mod 32). This results in a final mask as follows:

        Right mask 20        11111111111111110000000000000000
        Left mask 7          00000000000000000000000011111111
AND them together            --------------------------------
        Final mask           00000000000000000000000000000000


The control section of the processor consists of a 14-bit program counter (the PC), a 32-location PC stack (SPC) and stack pointer (SPCPTR), and a 2K dispatch memory, used during the DISPATCH instruction. Unlike some microprocessors, and like most traditional machines, the normal mode of operation is to execute the next sequential instruction by incrementing the PC.

The processor uses single instruction lookahead, i.e. the lookup of the next instruction is overlapped with execution of the current one. This implies that after branching instructions the processor normally executes the following instruction, even if the branch was successful. Provision is made in these instructions to inhibit this execution (with the N bit), but the cycle it would have used will then be wasted.

               (I2 is a branch instruction to the location of I8)

TIME ===>

|              |              |              |              |              |
|  fetch I1    |  fetch I2    |  fetch I3    |  fetch I8    |  fetch I9    |
|  execute I0  |  execute I1  |  execute I2  |  execute I3  |  execute I8  |
|              |              |              |              |              |
                  |              |              |              |
Fetch of branch---'              |              |              |
                                 |              |              |
Execution of branch--------------'              |              |
                                                |              |
Execution (optionally inhibited)                |              |
of following instruction------------------------'              |
Execution of instruction branched to---------------------------'

Two types of instruction affect the flow of control in the machine. The conditional JUMP specifies a new PC and transfer type in the instruction itself, while the DISPATCH instruction looks up the new PC and transfer type in the 2K dispatch memory. In either case, the new PC is loaded into the PC register, and the operation is specified by the 3-bit transfer type performed. These operations are:

N bit
If on, inhibits execution of the next instruction, i.e. the instruction at the address one greater than that of the transfer instruction. (This instruction needn't actually be at the address one greater, if a transfer of control was already in progress.) The cycle that would have executed that instruction is wasted.
The P and R bits are decoded as follows:
Normal program transfer.

P=1 R=0 CALL
Save the correct return address on the SPC stack, and jump to the new PC address.

Ignore new PC; instead pop PC of the SPC stack.

In a DISPATCH instruction, do not dispatch.
In a JUMP instruction; write into the instruction memory and do not jump.

The BRANCH transfer type is the normal program transfer, without saving a return address.

The CALL transfer type pushes the appropriate return address onto the SPC stack. This stack is 32 locations long. It is the responsibility of the programmer to avoid overflows. The return address is PC+2, or PC+1 if the N bit is also on. Actually, if the N bit is on the address of the instruction NOP'ed is saved, which may not be identical to PC+1 if a transfer of control is already in progress. If the N bit is not on, 1 + the address of that instruction is saved. In the case of a dispatch, if the N bit is not on and bit 25 of the instruction is on, save PC, the address of the dispatch instruction itself; this allows the dispatch to be re-executed upon return. (Actually, due to pipelining, when the above paragraph says PC it doesn't really mean PC.)

The RETURN transfer type pops a return PC from the SPC stack, ignoring the PC specified in the instruction or dispatch table.

The FALL THROUGH transfer type for dispatches allows some entries in a dispatch table to specify that the dispatch should not occur after all. The following instruction is executed (unless inhibited), followed by the one after that (unless the first following one branches and inhibits it!).

The I-MEM WRITE transfer type is the mechanism for writing instructions into the microprogram instruction memory, and is described in a later section. (The dispatch memory, unlike the instruction memory, is not written into by setting the P and R bits (after all, in a dispatch instruction these bits come from the dispatch memory!); instead the Miscellaneous Function field is used.)

An additional bit in every instruction, including ALU and BYTE instructions, called the POPJ bit, allows specification of simultaneous execution of a RETURN transfer type along with execution of any instruction. That is, it does the same thing as if this instruction, in addition to whatever else it does, had executed a RETURN transfer type jump without the N bit on. It is the responsibility of the programmer to avoid conflicts in the use of this bit simultaneously with other types of transfers.

The POPJ bit should be used in a JUMP instruction only in conjunction with the RETURN transfer type. This will cause a RETURN operation in either case, but execution of the following instruction is conditional, controlled by the N bit and the conditional JUMP instruction. The POPJ bit, when used in a DISPATCH instruction, is specially over-ruled by the JUMP and CALL transfer types. This allows you to RETURN normally, but jump off to other code in exceptional cases, using the same dispatch table as other dispatch instructions which do not want to return. The POPJ bit should not be used in conjunction with writing of dispatch or instruction memory, nor with the SPC pop and push functional source and destination. The machine doesn't bother to do anything reasonable in these cases.

The DISPATCH Instruction

The dispatch instruction allows selection of any source available on the M bus [see description of M bus sources in the Data Path section], and the dispatch on any sub-field of up to 7 bits from the selected word. The selected subfield is ORed with the "dispatch address" field of the instruction to produce an 11 bit address. This address is used to look up a 14 bit PC and 3 bit transfer type in the dispatch memory. The SPC-pointer-and-data-pop source will not operate reasonably in conjunction with the dispatch instruction.

IR<44-43> = 2 (DISPATCH operation)
IR<41-32> = Dispatch constant (also A source when writing D-MEM)
IR<31-26> = M source
IR<25>    = Alter return address pushed on SPC by the CALL transfer
            type, if the N bit is net, to be the address of this
            instruction rather than the next instruction.
IR<24>    = Enable instruction-stream hardware (described later).
IR<23>    = Unused
IR<22-12> = Address in dispatch memory
IR<9-8>   = Control dispatching off the map, see below.
IR<7-5>   = Length of byte (not minus 1!) from M source to dispatch on
IR<4-0>   = Rotation count (to the left) of M source

The dispatch operation takes the specified M source word and rotates it to the left as specified by the rotation count. All but the low K bits are masked out, where K is the contents of the length field. The result is OR'ed with the dispatch address, and this is used to address the 2K dispatch memory, which supplies the new PC and the R, P, and N bits.

If bits 8 and 9 of IR are not zero, the bottom bit of the dispatch address comes from the virtual memory map rather than the rotator and masker. The address inputs to the map in this case come from MD. This is primarily useful for testing pointers just fetched from main memory for validity with respect to the garbage collector's conventions. IR<8> selects bit 14 of the second level map, and IR<9> selects bit 15. Selecting both bits ORs them together.

The dispatch constant field is loaded into the DISPATCH CONSTANT register on every dispatch instruction. This register is accessible as an M source. The dispatch constant field has nothing whatsoever to do with the operation of dispatching; it is merely a convenient device for loading a completely random register while doing something else. (Uses for this feature are discussed in a later section.)

Miscellaneous function 2 inhibits the normal action of the instruction and instead loads the dispatch memory with the low order contents of the A memory scratchpad location specified in the A source. Note that the A source address is the same as the dispatch constant field. The dispatch constant is loaded anyway, but this can be ignored. The parity bit (bit 17) is also loaded, and it is the responsibility of the programmer to load correct (odd) parity into the memory. Normal addressing of the dispatch memory is in effect, so it is advisable to have the length field contain 0 so that the dispatch memory location to modify is uniquely specified by the dispatch address in the instruction.

The Jump Instruction

The Jump instruction allows conditional branching based on any bit of any M source or on a variety of internal processor conditions, including ALU output. (While DISPATCH could also be used to test single M source bits, the use of JUMP saves dispatch memory.) The JUMP operation is also used, by means of a trick, to write into instruction memory.

IR<44-43> = 1 (JUMP operation)
IR<41-32> = A source
IR<31-26> = M source
IR<25-12> = New PC
IR<9>     = R bit (1 means pop new PC off SPC stack)
IR<8>     = P bit (1 means push return PC onto SPC stack)
IR<7>     = N bit (1 means inhibit next instruction if jump successful)
IR<6>     = If 1, invert sense of jump condition
IR<5>     = If 0, test bit of M source; if 1, test internal condition
IR<4-0>   = If IR<5>=0, rotation count for M source.
            If IR<5>=1, condition number:
                0  Low bit of shifter output (illegal)
                1  M source < A source
                2  M source <= A source
                3  M source = A source
                4  Page fault
                5  Page fault or interrupt pending
                6  Page fault or interrupt pending or sequence break flag
                7  Unconditionally true

Page faults, interrupts, and sequence breaks are documented in later sections.

The jump condition is determined as follows. If IR<5>=0, then the M source is rotated left by the rotation count; the low-order bit of the result is then tested. Thus, to test the sign bit, a rotation count of 1 should be used. The jump condition is true if the low-order bit is 1. If IR<5>=1, then the specified internal condition is tested. In either case, the sense of the jump condition is inverted if IR<6>=1. In particular, this allows testing of all six arithmetic relations between the M and A sources.

If the final jump condition, possibly after inversion, is true, then the new PC field and the R, P, and N bits are used to determine the new contents of the PC. If the condition is not true, execution continues with the next instruction, modulo the POPJ bit.

If both the R and P bits are set (WRITE), then A and M sources are (conditionally!) written into the instruction memory. Bits <47-32> are taken from A source bits <15-0>; bits <31-0> are taken from M source <31-0>. Notice that this is not the same alignment of bits as is used for the "next instruction modify" functional destinations (16 and 17). The reason for the odd location of WRITE in the instruction set is due to the way it which it operates. It causes the same operations as the CALL transfer type, resulting in the old PC plus 1 or 2 being saved on the SPC stack and the PC register being loaded with the address to be modified. Then, when the instruction memory would normally be fetching the instruction to be executed from that location, a write pulse is generated, causing the saved data from the A and M sources to be written into the instruction memory. Meanwhile, the machine simulates a RETURN transfer instruction, causing the SPC stack to be popped back into the PC and instruction execution to proceed from where it left off. Note that this extra instruction requires use of a word on the SPC stack and requires an extra cycle. It is highly recommended that the N bit also be on in the JUMP instruction, since the processor will be executing a RETURN transfer type unconditionally during what should be the execution of the instruction following the write. If, however, this does not conflict with other things that this following instruction specifies, then the following instruction may be executed. Care is required.

Program Modification

A novel technique is used for variabilizing fields in the program instruction. Two of the "functional destinations" of the output bus are (conceptual) registers (sometimes collectively referred to as the OA register), whose contents get OR'ed with the next instruction executed. Combined with the shifter/masker ability to move any contiguous set of bits into an arbitrary field, this feature provides, for example, variable rotation counts and the ability to use program determined addresses of registers; for example, it can be used to index into the A scratchpad memory.

Function destination 16 (OA-REG-LOW), when written into, effectively OR's bits <25-0> into bits <25-0> of the next instruction; functional destination 17 (OA-REG-HIGH) effectively OR's bits <21-0> into bits <47-26> of the next instruction. The place between bits <26> and <25> is a natural dividing line for all classes of instructions. Note that only one half of a particular instruction can be modified, since it is impossible to write into both functional destinations simultaneously.

When this feature is used, parity checking is disabled for the word fetched from the instruction memory, since the OA "register" is OR'ed into the output of the memory before parity is checked.

This feature is particularly useful for supplying the address of a location of instruction memory or dispatch memory to be written into, for specifying variable addresses in the A and M memories, and for operations on bytes of variable length or position. Examples of these are detailed in a later section.


THe CADR processor uses only one clock signal. This clock loads output data into the designated registers, and a new PC and instruction are also loaded. The only events which do not take place synchronous with the clock are the control signals for the A, M, and PDL scratchpads and the SPC stack. For these devices, a two stage cycle is performed. During the first phase, the source address of the respective devices are gated into the address inputs. After the output data has settled, the outputs of these devices are latched. Then, the address is changed to that specified as the write location from the previous insttruction. After the address has settled, a write pulse is generated for the scratchpad memory to perform the write. Pass-around paths are provided (invisibly to the programmer) for the A and M memories, which notice and correct read references to a location which was written into on the previous cycle but has not yet actually been written into the scratchpad. No such pass-around path is provided for the PDL memory, because on any cycle in which the PDL memory is written into, the M scratchpad must also be written into, and so the next instruction can refer to that M scratchpad location, thereby using the M pass-around path. THe SPC stack has a pass-around path when used by the RETURN transfer type, but does not have a pass-around path when used as an M source. The RETURN pass-around path makes it possible to have a subroutine only two instructions long. It would take extra hardware to provide the missing pass-around paths, and examination of actual microprograms showed that they would be very rarely used.

The clock cycle is of variable length. The duration of the first half of the cycle (the "read phase") is controlled by the ILONG bit of the instruction (IR <45>) and by two "speed" bits from the diagnostic interface. The duration of the second half (the "write phase") is normally fixed. This clock serves as both the processor clock and a clock for the bus interface, memory and external devices.

The clock can be stopped at the end of either phase, for several reasons. Usually the clock stops at the end of the read phase, referred to as "wait". This leaves the clock in the inactive high state, and leaves the latches on the memories open. The clock can wait because the machine was commanded to halt by the diagnostic interface, because a single-step commanded by the diagnostic interface has completed, because of an error such as a parity error, because of the statistics counter overflowing, or because of a memory-wait condition. This latter condition happens if a main memory cycle is initiated while a previous cycle is in progress, or if the program calls for the result of a main memory read before the bus controller has granted the bus access needed to perform that read cycle. During a clock wait, the processor clock stops, but the clock to the rest of the system (the bus interface and XBUS devices), continues to run, allowing them to operate. When the processor finishes waiting the processor clock starts up in synchrony with the external clock.

The clock can also stop at the end of the write phase, referred to as "hang". This is used only during memory reads. If the processor calls for the result of a read which is in progress but has not yet completed, it hangs until the data has arrived from memory and sufficient time has passed for the data to flow through the data paths and appear on the output bus. This is also sufficient time for the parity of the data to be checked. In the case of a hang, both clocks stop, which allows them to restart synchronously without any extra delay. In this way, the speed of the processor is adjusted to exactly match the speed of the memory. [Ed note: Figure: CADR cycle timing omitted ]

Accessing memory

Access to main memory is accomplished through use of several functional sources and destinations. These perform three functions; first, they allow access to two registers, VMA (virtual memory address) and MD (memory data). Secondly, they can initiate memory operations. Thirdly, they can wait for a memory operation to be completed. Actually, this facility is not just for accessing main memory; it is used to access any device on the Xbus or the Unibus, which includes not only memory but peripheral equipment. For simplicity the term "memory" will be used, however.

There are eight functional destinations associated with the memory system. Four of these load data into the VMA, the other four load data into the MD. Each group of four consists of one with no other side effects, one which starts a read cycle, one which starts a write cycle, and one which writes into the virtual address map.

In a memory read operation, data from memory is placed in the MD register when it arrives, and can then be picked up by the program (using a functional source). In a memory write operation, the program places the data to be written into the MD register (by using a functional destination), whence it is passed to the memory.

The VMA register contains the virtual address of the location to be referenced. This is 24 bits long; the high 8 bits of the register exist but are ignored by the hardware. The VMA contains a "virtual" address; before being sent to the memory it is passed through the "map", which produces a 22 bit physical address, controls whether permission for the read or write operation requested is allowed, and remembers 8 bits which the software (microcode) can use for its own purposes.

Except when starting a memory cycle, the address to be mapped comes from bits <23-0> of the MD register, rather than the VMA register. The reason for this is to simplify the use of the map for checking what "space" a pointer being read from or written into memory points at, a frequently-needed operation in the Lisp machine garbage-collection algorithm.

The map consists of two scratchpad memories. The First Level Map contains 2048 5-bit locations, and is addressed by bits <23-13> of the VMA or MD. The Second Level map contains 1024 24-bit locations, and is addressed by the concatenation of the output from the First Level Map and bit <12-8> of the VMA or MD. The virtual address space consists of 2048 blocks, each containing 32 pages. Each page contains 256 words (of 32 bits, of course). Each block of virtual address space has a corresponding location in the First Level Map. Locations in the Second Level Map are not permanently allocated to particular addresses; instead, the First Level map location for a block of virtual addresses indicates where in the Second Level Map those addresses are currently described. The Second Level Map contains sufficient space to describe 32 blocks, so at any given time most blocks must be described as "no information available." This is done by reserving the last 32 locations in the Second Level Map for this purpose and filling them with "no information available" page descriptors; most Frist Level map locations will point here.

The output of the Second Level Map consists of:

MAP<23>    = access permission
MAP<22>    = write permission
MAP<21-14> = available to software.  Note that bits 15 and 14 can
             be tested by the DISPATCH instruction.
MAP<13-0>  = physical page number
The physical address sent to memory is the concatenation of the physical page number and bits 7-0 of the VMA.

The two maps can be read by putting an appropriate address in the MD, and reading the functional source MEMORY-MAP-DATA (11):

MAP<31> = 1 if the most recent memory cycle was not performed because it
          was an attempt to write without write permission, i.e. a 1 in
          bit 22 of the second level map.
MAP<30> = 1 if the most recent memory cycle was not performed because there
          was no access permission, i.e. a 1 in bit 23 of the second level map.
          MAP<30> is 0 if no access fault exists, although a write fault may
          exist.  Note that bits <31-30> apply to the last attempted memory
          cycle, and have nothing to do with the map locations addressed by
          the contents of MD.
MAP<29> = 0 always.
MAP<28-24> = First Level Map
MAP<23-0> = Second Level Map

The maps can be written by using one of the functional destinations VMA-WRITE-MAP (23), MEMORY-DATA-WRITE-MAP (33). The MD supplies the address of the map location to be written, and the VMA supplies the data to be written, and tells which level of the map is being written. One register must be set up in a previous instruction, the other is written via the functional destination, and the actual writing into the map happens on the following cycle. There is no pass-around path and no latch for the map, so the following instruction must not use it.

The first level map is written from bits <31-27> of the VMA, if VMA<26> is a 1. (These are not the same bits as it reads into when using the MEMORY-MAP-DATA functional source.) The second level map is written from VMA<23-0>, if VMA<25> is a 1. Note that when writing the second level map the first level map supplies part of the address, and must have been written previously. Therefore it is not useful to write both at the same time, although it is possible to set both bits to 1.

Main memory operations are initiated by using one of the functional destinations VMA-START-READ (21), VMA-START-WRITE (22), and MEMORY-DATA-START-WRITE (32). There is also MEMORY-DATA-START-READ (31), but it is probably useless. In the case of a write, the VMA supplies the address and the MD supplies the data, so one register must be set up in advance and the other is set up by the functional destination that starts the operation. A main memory read can also be started by the macro instruction-stream hardware, described later.

The register named (VMA or MD) is loaded with the result of the instruction (from the Output Bus) at the end of the cycle during which that instruction is executed. During the following cycle, the map is read. The instruction executed during this cycle should be a JUMP instruction which checks for a page fault condition. At the end of this cycle, if no page fault occurs, the memory operation begins. The processor continues executing while the memory operation happens, but if any operation which conflicts with the memory being busy is attempted, the machine waits or hangs until the memory operation has been completed. Such references include asking for the results of a read cycle by using the MEMORY-DATA (12) functional source, using any functional destination that refers to the VMA, MD or MAP, or attempting to start a read cycle via the instruction stream hardware.

The presence or absence of a page fault is remembered until the next time a memory cycle is started, so it is not strictly necessary to check for a page fault immediately after starting a cycle, but is good practice.

The MEMORY-DATA-START-WRITE destination is useful for doing the second half of a read-followed-by-write operation, since the correct value is still in the VMA. Note that it is still necessary to check for a write fault after starting the write, since you may have read permission but not write permission.

There is a feature by which main memory parity errors can be trapped to the microcode. A bit in the diagnostic interface controls whether or not this is enabled. When the MEMORY-DATA functional source is used, and the last thing to be loaded into the MD was data from memory which had even parity, a main memory parity error has occurred. If trapping is enabled, the current instruction is NOPed and a CALL transfer to location 0 is forced. The following instruction is also NOPed. The trap routine must use the OPC registers to determine just where to return to if it plans to return, since if a transfer operation was in progress the address pushed on the SPC stack by the trap may have nothing to do with the address of the instruction which caused the trap. This is also true of the error-handler for microcode-detected programming errors. If a main memory parity error occurs, and trapping is not enabled, the machine halts if error-halting is enabled, just as it does in response to a parity error in an internal memory.

When using semiconductor main memory, which has single-bit error correction, a parity error trap indicates that an uncorrectable multiple-bit error occurred. Single-bit errors are corrected automatically by the hardware, and cause an interrupt so that the processor may, at its leisure, log the error and attempt to rewrite the contents of the bad location.

The Instruction-Stream Feature

The CADR processor contains a small amount of hardware to aid in the interpretation of an instruction stream which comes in units smaller than the CADR word size. For example, the Lisp-machine macrocompiled instructions set uses 16-bit units. The hardware speeds up both fetching and decoding of instructions by relieving the microcode of some routine bookkeeping.

Both 8-bit (byte) and 16-bit (halfword) instructions are supported, depending on a mode bit (bit 29 of the "Interrupt Control" register, functional destination 2.) The hardware decides when it is time to fetch a new main-memory word, containing the next 2 or 4 units of the instruction stream, and alters the flow of microprogram control. The hardware provides a feature by which the rotator control can be made to select the current unit of the instruction stream; this is used when dispatching on the instruction being interpreted, and when extracting fields of the instruction via the BYTE microinstruction.

There is a 26-bit register called the Location Counter (LC), which can be read by functional source 13 and written by functional destination 1. It always contains the address of the next instruction stream unit, in terms of 8-bit bytes. In halfword mode LC<0> is forced to zero. The LC is capable of counting by 1 or 2 (depending on byte vs. halfword mode) and has a special connection to the VMA; the VMA is loaded from the LC, divided by 4, when an instruction-fetch occurs.

The high 6 bits of functional source 13 are not part of the LC per se, but contain various associated status as follows:
31Need Fetch. This is 1 if the next time the instruction stream is advanced, a new word will be fetched from main memory. This is a function of the low bits of LC, of byte mode, and of whether the LC has been written into since an instruction word was last fetched from main memory.
30not used, zero.
29LC Byte Mode. 1 if the instruction stream is in 8-bit units, 0 if it is in 16-bit units. This reflects bit 29 of the Interrupt Control register.
28Bus Reset. This reflects bit 28 of the Interrupt Control register, which is set to 1 to reset the bus interface, the Unibus, and the Xbus.
27Interrupt Enable. 1 if external interrupt requests are allowed to contribute to the JUMP condition. This reflects bit 27 of the Interrupt Control register.
26Sequence Break. 1 if a sequence break (macrocode interrupt signal) is pending. This flag does nothing except contribute to the JUMP condition. This reflects bit 26 of the Interrupt Control register.

Bit 14 of the SPC stack is used to flag the return address containing it as the address of the main instruction-interpretation loop. The hardware recognizes a RETURN transfer with SPC<14>=1 as completing the interpretation of one instruction and initiating the interpretation of the next. The instruction stream will be advanced to its next unit (byte or halfword) in the cycle following the RETURN transfer. (It is delayed one cycle for obscure timing reasons.) This cycle is free to also execute a useful microinstruction, provided it does not use the LC, VMA, MD, and associated hardware.

Advancing the instruction stream increments the LC, by 1 or 2. If a new word needs to be fetched from main memory, the unincremented LC, divided by 4, is transferred to the VMA and a read cycle is started. A fetch can be required either because the LC points at the first unit of a word or because the LC has been modified since the last instruction stream advance (a branch has occurred). It is legal for the instruction which does the RETURN transfer to modify the LC, and a fetch will always be required. If no fetch is required, the RETURN transfer is altered by forcing SPC<1> to 1, skipping over two microinstructions which, in the fetch case, check for a page fault (or interrupt or sequence break) and transfer the new instruction stream word from MD into a scratchpad location.

The instruction stream can also be advanced by a DISPATCH instruction with bit 24 set. In this case, no alteration of the SPC return address occurs. The dispatch should check the NEEDFETCH signal, which is available as bit 31 of the LC functional source, to determine whether a new word is going to be fetched. If a fetch occurs, the DISPATCH should call a subroutine to check for page fault and transfer the new instruction stream word from MD to a scratchpad location. If no fetch occurs, the DISPATCH should drop through. The instruction after the DISPATCH may then operate on the next unit of the instruction stream. This feature is provided to facilitate the use of multi-unit instructions.

The remaining hardware associated with the instruction stream feature implements miscellaneous function 3, which alters the M-rotate field to select the current unit of the instruction stream from the current word, which should be supplied an the M-source. This applies to any operation which uses the rotator: BYTE instructions, DISPATCH instructions, and JUMP instructions which test a bit. The instruction should be coded for the unit (byte or halfword) at the right-hand end of the word. In half-word mode, IR<4> is XOR'ed with LC<1> to produce the high-order bit of the rotate count. In byte mode, IR<4> is XOR'ed with (LC<1> XOR LC<0>), and IR<3> is XOR'ed with LC<0>. The effect, since the LC always has the address of the next instruction, and the bits are numbered from right to left, is as desired. In halfword mode, the low half of the M source is accessed for the even instruction, when LC<1>=1, and the high half is accessed for the odd instruction, when LC<1>=0.

Multiplication, Division, and the Q register

The Q register is provided in CADR primarily for multiplication and division. It is occasionally useful for other things because it is an extra place to put the results of an ALU instruction, and because it can be used to collect the bits which are shifted out when the OUTPUT-SELECTOR-RIGHTSHIFT-1 operation is used in an ALU instruction.

The Q register is controlled by two bits (IR<1-0>) in the ALU instruction. The operations are do nothing, shift it left, shift it right, and load it from the output of the ALU. (It loads from the ALU rather than the Output Bus for electrical reasons.) When the Q register shifts left, Q<0> receives -ALU<31>, the complement of the sign of the ALU output. When the Q register shifts right, Q<31> receives ALU<0>, the low bit of the ALU output. The Q register is also connected to the Output Bus shifter; when the Output Bus is shifted left, OB<0> receives Q<31>, the sign of the Q. These interconnections are dictated by the needs of multiplication and division.

Multiplication in CADR is a simple, 1 bit at a time, shift-and-add affair. The hardware provides a conditional-ALU operation, MULTIPLY-STEP, whis is ADD if Q<0>=1, and SETM otherwise. This is used in combination with SHIFT-Q-RIGHT and OUTPUT-SELECTOR-RIGHTSHIFT-1. Initially the multiplicand is placed in an A-scratchpad location and the multiplier is placed in Q. 32 MULTIPLY-STEP operations are executed; as Q shifts to the right each of the bits of the multiplier appear in Q<0>. If the bit is 1, the multiplicand gets added in. The results of each operation go into an M-scratchpad location, which is fed back into the next step. The low bit of each result is shifted into Q. Thus, when the 32 steps have been completed, the Q contains the low 32 bits of the product, and the M-scratchpad location contains the high 32 bits.

This algorithm needs a slight modification to deal with 2's complement numbers. The sign bit of a 2's complement number has negative weight, so in the last step if Q<0>=1, i.e. the multiplier is negative, a subtraction should be done instead of an addition. The hardware does not provide this, so instead we do a subtraction after the last step, which is adding and then subtracting twice as much, which has the effect of subtracting. Note that this correction only affects the high 32 bits of the product, and can be omitted if we are only looking for a single-precision result. Consider the following code. (The CONSLP assembler format used is explained later in this document.)

; Multiply Subroutine.  A-MPYR times Q-R, low product to Q-R, high to M-AC.

MPY       ((M-AC) MULTIPLY-STEP M-ZERO A-MPYR))     ;Partial result = 0 in first step
(REPEAT 30. ((M-AC) MULTIPLY-STEP M-AC A-MPYR))     ;Do 30 steps
          (POPJ-IF-BIT-CLEAR-XCT-NEXT               ;Return after next if A-MPYR positive
                    (BYTE-FIELD 1 0) Q-R)
          ((M-AC) MULTIPLY-STEP M-AC A-MPYR)        ;The final step
             (M-AC) SUB M-AC A-MPYR)                ;Correction for negative multiplier
          (NO-OP)                                   ;Jump delay

To multiply numbers of less than 32 bits is also possible. With the same initial conditions, after n steps the high n bits of the Q contain the low n bits of the product, and the remaining bits of the product are in the low bits of the M-scratchpad location. Two BYTE instructions can be used to extract and combine these bits to produce a right-adjusted product, if the numbers are unsigned.

Division is a little more complex than multiplication. It too goes a bit at a time, using a non-restoring algorithm which either adds or subtracts at each stage. The basic idea is to keep subtracting the divisor from the dividend, shifted over by different amounts, as in long division by hand. If the subtraction produces a positive result, it "goes in" and a quotient bit of 1 is produced. If the subtraction produces a negative result, it "fails to go in" and a quotient bit of 0 is produced. Instead of backing up and not doing the subtraction, we set a flag that too much has been subtracted, and add instead the next time. This works since the weight of the divisor on the next step is half as much, and B - (A / 2) = B - A + (A / 2). The "flag" is simply the complement of the quotient bit produced, except for the first step when the flag must be forced to OFF.

Division does not handle 2's complement numbers as easily as multiplication does. The algorithm essentially requires all positive numbers, however the hardware automatically takes the absolute value of the divisor by interchanging addition and subtraction if the divisor is negative. It is up to the microcode to make the dividend positive beforehand, and to determine the correct signs for the quotient and remainder afterward. The sign of the quotient should be the XOR of the signs of dividend and divisor. The sign of the remainder should be the same as the sign of the dividend.

Initially the positive dividend is in the Q register and the signed divisor is in an A-scratchpad location. Appropriate conditional-ALU operations are used in conjunction with the SHIFT-Q-LEFT and OUTPUT-SELECTOR-LEFTSHIFT-1 functions. An M-scratchpad location receives the result of each step, and is fed back to the next step. This location initially contains the high 32 bits of the double-length dividend, or 0 if the dividend is single-precision. At each step, the OUTPUT-SELECTOR-LEFTSHIFT-1 operation brings the high bit of the Q into the low bit of the M-scratchpad, bringing up another bit of the dividend. At each step, the complement of the sign of the ALU output represents a bit of the quotient and is shifted into the low end of Q. After 33 steps, Q contains the positive quotient (which is why it is called the Q-for-quotient register). The reason why it takes 33 steps rather than 32 is a little difficult to explain. The quotient bit produced by the first step, if 1, indicates "divide overflow", and is not really part of the quotient. When using a single-precision dividend, "divide overflow" can only happen if the divisor is zero, since the initial operation is zero minus the absolute value of the divisor, which is negative unless the divisor is zero.

What is left of the dividend after all the subtractions is the positive remainder. The last step does not use OUTPUT-SELECTOR-LEFTSHIFT-1, so that the M-scratchpad will receive the remainder rather than the remainder times 2. If the "too much has been subtracted" flag is set, it is necessary to do one final addition to correct the remainder. This addition simply undoes the previous subtraction, not also doing a new subtraction, because of the omission of the left shift.

The ALU operations for division are:

The conditional add or subtract described above, SHIFT-Q-LEFT, and OUTPUT-SELECTOR-LEFTSHIFT-1. Q<0>=0 serves as the "too much has been subtracted" flag.
Identical to DIVIDE-STEP except that the "too much has been subtracted" flag is forced to be off.
Identical to DIVIDE-STEP except that the OUTPUT-SELECTOR-LEFTSHIFT-1 is omitted.
The conditional add or subtract logic is used, except subtract is turned into SETM by invoking part of the multiply logic. The exchanging of add and subtract if the divisor is negative then applies, doing the right thing. No shifting occurs and Q is unchanged.

Division of number smaller than 32 bits can be accomplished in less than 33 steps by sufficiently careful shifting of the inputs and outputs.

To illustrate how it all fits together, and show how to do the sign-correction, here is the code for 32-bit division, with a double-precision dividend, in the CONSLP format explained later in this document:

; Division Subroutine
; M-AC and M-1 are the high and low words of the dividend, respectively.
; M-2 is the divisor.  The quotient is in M-AC, the remainder in M-1.

DIV       (JUMP-GREATER-OR-EQUAL M-AC A-ZERO DIV1)         ;Check for negative dividend
          (JUMP-NOT-EQUAL-XCT-NEXT M-1 A-ZERO DIV0)        ;If so, change sign
         ((M-1 Q-R) SUB M-ZERO A-1)
          ((M-AC) SUB M-AC (A-CONSTANT 1))                 ;Borrow from high if low is zero
DIV0      ((M-AC) SETCM M-AC)                              ;1's complement high dividend
          (CALL DIV2)                                      ;Now, call positive-dividend case
          (POPJ-AFTER-NEXT (M-1) SUB M-ZERO A-1)           ;Make the remainder negative
         ((M-AC) SUB M-ZERO A-AC)                          ;and change the sign of the quotient

; Divide routine for positive dividend.
DIV1      ((Q-R) M-1)                                      ;Low dividend to Q-R
DIV2      ((M-1) DIVIDE-FIRST-STEP M-AC A-2)               ;First division step
          (JUMP-IF-BIT-SET (BYTE-FIELD 1 0) Q-R DIVIDE-OVERFLOW) ;Error check
(REPEAT 31. ((M-1) DIVIDE-STEP M-1 A-2)                    ;Middle division steps
          ((M-1) DIVIDE-LAST-STEP M-1 A-2)                 ;Final step, quotient in Q-R
          ((M-1) DIVIDE-REMAINDER-CORRECTION-STEP M-1 A-2) ;M-1 gets remainder
          ((M-AC) Q-R)                                     ;Extract quotient from Q-R
          (POPJ-AFTER-NEXT                                 ;Return after next, but if
           POPJ-GREATER-OR-EQUAL M-2 A-ZERO)               ; divisor is negative,
         ((M-AC) SUB M-ZERO A-AC)                          ; change sign of quotient

The Bus Interface

The Bus Interface connects the CADR machine to two busses, the Unibus and the Xbus. The Unibus is a regular pdp11 bus, used to attach peripheral devices, especially commercial devices designed for the PDP11 line. The Xbus is a 32-bit bus used to attach memory and high-performance peripheral devices, such as disk. The bus interface also includes the diagnostic interface, which allows a unibus operator, such as a pdp10, a pdp11, or another lisp machine, to control the operation of the machine, hardware to pass interrupts from the Unibus and the Xbus to the processor, the logic which arbitrates the Xbus, and the logic which arbitrates the Unibus in the absence of a pdp11 on that bus.

The Bus Interface allows the CADR machine to access memory on the Xbus and devices on the Unibus, allows independent devices on the Xbus to access the Xbus (only), and allows the Unibus devices to access Xbus memory (through a map since the Unibus address space is not big enough.) Buffering is provided when the Unibus accesses the Xbus, to convert a 32-bit word into a pair of 16-bit words.

The CADR machine sees a 22-bit physical address space of 32-bit words. The top 128K of this, locations 17400000-17777777, reference the Unibus. Each 32-bit word has a 16-bit Unibus word in bits 0-15, and zero in bit 16-31. There is no provision for using byte addressing on the Unibus, nor for read-pause-write cycles. The 128K immediately below the Unibus, locations 17000000-17377777, are reserved for Xbus I/O devices. Locations 0-16777777 are for Xbus memory.

The bus interface includes a number of Unibus registers which control its various functions:

Spy Feature

Unibus locations 766000-766036 are used for the Spy feature, which is described in detail elsewhere. These locations read and write various internal signals in the CADR machine, and provide the necessary hook for microcode loading and diagnostics.

Two-Machine Lashup

Two bus interfaces may be cabled together with a single 50-wire flat cable for maintenance purposes. One machine, the debugger, is able to perform reads and writes on the other machine's, the debuggee's, Unibus. Through registers on the Unibus (such as the Spy feature), the debuggee may be diagnosed and exercised. By using the debuggee's Unibus map (described below), the debuggee's Xbus can be exercised. The following locations on the debugger's Unibus control this feature:
766100Reads or writes the debuggee-Unibus location addressed by the registers below.
766114(Write only) Contains bits 1-16 of the debuggee-Unibus address to be accessed. Bit 0 of the address is always zero.
766110(Write only) Contains additional modifier bits, as follows. These bits are reset to zero when the debuggee's Unibus is reset.
1Bit 17 of the debugee-Unibus address.
2Resets the debuggee's Unibus and bus interface. Write a 1 here then write a 0.
4Timeout inhibit. This turns off the NXM timeout for all Xbus and Unibus cycles done by the debuggee's bus interface (not just those commanded by the debugger).
766104(Read only) These contain the status for bus cycles executed on the debuggees's busses. These bits are cleared by writing into location 766044 (Error Status) on the debuggee's Unibus. They are not cleared by power up. The bits are documented below under "Error Status".

Error Status

766044Reading this location returns accumulated error status bits from previous bus cycles. Writing this location ignores the data written and clears the status bits. Note that these bits are not cleared by power up.
1 Xbus NXM Error. Set when an Xbus cycle times out for lack of response.
2 Xbus Parity Error. Set when an Xbus read receives a word with bad parity, and the Xbus ignore-parity line was not asserted. Parity Error is also set by Xbus NXM Error.
4CADR Address Parity Error. Set when an address received from the processor has bad parity. Indicates trouble in the communication between the processor and the bus interface.
10Unibus NXM Error. Set when a Unibus cycle times out for lack of response.
20CADR Parity Error. Set when data received from the processor has bad parity. Indicates trouble in the communication between the processor and the bus interface.
40Unibus Map Error. Set when an attempt to perform an Xbus cycle through the Unibus map is refused because the map specifies invalid or write-protected.

The remaining bits are random (not necessarily zero).


The bus interface allows the CADR machine to field interrupts on the Unibus, if no pdp11 is present. If a pdp11 is present, its program can forward interrupts to the CADR machine in a transparent way. The Xbus can interrupt the CADR machine. The following Unibus locations control interrupts and the Unibus arbitrator:
766040Reading this location returns interrupt status, as follows:
1Disable Interrupt Grant. If this is set, the Unibus arbitrator will not grant BR4, BR5, BR6, and BR7 requests. It will continue to grant NPR requests. Powers up to zero.
2Local Enable (read only). 1 means that the bus interface is arbitrating the Unibus. 0 means that a pdp11 is present on the bus and is doing the arbitration.
1774Bits 9-2 contain the vector address of the last Unibus interrupt accepted by the bus interface or simulated by the pdp11 program.
2000Enable Unibus Interrupts. A 1 here causes bit 15 (Unibus interrupt) to be set when the bus interface accepts a Unibus interrupt. This bit is not reset by power-up.
4000Interrupt Stops Grants. A 1 here causes bit 0 (Disable Interrupt Grant) to be set when the bus interface accepts a Unibus interrupt, thus preventing further interrupts until the CADR machine has processed the first interrupt. This bit is not reset by power-up.
30000Bits 13-12 are the "interrupt level" for purposes of Unibus granting. The mapping to normal pdp11 levels is : 0->0, 1->4, 2->5, 3->6. To simulate level 7, turn on Disable Interrupt Grant. These bits are not reset by power-up.
40000Xbus Interrupt (read only). This bit is the interrupt-request line on the Xbus.
100000Unibus Interrupt. A 1 indicates that a Unibus interrupt has been accepted by the bus interface or simulated by a pdp11 program, and is awaiting processing by the CADR program. This bit clears on power-up. Note that the interrupt-request signal to the CADR machine is the OR of bits 14 and 15.
766040Writing this location writes into bits 0 and 10-13 (mask 36001) of the above register. This is used to change the "interrupt level" and to re-enable acceptance of Unibus interrupts after processing an interrupt.
766042Writing this location writes into bits 2-9 and 15 (mask 101774) of the above register. This is used to simulate Unibus interrupts and to clear bit 15 (Unibus Interrupt) after processing an interrupt.

Locations between 766040 and 766136 not mentioned above are duplicates of other locations, and should not be used.

Unibus Map

Unibus locations 140000-177777 are divided into 16 pages which can be mapped anywhere in Xbus physical address space. Each page is 512 16-bit words or 256 32-bit words long, the same size as the pages of the CADR virtual memory. The first 8 pages can be addressed by a pdp11, while the second 8 are hidden under the pdp11 I/O space. The Unibus map is intended to be used both as a diagnostic path to the Xbus and for operating Unibus peripherals that access memory.

Each Xbus location occupies 4 Unibus byte addresses. It takes two 16-bit Unibus cycles to read or write one 32-bit Xbus location. 16 buffers (one for each page) are provided to hold the data between the two Unibus cycles. As long as each page is only in use by a single bus-master, the right thing will happen.

An additional feature is that writing an Xbus address of 17400000 or higher through the Unibus map writes into CADR's MD register. This provides a 32-bit parallel data path into the processor for diagnostic purposes. These Xbus addresses are otherwise unusable, because they are used by the processor to address the Unibus.

Unibus locations 766140-766176 contain the 16 mapping registers. Note that these power up to random contents, and should be cleared by an initialization routine. The bit layout is:
100000Bit 15 is the map-valid bit. If this is 0, this mapping register is not set up, and will not respond to the Unibus; NXM timeout will occur and an Error Status bit will be set.
40000Bit 14 is the write-permit bit. If this is 0, this mapping register will not respond to Unibus writes; NXM timeout will occur and an Error Status bit will be set.
37777Bits 13-0 contain the Xbus page number. These bits are concatenated with bits 9-2 of the Unibus address to produce the mapped Xbus address.

The Xbus

The Xbus is the standard 32 bit wide data bus for the CADR processor. Main memory and high speed peripherals such as the disk control and TV display are interfaced to the Xbus. Control of the Xbus is similar to the Unibus, in that transfers are positiely timed and (as far as the devices are concerned) asynchronous. The bus is terminated at both ends with resistive pullups of 390 ohms to ground and 180 ohms to +5 volts, for an effective 123 ohm termination to +3.42 volts. At ground, each termination draws 28mA for a total load of 56 mA. The bus is open collector, and may be driven with any device capable of handling the 56 mA. load. The recommended driver is the AMD 26S10, which also provides bus receivers.

A typical read cycle begins with placing the address for the transfer on the -XADDR lines and the parity of the address on the -XBUS.ADDRPAR line. The -XBUS.RQ line is then lowered, initiating the request. The responding device places the requested data on the 32 -XBUS lines and the parity of the data on the -XBUS.PAR line. Should it not be convenient for the device to produce parity (as in the case of I/O registers), the device may assert -XBUS.IGNPAR to notify the bus master that the transfer should not be checked for correct parity. The responding device then asserts -XBUS.ACK, which remains asserted until the -XBUS.RQ signal is removed by the master.

Write requests proceed identically, except that the master asserts -XBUS.WR and the data to be written on the -XBUS lines along with the address lines. All bus masters are required to produce good parity data on writes.

Deskewing delays are the responsibility of the bus master. In particular, it is the responsibility of the bus master to assert good address, write, and data lines 80 ns. prior to asserting -XBUS.RQ, and these lines must be held until the -XBUS.ACK signal drops in response to the master dropping -XBUS.RQ. Responding devices are allowed to assert -XBUS.ACK at the same time they drive read data onto the -XBUS lines. Thus, masters should delay 50ns. after receiving -XBUS.ACK before dropping -XBUS.RQ and strobing the data. Responding devices are required to drop -XBUS.ACK immediately after -XBUS.RQ is no longer asserted.

Normal bus master arbitration between the CADR processor and the Unibus requests is handled by the bus interface. Devices on the Xbus which must become bus master, such as the disk control, do so by asserting the -XBUS.EXTRQ signal. When the bus becomes free, the bus interface responds by asserting -XBUS.EXTGRANT. This signal is daisy chained between bus master devices on the Xbus, coming in on the -XBUS.EXTGRANT.IN pin and leaving on the -XBUS.EXTGRANT.OUT pin. Within each device, the decision is made whether or not to pass the grant onto the next device. Unlike the Unibus structure, the decision on whether to pass grant and the act of becoming bus master happen synchronously with a master clock signal distributed on the -XBUS.SYNC line.

When an device initiates a request, it immediately asserts -XBUS.EXTRQ. At the falling edge of -XBUS.SYNC it clocks the request signal into a D flip flop which we will call REQ.SYNC. When -XBUS.EXTGRANT.IN goes low, the device asserts -XBUS.EXTGRANT.OUT unless it has either the REQ.SYNC flip flop set, or is already the bus master. At the next falling edge of -XBUS.SYNC the device which has both -XBUS.EXTGRANT.IN and REQ.SYNC set becomes bus master. The device should immediately assert -XBUS.BUSY and may immediately begin asserting address lines for a transfer. -XBUS.BUSY may be dropped asynchronously, after the slave device drops -XBUS.ACK in response to the master's request.

The -XBUS.EXTGRANT.IN signal must be terminated with a resistive pullup of 180 ohms to +5 volts within each device which does not simply connect it to -XBUS.EXTGRANT.OUT.

XBUS Signal Review:

Data Lines:
-XBUS<31:0>32 data lines, low when data is a one.
-XBUS.PARParity of the 32 data lines, required for writes.
-XBUS.IGNPARIgnore parity signal, may be asserted by any device for a read.
Address Lines:
-XADDR<21:0>22 address lines, low for address bit a one.
-XADDR.PAROdd parity for the address.
Cycle control lines:
-XBUS.RQAsserted by the master to request a read or write. Minimum of 80 ns following stable -XADDR, -XBUS.WRITE, and -XBUS data.
-XBUS.ACKAsserted by the slave in response to -XBUS.RQ No delay necessary following assertion of good read.
-XBUS.WRAsserted by the master during a write cycle.
Mastership control lines:
-XBUS.BUSYAsserted when a device other than the bus interface is bus master. Only the bus interface examines this line. Asserted on a -XBUS.SYNC clock edge, dropped asynchronously after -XBUS.ACK drops.
-XBUS.EXTRQAsserted when a device other than the bus interface wishes to become bus master. Asserted asynchronously, may be removed asynchronously after the device becomes master, but before dropping -XBUS.BUSY.
-XBUS.EXTGRANT.INThe daisy-chained mastership grant signal. Must be pulled up to +5V with a 180 ohm resistor.
-XBUS.EXTGRANT.OUTAsserted initially by the bus interface, synchronously with the -XBUS.SYNC edge. The signal may be subject to synchronizer lossage, since it is a clocked version of -XBUS.EXTRQ which is not synchronous with -XBUS.SYNC.
-XBUS.INITWhen low, resets all devices. This is low during power on and off, and when the machine is reset.
-XBUS.SYNCSynchronization clock for mastership passing and other desired purposes. Devices become bus master synchronous with the edge of this signal. The request will normally follow the edge by 80 ns.
-XBUS.INTRDriving this low requests an interrupt. All devices are required to initialize to a non-interrupt enable condition, and are required to have interrupt enable and disable bits which can selectively enable interrupts from that device. The "requesting interrupt" state must be readable in one of the device control register bits.
-XBUS.POWER.OKThis line is HIGH when power is stable. It remains low for 3 seconds after power comes on, and goes low 3 seconds before power is turned off.

Error Checking

All internal memories in the CADR machine have parity checking. If bad parity is detected, the machine is halted, if that is enabled. The processor always completes the current instruction, and clocks the next one into the IR, before halting. This is done to simplify the timing and to ensure that it halts with the scratchpad memory latches open. It means that the data with bad parity will no longer be on the busses once the machine stops. Furthermore, one incorrect instruction will have been executed. The OPC registers can be helpful in reconstructing what must have happened.

Upon initial power-on, error halting is disabled, but it is expected that as soon as the bootstrap program has initialized all internal memories it will enable error halting.

Main memory parity is checked and can either halt the machine, cause a microcode trap, or be ignored, depending on mode flags in the diagnostic interface.

The data paths do not have any redundant checking. When the machine is bootstrapped it runs some simple diagnostics designed to detect solid failures in the memories and data paths.

Self Bootstrapping

When the machine is powered on it resets itself and the Unibus but does not automatically start up. A bootstrap sequence can be initiated in one of several ways. The diagnostic interface can command one. The diagnostic display panel, by grounding one wire, can start one. This is intended to be connected to a push button. The bus interface can start a bootstrap by grounding one wire. The chaos network interface, if it receives a certain sequence of messages from the network, will "push the boot button." The I/O board recognizes a special set of keyboard commands (left and right control-meta) as a boot signal. The character typed along with the left-right control-meta is available to the bootstrap for selection of software options.

The bootstrap sequence starts by resetting the machine, which will halt it if it is running. It turns on RUN, which will not do anything yet since the clock is stopped. It sets the machine to its slowest speed, disables parity traps, error halts, and the statistics counter, and enables the PROM (read-only) control memory. The trailing edge of the boot signal allows the clock to start, causing a trap to microcode location 0, just like the memory parity trap. Location 0 of the PROM receives control. It must clear all internal memories (filling them with good parity), reset the Unibus (before first using it), enable error halts, set the machine speed to its normal value, run some diagnostic checks to be sure the machine is working to some extent, load the microcode from the disk, load the initial contents of main memory from the disk, and transfer control to the normal microcode at its start address by going over the Unibus and manipulating the diagnostic interface.

If the diagnostic self-test fails, the microcode goes into a loop, and the value of the PC can be read from the diagnostic display to determine what the problem seemed to be.

Interrupts and Sequence Breaks

Interrupts are hardware signals to the microcode - typically the microcode transfers data in or out of a buffer in main memory. When the signal requires the attention of full Lisp code, a sequence break is triggered. This consists of setting a sequence-break pending flag in A-memory, and, if a defer-sequence-break flag (also in A-memory) is not set, setting the hardware sequence-break flag. This flag is tested at various convenient points such as macroinstruction fetch, and causes the microcode to turn off the flag and enter the sequence-break routines. The sequence-break flag is tested by the same jump instruction that tests for page faults and interrupts.

Interrupts can be generated by both the Xbus and the Unibus. The exact protocol is documented in the section on the bus interface.

Sequence-breaks are software signals indicating the need to run the scheduler (a Lisp program). A sequence-break suggests that the condition for which some process is waiting may have become true. The scheduler checks all processes for runnability, and also checks if it is time to perform periodic actions which are not full processes. Lisp programs can defer sequence-breaks to protect critical areas, while still allowing interrupts so that real-time response at the lowest level is preserved.

Access to virtual memory in the Lisp Machine software environment is viewed as a primitive operation. Regardless of the actual location of a memory datum, the fetch of that item is continued. This view considerably simplifies coding of the system, but imposes moderately high potential latencies in responding to sequence breaks. Interrupts are handled entirely at the microcode level, and the response time for these will be quite short.

The interrupt-control register, writable by functional destination 2, and readable in the high bits of LC (functional source 13), contains three bits relevant to interrupts. Bit <27>, INTERRUPT ENABLE, allows the external interrupt signal from the bus interface to be seen by the JUMP instruction. Bit <26>, SEQUENCE-BREAK, is the sequence-break flag which is testable by the JUMP instruction.

Bit <28>, BUS-RESET, generates a RESET signal on the Unibus (BUS INIT L) and on the Xbus (XBUS.INIT L), and resets the bus interface, when it is written 1 and then 0. The machine also resets the busses when it is powered up.

Bit <29> is used by the Instruction-Stream feature.

The Statistics Counter

The statistics counter is a 32-bit counter, which increments whenever an instruction with bit 46 = 1 is executed. When the counter overflows from -1 to 0 the machine stops, after completing execution of the instruction which caused the overflow. (The stopping is under control of an enable bit in the diagnostic interface.) Bit 46 is always 0 in instructions from the PROM.

The statistics counter can be read and written using the diagnostic interface. It provides several facilities.

It can be used for metering, to measure how many instructions are executed, possibly restricted to a certain subset of the microprogram. The microcode debugger and console program has commands to set and clear the statistics bits in areas of control memory.

It can be used for breakpointing, by setting the counter to -1 and turning on the statistics bit in those instructions which have breakpoints set on them.

It can be used to find obscure bugs, by setting the statistics bit in all locations of control memory, and setting the appropriate number in the statistics counter to cause the machine to halt just before the point where the error appears, so that it can be single-stepped through the suspect microcode.

The statistics counter is loaded from the Instruction Write Register, rather than the normal diagnostic bus, because of its 32-bit width. Effectively it loads from the M bus with a 1-cycle delay. It is probably not possible for the machine to use the statistics counter on itself, although clever ways might be found.

The Diagnostic Interface

The diagnostic interface occupies 16 Unibus addresses. It includes a 16-bit diagnostic bus which can be used to read and write various portions of the machine. There are 16 readable locations, and 8 writable locations. A readable location and a writable location at the same address have no relation to each other. The diagnostic bus is used by debugging and maintenance programs, including the "console" program, and in a few cases by the machine itself during bootstrapping.

First we will describe the readable locations. These are sometimes called the "spy feature." Naturally, most of these are somewhat meaningless if read while the machine is running.

766000 IR<15-0>.The low 16 bits of the currently-executing instruction.
766002 IR<31-16>.The middle 16 bits of the currently-executing instruction.
766004 IR<47-32>.The high 16 bits of the currently-executing instruction.
766006 not used
766010 OPC.The OPCs are described below.
766012 PC.The current program counter, which is the address of the next instruction to be executed.
766014 OB<15-0>.The low half of the output bus.
766016 OB<31-16>.The high half of the output bus.
766020 Flag Register 1.This provides various signals associated with starting and stopping the machine. When the machine stops due to a hardware error, this register tells what happened.
The bits are:
<15> = -WAIT.1 if the machine is running or runnable, 0 if it is waiting for memory. See the discussion of Clocks for the exact meaning of WAIT.
<14> = -V1PE.Normally 1, 0 if the level-2 map had a parity error at the last clock.
<13> = -V0PE.Normally 1, 0 if the level-1 map had a parity error at the last clock.
<12> = HIGHOK.1 if the high runs in the machine are all valid, 0 if some are not. This is essentially a power-supply check, and a check for broken wires.
<11> = -STATHALT.Normally 1, 0 if the machine has been stopped by the statistics counter.
<10> = ERR.1 if an error condition is present. If ERRSTOP is on in the mode register, the machine is stopped.
<9> = SSDONE.1 if a single-step operation has been completed.
<8> = SRUN.1 if the machine is trying to run (but it may be stopped by a parity error, by a wait condition, or by the statistics counter).
<7> = -HIGHERR.1 if there was HIGHOK at the last clock.
<6> = -MEMPE.Normally 1, 0 if there was a main memory parity error that was not caught by a trap at the last clock.
<5> = -IPE.Normally 1, 0 if there was a control memory parity error at the last clock.
<4> = -DPE.Normally 1, 0 if there was a dispatch memory parity error at the last clock.
<3> = -SPE.Normally 1, 0 if there was an SPC stack parity error at the last clock.
<2> = -PDLPE.Normally 1, 0 if there was a PDL-buffer parity error at the last clock.
<1> = -MPE.Normally 1, 0 if there was an M-scratchpad parity error at the last clock.
<0> = -APE.Normally 1, 0 if there was an A-scratchpad parity error at the last clock.
766022 Flag Register 2.This register contains flags associated with pipelining and some miscellaneous control signals which the debugging program likes to see.
The bits are:
<15> = unused
<14> = unused
<13> = WMAPD.The previous cycle said to write the map, and this cycle will.
<12> = DESTSPCD.The previous cycle wrote into the SPC stack by using a functional destination (as opposed to a CALL transfer).
<11> = IWRITED.The previous cycle did an I-MEM WRITE type of JUMP instruction, and this cycle will write control memory, do a RETURN transfer, and NOP the following cycle.
<10> = IMODD.The previous cycle used the "OA register" to modify this cycle's instruction, or this cycle's instruction came from the DEBUG-IR (see below). This flag inhibits parity checking of the IR.
<9> = PDLWRITED.The previous cycle caused a write into the PDL-buffer, and this cycle will do it.
<8> = SPUSHD.The previous cycle caused a write into the SPC stack, and this cycle will do it.
<7> = unused
<6> = unused.
<5> = IR<48>.This is the parity bit of the IR.
<4> = NOP.The instruction currently in the IR is not really being executed; this cycle is a NOP cycle.
<3> = -VMAOK.The last attempt to start a main memory cycle was not successful because the map indicated a page fault.
<2> = JCOND.1 if the jump-condition is satisfied. Meaningless if the instruction in IR is not a JUMP instruction.
<1-0> = PCS1-0.These 2 bits select the next PC (the address of the instruction after next.)
The encoded values are:
0 = SPC<13-0> the SPC stack.
1 = IR<25-12>the address specified by a JUMP instruction.
2 = DPC<13-0>the dispatch memory.
3 = IPC<13-0> the PC+1.
766024 M<15-0>.The low half of the M-source selected by the instruction currently in IR.
766026 M<31-16>.The high half of the M-source.
766030 A<15-0>.The low half of the A-source selected by the instruction currently in IR.
766032 A<31-16>. The high half of the A-source.
766034 ST<15-0>. The low half of the statistics counter.
766036 ST<31-16>.The high half of the statistics counter.

Here is a description of the writable registers of the diagnostic interface.

766000 DEBUG-IR<15-0>.The low 16 bits of an instruction supplied by the diagnostic interface.
766002 DEBUG-IR<31-16>.The middle 16 bits.
766004 DEBUG-IR<47-32>.The high 16 bits.
766006 Clock control register.Resetting the machine sets this to zero.
The following bits exist:
<4> = LDSTAT.Setting this to 1, then clocking the machine, causes the statistics counter to load from IWR<31-0>, which loaded from the M bus on the previous clock.
<3> = IDEBUG.Setting this to 1 causes the IR to load from the DEBUG-IR instead of the PROM or the control memory, when the machine is clocked. The primary way that the machine can be manipulated through the diagnostic interface is by executing instructions using this mechanism.
<2> = NOP11.Setting this to 1 forces NOP. This allows you to clock the machine, for instance to transfer DEBUG-IR into IR, without the present contents of the IR causing unwanted side-effects by getting executed as an instruction. NOP11 does not prevent the PC from getting changed (in fact it will be incremented), and it does not prevent previously-scheduled pipelined writes from happening.
<1> = STEP.Setting this to 1, when SSDONE is 0, causes the processor clock to run for one cycle, and then set SSDONE. Setting STEP to 0 clears SSDONE. (Both of these operations really take several cycles of the clock to complete.) STEP is the way that the diagnostic interface "clocks" the machine. Note that the main clock is running all the time, even when the machine is stopped. STEP generates a single processor clock, in synchronism with the main clock.
<0> = RUN.Setting this to 1 causes the machine to start running. You first use STEP to set up the state of all the registers and memories, the PC, and the IR, then turn on RUN. The first instruction executed is the one you left in the IR.

766010 OPC control register.Resetting the machine sets this to zero. This register contains some bits which need to be used by the console program in order to completely restore the state of the machine from a saved state.
The bits are:
<2> = OPCINH.Setting this to 1 inhibits the OPCs from being clocked by the processor clock. This bit must not be changed except when the clock is high (i.e. the machine is stopped). The process of restoring the OPCs consists of setting OPCINH, then getting the 8 values into the PC by executing JUMP instructions, and transferring those values into the OPCs via the OPCCLK bit. Once the OPCs have been restored, OPCINH remains set so that they will be undisturbed while the rest of the machine state is restored. Just before starting the machine, set OPCINH to 0.
<1> = OPCCLK.Setting this to 1 and then to 0 generates a clock to just the OPCs. This is used to read out the 8 OPC registers without disturbing the state of the rest of the machine.
<0> = LPC.HOLD.Setting this to 1 prevents the LPC register from loading from the PC register when the machine is clocked. This is used in restoring the LPC. The LPC is a duplicate copy of the first OPC register, used by the IR<25> feature of the DISPATCH instruction.
766012 Mode register.Resetting the machine sets this to zero. This register enables various features and controls the speed of the clock.
The bits are:
<7> = PROG.BOOT.Setting this to 1 starts a bootstrap sequence.
<6> = PROG.RESET.Setting this to 1 resets the machine. Reset stops the machine by clearing RUN, forces the clock to stop until the RESET operation is over, clears the pipeline flags which cause things to happen in the next instruction, and clears the Clock, Mode, and OPC registers of the diagnostic interface.
<5> = PROMDISABLE.A 1 here disables the PROM. A 0 here replaces the first 1K locations of control memory with the PROM.
<4> = TRAPENB.A 1 here enables main memory parity errors to cause microcode traps to location 0. A 0 here causes main memory parity errors to be treated the same as other parity errors.
<3> = STATHENB.A 1 here enables overflow of the statistics counter to halt the machine.
<2> = ERRSTOP.A 1 here enables hardware errors (HIGHERR and various parity errors) to halt the machine. A 0 causes it to continue blithely on.
<1-0> = SPEED<1-0>.These bits control the speed of the clock. The ILONG bit in the microinstruction also affects the speed, slowing it down by 40 nanoseconds.
The speed codes are:
0 = Extra Slow
1 = Slow
2 = Normal
3 = Fast
766014 not used.
766016 not used.

The OPCs are a set of 8 registers which remember the last 8 values of the PC. This provides a useful history for debugging. It is also used by the microcode itself in certain trap-handling routines. You can only read the last of the 8 OPCs, which is what the PC was 8 clocks ago. Special control is provided over the clocking of the OPCs so that they can be read out without di so that they can be saved and restored by the microcode debugger. This is described above under 766010.

The OPCs can be read both by the diagnostic interface and as a functional source, for maximum flexibility.

The bus interface provides a special path by which the MD register may be loaded. This provides a parallel source of diagnostic input data. After loading MD, instructions can be executed via the DEBUG-IR to transfer the data to the desired destination.

There are several maintenance indicators (light-emitting diodes) scattered around the machine. Inside the front door, near the lower-left-hand corner, are 5 octal displays. These show the current value of the PC. The decimal points on these displays show various interesting conditions. From left to right:

1 - PROMENABLE.Indicates that the current instruction is coming from the PROM rather than the writable control memory.
2 - IPE.Indicates that control memory had a parity error at the last clock.
3 - DPE.Indicates that dispatch memory had a parity error at the last clock.
4 - TILT0.Indicates that the map or main memory had a parity error at the last clock.
5 - TILT1.Indicates that the A-scratchpad, the M-scratchpad, the PDL-buffer, or the SPC stack had a parity error at the last clock.

There is also provision for indicators for the various error conditions, "the machine is really running," and the status of the disk interface. The location of this indicator panel, and whether or not all machines will have one, is not yet determined.

The Disk Controller

The Lisp machine disk controller attaches from 1 to 8 disk units of the "Trident" family to the CADR machine's XBUS. The 1-unit version consists of one board, and a second board is added when more than one disk unit is to be used. The two versions are almost program compatible.

Interface Registers

The disk controller is operated by reading and writing four 32-bit registers which are on the XBUS. These are normally at physical addresses 17377774-17377777, which is just below the Unibus. The address can be changed by changing jumpers. Many bits in these registers refer to the "selected unit", which is that disk unit whose number is currently in bits <30:28> of the disk-address register.

When read, the registers are:

<24:31>The block-counter of the selected unit. This tells you its current rotational position. Reading of this register is not synchronized to its incrementation, so you must read it twice and check that it came out the same both times.
<23>Internal Parity Error. This indicates that parity of the bits seen at the disk and parity of the bits seen at the memory failed to agree; something must have been lost inside the controller someplace. The Read All and Write All commands cause spurious internal parity errors. The Read Compare command causes a spurious internal parity error if it sets Read Compare Difference (bit 22) and the the disk data and the memory data differ in parity. This error does not stop the transfer.
<22>Read Compare Difference. This indicates that data from memory and data from the disk failed to agree. This bit is undefined unless the command is read-compare. This error does not stop the transfer.
<21>CCW Cycle. This bit being on in combination with Memory Parity Error or Nonexistent Memory Error indicates that the error happened while fetching a CCW, rather than while reading or writing data.
<20>Nonexistent Memory Error. Indicates that memory (or other XBUS device) failed to respond within 15 microseconds. This error stops the transfer.
<19>Memory Parity Error. Indicates that even parity was read from memory (or other XBUS device). This error stops the transfer.
<18>Header Compare Error. Indicates that a block-header read from disk failed to have the expected value. This may be because the disk head is not positioned at the proper place, because the disk is not correctly formatted, or because the header wasn't read correctly. This error stops the transfer.
<17>Header ECC Error. Indicates that the error-correcting code of a block header failed to check. Unfortunately most header ECC errors show up as header compare errors instead. Maybe this can be fixed? This error stops the transfer. Header ECC Error also happens if an attempt is made to continue a read or write operation past the end of the disk.
<16>ECC Hard. Indicates that the error correcting code discovered an error, and was unable to correct it. The data read from disk is wrong, try reading again. This error stops the transfer.
<15>ECC Soft. Indicates that the error correcting code discovered an error, and was able to determine which data bits were in error. The program can correct it, see the ECC Register for how. The error correcting code will correct any single burst of up to 11 erroneous bits. This error stops the transfer.
<14>Read Overrun. Indicates that data arrived from the disk faster than it could be stored into memory. This error stops the transfer.
<13>Write Overrun. Indicates that memory did not supply data fast enough for the disk. This error stops the transfer.
<12>Start Block Error. Indicates that a start-of-block (sector pulse) happened at a time when it should not have. Either the disk is incorrectly formatted or it is generating spurious sector pulses. This error stops the transfer.
<11>Timeout Error. Indicates that a disk operation took longer than 2.5 seconds. This error stops the transfer.
<10>Selected Unit Seek Error. The selected unit is reporting failure of a seek operation. This error stops the transfer. Reset the error by using the Recalibrate command.
<9>Selected Unit not On-line. The heads are not loaded, the disk is not powered on, or there is no disk at the specified unit number. This error stops the transfer.
<8>Selected Unit not On-Cylinder. Generally indicates that a seek is in progress on the selected unit. Not an error. If the disk goes off-cylinder during a write operation, a fault will occur. If it goes off cylinder during a read, presumably a header-compare error or an ECC error will occur.
<7>Selected Unit Read-Only. The status of a switch on the disk. Note that the read-only status can only change to reflect a change in the switch when the drive is not selected. Storing into the Disk Address register momentarily deselects the current unit so that it may update its read-only status from the switch. Writing while the disk is read-only causes a fault.
<6>Selected Unit Fault. Indicates either trouble with the disk or a programming error, see the Trident manual. This error stops the transfer. Reset by using the Fault Clear and/or Recalibrate commands. This error lights the Device Check light on the drive.
<5>No Unit Selected. This error stops the transfer. Happens if no disk is plugged into the selected unit number, or the disk unit is powered off or "degated".
<4>Multiple Units Selected. This error stops the transfer. This indicates that more than one disk drive is selected, or the wrong drive is selected.
<3>Interrupt Request. 1 means the disk controller is asserting -XBUS.INTR.
<2>Selected Unit Attention. Reset using the At Ease command. Attention indicates seek completion, recalibrate completion, initial loading of the heads, seek incomplete error, or an emergency head retract. "Implicit" seeks do not cause attention.
<1>Any Attention. Some unit has an attention, you have to select them one after another to find out which.
<0>Not Active. 0 means the controller is busy, 1 means it is ready to accept a command.
<31:24>not used
<23:22> Disk type. 00 Trident 01 Marksman 10 unused 11 Trident (old control)
<21:0> the address of the last memory reference made by the disk control. This is the address of a CCW if CCW Cycle is on in the status register, otherwise the address of a data word.
<31>not used
<30:28>Unit number. In the 1-unit version, always zero.
<27:16>Cylinder number. A T-80 has 815. cylinders.
<15:8>Head number. A T-80 has 5 heads. As it turns out, only the bottom 6 bits of the head number can work (this is a feature of the Trident.)
<7:0>Block number. A T-80 is usually formatted with 17. blocks per track. "Block" is mostly synonymous with "sector".

When a transfer is terminated by an error, the disk address register contains the address of the block being transferred when the error occurred. When a transfer terminates normally, the disk address register has the address of the last block transferred.

<31:16>Error pattern bits.
<15:0>Error bit position+1.

When a soft ECC error occurs, this register tells where in the last block transferred the error was. The disk address register has the disk address of the block containing the error, and the command list pointer points to the CCW which points to the memory page containing the error. The error pattern should be XOR'ed into the contents of memory at the specified bit address; it may overlap across a word boundary. Note that the bit position is off by 1; the first bit in the block is bit 1.

You should not write any register while a transfer is active, except for using the Reset command to stop a hung transfer, and even then you should expect to lose.

When written, the registers are:

Writing the command register does NOT initiate a transfer, unlike most disk controllers. Use register 3 (START) to initiate a transfer, after setting up the other registers. However, writing the command register does reset the various error flags. Note that the command register cannot be read back.
<31:12> not used
<11> Done Interrupt Enable.Enables not-active (bit 0 of the status register) to cause an interrupt. The interrupt will keep happening until you clear this bit. (This is really an idle interrupt rather than a done interrupt.)
<10> Attention Interrupt Enable.Enables any-attention (bit 1 of the status register) to cause an interrupt. (The interrupt will only happen if the controller is not active. While the controller is active you couldn't do anything about it anyway.) The interrupt will keep happening until you select the drive and give an at-ease command, or clear this bit.
<9>Recalibrate.In combination with command 5, causes the disk to return the heads to cylinder 0.
<8> Fault Clear.In combination with command 5, resets most fault conditions in the disk.
<7> Data Strobe Late.For recovery of marginal data.
<6> Data Strobe Early.For recovery of marginal data.
<5> Servo Offset.For recovery of marginal data, offsets the heads slightly. Bit 4 controls which direction. Note that this is somewhat kludgey, if you try to seek while the heads are offset you get a fault (use command 6 first to clear the offset.) Transferring more than one block at a time while in servo offset mode, or even retrying a transfer without first doing an offset clear, will probably cause a fault. Of questionable worth anyway. Writing while the heads are offset causes a fault.
<4> Offset forward.1 means offset forward, 0 means offset backward.
<3> I/O Direction.1 means from-memory, 0 means to-memory. See below for valid combinations.
<2:0> Command code.The following combinations of bits are valid commands (here expressed in octal). Note that bits 10 and 11 may always be turned on, and bits 4 through 7 may be turned on in any reading command.
0000 Read.
0010 Read-compare.Reads from both disk and memory, and sets bit 22 of the status register if they don't agree.
0011 Write.
0002 Read All.Reads all bits of the disk starting at the specified rotational position. Note that internal parity errors will occur spuriously during this command, and that it will not automatically advance heads and cylinders. See the description of disk formatting below.
0013 Write All.Writes all bits of the disk starting at the specified rotational position. This is intended for formatting the disk, see below. The caveats under READ ALL apply to WRITE ALL also. In addition, it doesn't really write quite all of the last page; somewhere between zero and seventeen words will be lost.
0004 Seek.Initiates a seek to the cylinder specified in the disk address register. An attention will occur when the seek completes. Note that this command is not logically necessary; the controller always initiates a seek if necessary at the start of a data transfer command. The read, read-compare, and write commands also will seek in the middle of a transfer when necessary. The seek command is provided so you can overlap seeks on multiple units.
0005 At ease.Resets attention on the selected unit.
1005 Recalibrate.Seek to cylinder 0, without assuming the current position of the heads is correct. This is used to correct a seek error, and as part of error recovery. Recalibrate resets some error conditions in the drive, and causes an attention when complete.
0405 Fault clear.Resets most error conditions in the drive.
1405This probably does both a Recalibrate and a Fault Clear.
0006 Offset clear.Take the heads out of the offset state. This does not wait for completion, but the next command will.
xxx7This is a reserved command, and will currently hang the controller, causing a timeout error (bit 11 in the status register.)
0016 Reset.This stops the current transfer and resets the controller. This command takes effect as soon as it is stored in the command register; no store in START is required. After storing a Reset command you should store 0 in the command register to turn off the reset condition. Use of Reset while a transfer is in progress isn't guaranteed not to do strange things.

All commands except for the xxx5 group and Reset wait for completion of any previous seek operation on the selected unit before starting. Thus even the Seek and Offset Clear commands can take finite time before the controller is ready for the next command.

This is the address of a vector of Channel Command Words (CCWs) which specify what memory pages, and how many, are to be transferred to/from disk. Only bits <15:0> of the CLP can count, so if you try to carry across this boundary your command list will wrap around.

The format of a CCW is:
<31:24>not used
<23:8>Main memory address of a page
<7:1>not used
<0>More flag. If this bit is 0, this is the last CCW in the list. If this bit is 1, there is another CCW in the following location.

See the description of the disk address register under reading. Note that in the 1-unit version, the unit number bits <30:28> are ignored and regarded as always zero.

Writing anything at this address initiates the operation specified in the command, disk address, and command list pointer registers.

Disk Structure

Each disk block contains one Lisp machine page worth of data, i.e. 256. words or 1024. bytes. You can transfer up to 65536. consecutive disk blocks to non-consecutive memory locations in a single operation, or you could if the machine supported that much main memory. A T-80 has 815. cylinders, each with 5 heads (tracks), each with 16. or 17. blocks depending on how you feel like formatting it. A T-300 is the same except it has 19. heads.


The format is determined by the program that uses the Write All operation to format the disk, within the constraints determined by the hardware. A track contains (approximately) 20160. bytes (on a T-80 or a T-300). Jumpers in the disk are set to give 17. sector pulses per track, or one every 1164. bytes, with a little left over at the end of the track.
Everything goes low-order bit first and low-order byte first. Note that bits in the disk controller are the complement of bits seen by the drive. Thus all bits in the Trident manual should be thought of as complemented.
The format of a block is:

	(sector pulse here)
	PREAMBLE - 53. bytes of ones.
	VFO LOCK - 8. bytes of ones.
	SYNC - a byte containing octal 177
	HEADER - a 32-bit word as follows:
	   <31:30> next block address code:
		0 following block on same track
		1 block 0 on next track (next head)
		2 block 0 on head 0 of next cylinder
		3 end of disk
	   <29:28> not used, should be zero
	   <27:16> cylinder number, used to verify that the
		   disk is positioned to the correct cylinder.
	   <15:8> head number, used to verify the head selection.
	   <7:0> block number, used to verify the rotational position.
	HEADER ECC - a 32-bit checkword.
	VFO RELOCK - 20. bytes of ones.
	SYNC - a byte containing octal 177
	PAD - a byte containing octal 377, which is here to fix
		a bug in the logic for read-compare.  (Ugh)
	DATA - 1024. bytes of whatever you want.
	DATA ECC - a 32-bit checkword.
	POSTAMBLE - 44. bytes of ones.

To format the disk, you should do it one track at a time. Lay out in memory the bits to be written on the track. Truncate the length to a multiple of a page, but make sure that the last 17. words don't matter (in general you will be writing 19. pages, or 19456. bytes, leaving about 771. bytes at the end of the track which may not get written, depending on how full the fifo is when the operation terminates. Depending on the block length chosen, you may not get a chance to fully write the last block, but as long as you get into the data area it will be all right. Do a WRITE ALL command of this data, with a disk address whose block-number field (bits <7:0>) is zero. Ignore any internal parity error (bit 23 of the status register.) You can verify it by using the Read All command (but the internal parity and read-compare features will not work), or you can use the ordinary write and read commands. You must compute the ECC check-words manually. The polynomial is x^31+x^29+x^20+x^10+x^8+1 [if I understand this logic correctly.]

Note that, when using Read All, there is some ambiguity as to precisely where the data read starts. It is unlikely to line up the bytes on byte boundaries. The first several microseconds worth of data will be missing or corrupted.


Connector J11 is provided for a flat cable to an LED display, with the following useful signals on it. These are ground when inactive, 15 milliamps at +3 volts or so when active.

	1	Read Active.  The controller is active and bit 0
		of the command register is 0.
	2	Write Active.  The controller is active and bit 0
		of the command register is 1.
	3	Seek.  The selected unit is not on-cylinder.
	4	Transfer Lossage.  This is the IOR of Timeout, Read
		Overrun, Write Overrun, Memory Parity Error, and
		Nonexistent Memory Error.
	5	Format Lossage.  This is the IOR of Start Block Error,
		Header Compare Error, Header ECC Error, and Reset.
	6	ECC Lossage.  This is the IOR of Hard ECC Error and
		Soft ECC Error.
	7	Disk Lossage.  This is the IOR of Multiple Units Selected,
		No Units Selected, Selected Unit Fault, Selected Unit not
		On-Line, and Selected Unit Seek Error.
	8	Spare.  This probably does not light up.
<<Here insert a one-page table of instruction formats and so forth>>

The CONSLP Assembler

CONSLP is a symbolic assembler written in Maclisp which reads in source code for the CADR machine and produces a file loadable by the CC debugger. The source code is written in the form of LISP S-expressions; symbols are LISP atomic symbols, and instructions or data items are written as lists. Comments can thus be written using Maclisp's semicolon convention. The input radix for numbers is 8 (octal), except that a trailing decimal point forces radix 10 (decimal).


A program can specify data to be loaded into the instruction, dispatch, A, and M memories. To specify which of the memories to assemble data for, the LOCALITY pseudo-op is used:

	(LOCALITY I-MEM)	;following data goes into instruction memory
	(LOCALITY D-MEM)	;ditto, dispatch memory
	(LOCALITY A-MEM)	;ditto, A memory
	(LOCALITY M-MEM)	;ditto, M memory

Location Tags and Symbols

When an atomic symbol is encountered in the instruction stream being assembled, it is taken to be a location tag (label). The tag is defined, as usual, to be the value of the next location in the current locality to be assembled into, but shifted to put the tag value into its "normal" position. For A memory tags, the normal position is the A-source field of an instruction; similarly for M memory tags. For I memory tags, the normal position is the New PC field of a JUMP instruction; for D memory tags, the Dispatch Offset field of a DISPATCH instruction. Thus, if FOO is a tag for location 7 of dispatch memory, then the effective value of FOO is 70000 .

By convention, tags in A memory begin with the letters "A-", and in M memory with "M-", but this is not enforced by CONSLP.

Symbols can also be defined by means of the ASSIGN pseudo-operation:

(ASSIGN <symbol> <value>)

For example:

The <value> may be an expression program, in general. When a symbol is referenced, the expression program is evaluated to produce the symbol's value (which may be conditional on the context in which it appears). Expression programs are discussed in a later section.


In general, CONSLP assembles a list into a data item by evaluating all the elements of the list and adding them up. There is a fairly rich language for specifying complex expression programs and assigning symbolic names to them; for now, however, we will merely use the symbols predefined by CONSLP. CONSLP also allows the fields of an instruction to be written in almost any order, but we will describe only the conventional order for writing them.

The general form of an I-MEM instruction is:

	(<popj>  (<destinations>)  <operation>  <condition> 
	   <M-source>  <byte-descriptor>  <A-source>  <target-tag>  <other fields>)

The <popj> field is POPJ-AFTER-NEXT to specify that the POPJ bit be set.

The <destinations> field may be an A or M memory tag, or the name of a functional destination, or both an M memory tag and a functional destination.

The <operation> specifies the instruction type, and possibly other fields (such as the jump condition) as well.

The <condition> may also be a separate field, though it usually is encoded as part of the operation.

The <byte-descriptor> describes the byte to be used in a BYTE or DISPATCH instruction.

The <M-source> and <A-source> specify the sources; these may be tags in the appropriate memories, or, for the <M-source>, the name of an M multiplexor source.

The <target-tag> is an I-MEM tag for JUMP instructions, or a D-MEM tag for DISPATCH instructions.

The <other fields> can be such things as the Q control and Miscellaneous Functions.

Many of these fields can be omitted, and CONSLP will default them appropriately. If the <operation> is omitted, then ALU is assumed, unless a <byte descriptor> is present either implicitly or explicitly, in which case BYTE is assumed. If only one source is present in an ALU instruction, then an opcode of SETA is supplied for an A source, and SETM for an M source, thus causing a simple movement of data. If the A source is omitted in a BYTE instruction, then location 2 in A memory is assumed (which is supposed to contain zero).

Here are some examples of instructions, with commentary. We assume the convention described above for A and M memory tags.

	((A-FOO) M-BAR)	;move from BAR in M-MEM to FOO in A-MEM

(CALL ZAP) ;do a CALL transfer to instruction ZAP (N bit set)
((A-FOO) SUB M-BAR A-BAZ) ;subtract A-BAZ from M-BAR, put result in A-FOO
(JUMP-EQUAL-XCT-NEXT M-BAR A-FOO LOSE) ;jump to LOSE if M-BAR equals A-FOO; N bit is clear, ; so instruction after the JUMP is executed ; whether or not the JUMP succeeds
(POPJ-AFTER-NEXT (M-FOO) MEMORY-DATA) ;put data from memory into M-FOO, ; and also POPJ after next instruction
((M-SAVE MEMORY-DATA-START-WRITE) ADD MEMORY-DATA A-ZERO ALU-CARRY-IN-ONE) ;add one to the read memory data, ; transfer to write memory data and M-SAVE, ; and begin writing the data into main memory ; at the address already in the VMA


CONSLP provides a facility for specifying literals in the A and M memories. The constructs

	(A-CONSTANT <expression>)   and   (M-CONSTANT <expression>)

may appear as an A source or M source specification, causing CONSLP to allocate a word in the appropriate memory, assemble the literal expression there, and use the address of that location as the source location. If the same constant in the same memory is referenced many times, CONSLP will assemble only one copy of it. Two constants are considered the same if their final binary values are identical, regardless of the source expressions which reduced to those values. The zero constant is treated specially, and made to refer to location 2 of the appropriate memory (hence the user should reserve these locations as constant sources of zeros). Similarly the -1 constant is made to refer to location 3 of the appropriate memory.

Byte Specifications

Rather than requiring the user to calculate the rotation count and length (minus 1) fields for BYTE and DISPATCH instructions, CONSLP provides a uniform method for specifying a byte in terms of its size and position in the word; CONSLP then calculates the fields appropriately.

The simplest way to describe a byte is with the BYTE-FIELD construct:

	(BYTE-FIELD <size in bits> <position from right>)

For example, (BYTE-FIELD 5 0) is the low five bits of a word, and (BYTE-FIELD 7 5) is the seven bits above them. The two arguments to BYTE-FIELD must be constant integers.

Another way to describe a byte is:
	(LISP-BYTE <ppss>)

where the low two octal digits of <ppss> are the size and the next two are the position. The argument <ppss> is evaluated as a LISP form (see below under "Expression Programs").

When a byte specifier appears in an instruction, the op-code is defaulted to BYTE, and the type of byte instruction defaulted to "load byte". If specified elsewhere in the instruction, the op-code may be DISPATCH instead; the dispatch is based on the specified byte. The op-code may also be JUMP, but only if the byte is one bit wide; this means that the jump will test the specified bit of the M source.

When CONSLP assembles the final instruction, it constructs the rotation count and length minus 1 fields on the basis of the byte specifier and the operation to be performed. For JUMP, DISPATCH, and "load byte" type BYTE instructions, this involves subtracting the byte position from 32 to obtain the correct rotation count. (Recall that CADR rotates words to the left.) If Miscellaneous Function 3 (LOW PC BIT specifies half word) is enabled, then the position (which should be less than 16) is subtracted from 16 instead. For "deposit byte" and "selective deposit" type BYTE instructions, the byte position itself is used as the rotation count. The length minus 1 field for BYTE and JUMP is computed by subtracting 1 from the byte length, unless the byte length is zero, in which case zero is used. (Note that CADR cannot really handle zero-length bytes, but CONSLP allows them to be defined on the theory that the "next instruction modify" feature may be in use. Programs which use this feature must be aware of the hackery which the assembler pulls, and allow for the actual values of the fields at run time.) The DISPATCH instruction has a length field instead of a length minus 1 field, and so no subtraction of 1 is performed for it.

Here are some examples of the use of byte specifiers:

	((M-X) (BYTE-FIELD 7 4) M-Y)
			;extracts a 7-bit byte, 4 bits from
			; the right, from M-Y, and puts this
			; byte right-justified in M-X.  The
			; A source is defaulted to 1, which
			; should be a constant zero so that the
			; other bits in M-X will be zero.

(JUMP-IF-BIT-SET (BYTE-FIELD 1 3) M-ZAP QUUX) ;jump to QUUX if the "10" bit is set in M-ZAP
(DISPATCH (BYTE-FIELD 3 0) M-ZAP DTABLE) ;use the low three bits of M-ZAP to index ; into the dispatch table DTABLE

It is possible to create a symbolic name for a byte field by using the ASSIGN pseudo-operation:


Since this is a common operation, another pseudo-op exists for the purpose:
   (DEF-DATA-FIELD <symbol> <byte size> <byte position>)

For example:

It is also possible to associate a name with a byte field in a particular register. One way to do this is to sum the byte specifier and the name of the register:


This case too is common enough to warrant a special pseudo-operation for the purpose:
   (DEF-BIT-FIELD-IN-REG <symbol> <byte size> <byte position> <register>)

For example:

Note that the <register> had better be in the M-scratchpad. With this definition, it is only necessary to mention, say, PRIORITY, in an instruction to cause an appropriate byte reference to occur:
	((A-PRIORITY) PRIORITY)	;extract the PRIORITY byte from PDP-11-PS
				; and place it right-justified in A-PRIORITY

By special dispensation, it also works to use such symbols in the destination field. The appropriate DPB is assembled.

Two more pseudo-operations make it easy to define names for many consecutive bits or fields in a register.

   (DEF-NEXT-FIELD <symbol> <byte size> <register>)

This defines <symbol> to be a byte of the speicified size, in a position to the left of any fields already defined by DEF-NEXT-FIELD. If this is the first DEF-NEXT-FIELD for the specified register, then the field position is zero (at the low end of the word). For example:

would be entirely equivalent to:

The pseudo-operation:
   (DEF-NEXT-BIT <symbol> <register>)

is entirely equivalent to:
   (DEF-NEXT-FIELD <symbol> 1 <register>)

and so allocates a single bit. It may be intermixed freely with DEF-NEXT-FIELD. For example:

The construct:
   (RESET-BIT-POINTER <register>)

may be used to reset the pointer into <register> used by DEF-NEXT-FIELD and DEF-NEXT-BIT. This is useful if the data in <register> can have several different formats. For example:


Dispatch Tables

When assembling into the dispatch memory (i.e. (LOCALITY D-MEM)) it is necessary to use two special pseudo-operations, START-DISPATCH and END-DISPATCH, to allocate blocks of dispatch memory. These pseudo-operations specify the length of the block required, and CONSLP undertakes to pack the various odd-sized blocks into the dispatch memory in an appropriate manner.

The typical form for a dispatch block is:

(START-DISPATCH <log2 of size> <constant data>)
<dispatch table tag>
	<first word of table>
	<last word of table>

The <log2 of size> is the number of bits that will be dispatched on, that is, the logarithm base 2 of the size of the dispatch block. The <constant data> will be added into each of the words of the dispatch table; this is useful for the P, R, and N bits (which in CONSLP are called P-BIT, R-BIT, and INHIBIT-XCT-NEXT-BIT). The END-DISPATCH is logically not necessary, but is used for error checking. Exactly the correct number of words must be assembled between the START-DISPATCH and END-DISPATCH, or CONSLP will give an error message.

As an example of a dispatch table, consider this code:



Note that the use in I-MEM of the op-code DISPATCH-CALL-XCT-NEXT is purely for cosmetic purposes, to indicate that the P bit but not the N bit is a constant in all of the dispatch table entries; it is otherwise identical to the DISPATCH op-code.

Standard Operation Codes

CONSLP supplies a large number of initial symbols for various operations, particularly for the various conditional jumps. While it is possible to define different ones, use of these standard ones is naturally encouraged. (These symbols are defined in the file LISPM; CONSYM >.)

ALU Operations

The standard ALU operations supplied by CONSLP are:

	SETCM		set to complement of M
	ANDCB		AND together complements of both M and A
	ANDCM		AND complement of M with A
	SETZ		set to zeros
	ORCB		OR together complements of both M and A
	SETCA		set to complement of A
	XOR		XOR (exclusive OR) M and A
	ANDCA		AND M with complement of A
	ORCM		OR complement of M with A
	EQV		EQV M and A (complement of XOR)
	SETA		set to A
	AND		AND together M and A
	SETO		set to ones
	ORCA		OR M with complement of A
	IOR		OR M and A (inclusive OR)
	SETM		set to M

Arithmetic ADD M plus A (two's complement addition) SUB M minus A (two's complement subtraction) M+M M plus M (two's complement addition) M+M+1 M plus M plus 1 M+A+1 M plus A plus 1 M-A-1 M minus A minus 1 M+1 M plus 1

The conditional ALU operations for multiplication and division are explained in detail in a later section. The output bus selector field defaults to 1 (output bus gets ALU output). The other two choices must be specified explicitly:

The Q control field of an ALU instruction may be specified by using one of these symbols:
	SHIFT-Q-LEFT	shift Q left (shifts inverse of ALU<31> into Q<0>)
	SHIFT-Q-RIGHT	shift Q right (shifts ALU<0> into Q<31>)
	LOAD-Q		load Q from output bus

If none of these is present, the default is to do nothing to Q. (Instead of writing LOAD-Q, one may write Q-R in the destination portion of the instruction. This does not mean that Q is a functional destination; it merely forces the operation to be ALU, and forces the Q control field to be LOAD-Q.)

The carry field may be specified by ALU-CARRY-IN-ZERO or ALU-CARRY-IN-ONE. Note that the SUB, M+M+1, M+A+1, and M+1 operations have ALU-CARRY-IN-ONE as part of their definitions, so it is not necessary to specify it explicitly.

BYTE operations

If a byte specifier is present in an instruction and the op-code is not explicitly forced to be JUMP or DISPATCH, then the op-code is BYTE by default, performing a "load byte" type of operation.

To get a "deposit byte" type operation, the symbol DPB is used; similarly, to get a "selective deposit", SELECTIVE-DEPOSIT is used. For example:

			;a true PDP-10 style DPB; the low octal
			; digit of M-BAR replaces the third lowest
			; octal digit of A-FOO.

((A-ZAP) DPB M-BAR (BYTE-FIELD 3 6) A-FOO) ;similar, but the result is placed in ; A-ZAP. A-FOO is not altered.
((A-ZAP) SELECTIVE-DEPOSIT M-FOO (BYTE-FIELD 16. 8) (A-CONSTANT -1)) ;A-ZAP gets a copy of M-FOO with the high eight ; bits and the low eight bits replaced with all ones ; (alternatively, it gets a copy of the -1 ; with the middle 16. bits replaced with ; the corresponding bits from M-FOO)

DISPATCH Operations

Four op-codes are defined in CONSLP for dispatching:


These are provided purely for cosmetic purposes, since the actual dispatch action is controlled by the dispatch table. CONSLP makes no attempt to check that the "correct" op-code is used with a given dispatch table. By convention, the XCT-NEXT versions are used iff the instruction following the dispatch instruction will be executed (N bit not set), and the CALL versions are used if the P bit is set.

To specify the value of the 10-bit "immediate argument" which is loaded into the DISPATCH CONSTANT register, one may use

	(I-ARG <expression>)		;immediate argument

in the dispatch instruction.

There is a special pseudo-op to facilitate use of the DISPATCH CONSTANT to pass a small, constant number as an argument to a subroutine. The form

generates a DISPATCH instruction to a one-word table containing a CALL-type transfer to FOO, and puts BAR in the dispatch constant field of the dispatch instruction. FOO may then use the READ-I-ARG functional source to pick up and act on the argument.

Miscellaneous Function 2 (write into the dispatch memory) is specified by the symbol WRITE-DISPATCH-RAM.

JUMP Operations

CONSLP defines a large number of names for the various JUMP operations. These are all built out of a logical progression of pieces:

	<type>  <condition>  <xct next>

The <type> may be either JUMP, CALL, or POPJ, meaning that no bits, the P bit, or the R bit is set. The <condition> may be one of the following:

If omitted, the <condition> is assumed to be "always". The <xct_next>, if present, is XCT-NEXT; its absence denotes the presence of the N bit, which inhibits the instruction after the jump if the jump is successful. The three parts are connected by "-". Examples of these operations:

The POPJ-XCT-NEXT operation is not to be confused with POPJ-AFTER-NEXT, which may be used in any instruction to set the POPJ bit.

Jump instructions which perform an arithmetic comparison should have both an A and an M source; the sources are compared. Jump instructions which test a bit should have an M source and a byte specifier for a 1-bit byte to test.

Functional Sources

The following names are supplied by CONSLP for the various functional sources:

 0	READ-I-ARG			The dispatch constant
	MICRO-STACK-POINTER		Byte specifier for bits <28-24>
	MICRO-STACK-DATA		Byte specifier for bits <18-0>
14	MICRO-STACK-PNTR-AND-DATA-POP	Like 1, but pops SPC stack
	MICRO-STACK-POINTER-POP		Like 1, but pops SPC stack
	MICRO-STACK-DATA-POP		Like 1, but pops SPC stack
 2	PDL-BUFFER-POINTER		PDL-pointer register
 3	PDL-BUFFER-INDEX		PDL-index register
 5	C-PDL-BUFFER-INDEX		PDL-buffer addressed by index
25	C-PDL-BUFFER-POINTER		PDL-buffer addressed by pointer
24	C-PDL-BUFFER-POINTER-POP	PDL-buffer addressed by pointer, pop
 7	Q-R				Q register
10	VMA				VMA register

Functional Destinations

The following names are provided by CONSLP for functional destinations. Note that some of them are the same names used for sources; CONSLP distinguishes usage by context.

 2	INTERRUPT-CONTROL		Interrupt Control Register
10	C-PDL-BUFFER-POINTER		Pdl location addressed by PDL POINTER
11	C-PDL-BUFFER-POINTER-PUSH	Push data onto pdl, increment PDL POINTER
12	C-PDL-BUFFER-INDEX		Pdl location addressed by PDL INDEX
15	MICRO-STACK-DATA-PUSH		Push data onto SPC stack
16	OA-REG-LOW			Next instruction modify, bits <25-0>
17	OA-REG-HI			Next instruction modify, bits <47-26>
20	VMA				VMA register
21	VMA-START-READ			VMA, initiate read cycle
22	VMA-START-WRITE			VMA, initiate write cycle
30	MEMORY-DATA			MD register
31	MEMORY-DATA-START-READ		MD, initiate read cycle
32	MEMORY-DATA-START-WRITE		MD, initiate write cycle

The symbol Q-R may also be used as a destination; it causes an ALU instruction to have its Q control field to be set to "load Q from ALU output"; this is equivalent to specifying LOAD-Q in the instruction. Do not use the output bus shifter in connection with Q-R as a destination!

Operations Common to All Instructions

The symbol for the POPJ bit is POPJ-AFTER-NEXT.

Miscellaneous Function 3 is denoted by LOW-PC-BIT-SELECTS-HALF-WD. (This feature is described in greater detail in an earlier and a later section.)

Expression Programs in CONSLP

Wherever an expression may be used in CONSLP, the following arcane forms may be used. In particular, the value of a symbol is normally an expression instead of a simple number. Whenever an expression (or a symbol with an expression as its definition) is encountered, it is evaluated according to the following rules:

<number>Evaluates to itself.
(PLUS <exp1> <exp2>)Adds together the two expressions, and combines their properties (such as byte-specifier-ness).
(DESTINATION-P <exp>)A conditional: if encountered while assembling a destination, returns the value of <exp>, and otherwise NIL.
(SOURCE-P <exp>)A conditional: if encountered while assembling a source (M or A), returns the value of <exp>, and otherwise NIL.
(DISPATCH-INSTRUCTION-P <exp>)A conditional: if encountered while assembling a DISPATCH instruction, returns the value of <exp>, and otherwise NIL.
(JUMP-INSTRUCTION-P <exp>)A conditional: if encountered while assembling a JUMP instruction, returns the value of <exp>, and otherwise NIL.
(ALU-INSTRUCTION-P <exp>)A conditional: if encountered while assembling an ALU instruction, returns the value of <exp>, and otherwise NIL.
(BYTE-INSTRUCTION-P <exp>)A conditional: if encountered while assembling a BYTE instruction, returns the value of <exp>, and otherwise NIL.
(NOT <conditional>)Negation. <conditional> must be one of the above conditionsl forms.
(OR <cond1> ... <condn>)Like a LISP OR, returns the first non-NIL conditional.
(BYTE-FIELD <size> <pos>)As described earlier, defines a byte with the given size and position from the right.
(LISP-BYTE <ppss>)As described earlier; if ppss is written in octal, then this is like (BYTE-FIELD ss pp). If <ppss> is not a number, then it is a LISP expression (not a CONSLP expression!), and is evaluated in LISP.
(BYTE-MASK <byte specifier>)Value is a word which is zero everywhere except for being all ones in the specified byte. This is a kind of conditional, in that it returns NIL if the byte specifier doesn't really specify a byte.
(BYTE-VALUE <byte specifier> <value>)Value is a word which is zero everywhere, except that it contains <value> in the specified byte. This is a kind of conditional, in that it returns NIL if the byte specifier doesn't really specify a byte.
(OA-HIGH-CONTEXT <word>)Assembles <word> as an instruction, and returns the high half (bits <47-26>), as if for use by the OA register feature (next instruction modify, functional destination 17).
(OA-LOW-CONTEXT <word>)Assembles <word> as an instruction, and returns the low half (bits <25-0>), as if for use by the OA register feature (next instruction modify, functional destination 16).
(FORCE-DISPATCH <exp>)Returns value of <exp>, but also forces the instruction to be a DISPATCH instruction. A conflict causes an error.
(FORCE-JUMP <exp>)Returns value of <exp>, but also forces the instruction to be a JUMP instruction.
(FORCE-ALU <exp>)Returns value of <exp>, but also forces the instruction to be an ALU instruction.
(FORCE-BYTE <exp>)Returns value of <exp>, but also forces the instruction to be a BYTE instruction.
(FORCE-DISPATCH-OR-BYTE <exp>)Returns value of <exp>, but also forces the instruction to be a DISPATCH or BYTE instruction.
(FORCE-ALU-OR-BYTE <exp>)Returns value of <exp>, but also forces the instruction to be an ALU or BYTE instruction.
(I-MEM-LOC <tag>)Returns the address represented by <tag> in locality I-MEM as a right-justified value.
(D-MEM-LOC <tag>)Returns the address represented by <tag> in locality D-MEM as a right-justified value.
(A-MEM-LOC <tag>)Returns the address represented by <tag> in locality A-MEM as a right-justified value.
(M-MEM-LOC <tag>)Returns the address represented by <tag> in locality M-MEM as a right-justified value.
(EVAL <lisp exp>)Returns the result of evaluating in LISP the S-expression <exp>.
(FIELD <name> <value>)Makes a note that the field <name> has been specified, then multiplies together the values of <name> and <value>; if <name> has a LISP CONS-LAP-ADDITIVE-CONSTANT property, this is then added in. (This obscurity is the primitive from which all field specifications are made.)
(ERROR)Error if this is assembled. Useful in conditionals.

As examples of how conditionals might be used in expressions, consider these definitions (which are similar (but not identical) to the ones actually used in CONSLP):

		(FORCE-ALU 3)))


Miscellaneous Pseudo-Operations

Several identical words may be assembled consecutively by saying:

	(REPEAT <count> <word>)

The location counter within the current locality may be set by

	(LOC <value>)		;sets it to <value>
	(MODULO <n>)		;advances it to the next multiple of <n>

If the MODULO operation is used in A-memory, wastage is avoided by filling in the skipped-over locations with constants.

CADR Features and Programming Examples

In this section the various features of the CADR machine are examined and discussed in detail. An attempt is made to give some feeling for how each feature fits into the overall structure of the machine, and the purposes for which the feature is intended. Short programming examples using each feature are presented.

Timing - The N Bit and the POPJ Bit

Because CADR fetches the next instruction at the same time it is executing the current one, by the time the effect of a JUMP or DISPATCH is known the instruction following the JUMP or DISPATCH has already been fetched. Unless suppressed by the N bit, this instruction is executed before the instruction branched to. The effect of this on programming is that one should "code the branch one instruction sooner". The mnemonics CONSLP provides for the various branching operations normally set the N bit, thus doing the straightforward thing at the cost of wasted cycles; one must append "-XCT-NEXT" to the mnemonic to clear the N bit and so bum the code.

For example, consider these two pieces of code:



These both perform an XOR and conditionally jump to MUMBLIFY, but the first one wastes a cycle if the JUMP is successful. Notice the convention of "exdenting" an instruction which is under the influence of an XCT-NEXT to make it more visible.

If a CALL transfer type is executed, the return address saved on the SPC stack depends on the N bit:

	(CALL THE-SUBROUTINE)		;call, N bit set
	((A-FOO) XOR M-BAR A-FOO)	;return here after call

(CALL-XCT-NEXT THE-SUBROUTINE) ;call, N bit clear ((A-ARGUMENT) ADD M-BAZ A-FOO) ;do this before entering the subroutine ((A-FOO) XOR M-BAR A-FOO) ;return here after call
If the N bit is set, PC+1 is pushed on the SPC stack; otherwise PC+2 is pushed.

The POPJ bit may be set in any instruction. It causes a RETURN transfer, but only after the next instruction has also been executed:

ADD-THREE-WORDS		;subroutine to add together A-1, A-2, and A-3
	((M-RESULT) A-1)
       ((M-RESULT) ADD M-RESULT A-3)

Again, the idea is to specify the desired control "one instruction early".

Consider the following program:


When started at START, it will go into an infinite loop alternately executing FOO and BAR. Effectively it is in two "jump point" loops at the same time!

Byte Manipulation

By using M location 2 (by convention a source of zeros) with a BYTE instruction, one can clear any bit or field of bits in any A memory location:

	((A-FOO) DPB M-ZERO A-FOO (BYTE-FIELD 1 31.))	;clear sign bit

It is often convenient to reserve another M memory location to contain -1 (all ones), in order to be able to set bits easily:
	((A-FOO) DPB M-ONES A-FOO (BYTE-FIELD 1 31.))	;set sign bit

In a similar manner one can write a routine to extend a signed 24-bit number to 32 bits:
SIGN-EXTEND				;extend 24-bit number in M-NUM

Another way to do this, which doesn't require the use of POPJ, is to use OA modification to select whether the M source is M-ZERO or M-ONES:
	((OA-REG-HI) (BYTE-FIELD 1 23.) M-NUM)	;low M-source bit gets sign

This requires that M-ZERO and M-ONES be an even/odd pair.

Normally bytes can only be loaded from an M source. However, it is possible to load a byte from A-memory, provided that it is at one end of the word, by the following trick:



The Instruction Stream

<<Some new stuff should be written for this>>

The SPC Stack

The SPC stack is 32 locations long, each location containing 19 bits (plus parity). It is indexed by SPCPTR, a 5-bit up/down counter. It is used primarily as a microcode subroutine return stack, but besides the 14 bits needed to save a microcode PC there are 5 bits for software use, one of which is the bit used for the macroinstruction pair fetch feature mentioned above.

SPC Stack Location        18     15    12    9     6      3     0
                          |     |     |     |     |      |     |
                          |         |                          |
                          |    5    |            14            |
                          |         |                          |
                               |                  |
Software bits------------------'                  |
Saved return address------------------------------'

There are two ways in which to write into the SPC stack memory; both of them also increment SPCPTR, thus causing a push operation. A JUMP or DISPATCH performing a CALL transfer type (P bit set, R bit clear) causes a return address to be pushed on the stack as described earlier. The five software bits are set to zero. Writing into functional destination 15 (MICRO-STACK-DATA-PUSH) pushes the low 19 bits of the output bus data onto the SPC stack.

The SPC stack is read by a JUMP or DISPATCH performing a RETURN transfer type (R bit set, P bit clear); the low 14 bits popped off the stack are put in the PC, and the software bits are ignored, except for bit 14 which causes NEXT-INSTR. It can also be read as M functional sources 1 and 14. The first (MICRO-STACK-PNTR-AND-DATA) merely reads the data (and SPCPTR) on the top of the stack, while the second (MICRO-STACK-PNTR-AND-DATA-POP) pops the stack after reading the data. There is no way to explicitly set the contents of SPCPTR. However, a good trick is to use the following loop:


A better trick is to use the following loop, which not only is shorter, but is recursive rather than iterative, and has the important advantage of being more obscure:
This is a good thing to do on initialization so that the stack will begin in a known place, thus aiding debugging via the diagnostic interface.

There is no provision for detection of SPC stack overflow or underflow. It is the responsibility of the programmer to avoid nesting subroutines to a depth greater than 32.


The PDL BUFFER is intended to be used as a special-purpose cache in the Lisp machine to contain the top portion of the Lisp pushdown stack. It has 1024 locations of 32 bits, and can be indexed by either the PDL POINTER or the PDL INDEX. PDL POINTER is a 10-bit up/down counter, while PDL INDEX is simply a 10-bit register.

The PDL BUFFER is manipulated through various functional sources and functional destinations. The PDL POINTER and PDL INDEX registers may be read and written. (On CONS, these could only be read together, but on CADR they are read separately to facilitate doing arithmetic with them without the need to extract a byte first.) The contents of the PDL BUFFER location addressed by the contents of PDL INDEX may be read and written. The contents of the location addressed by the contents of PDL POINTER may also be read and written, and in this case the PUSH and POP operations may optionally be done by incrementing or decrementing the PDL POINTER. The pointer decrements after reading and increments before writing, so it always points to the topmost valid location.

It doesn't work to specify both C-PDL-BUFFER-POINTER-PUSH and C-PDL-BUFFER-POINTER-POP in the same instruction. On the other hand, the same effect can always be achieved simply by using C-PDL-BUFFER-POINTER for both source and destination instead.

There is no provision for automatic overflow or underflow detection on pushes and pops of the PDL BUFFER. In the Lisp machine, the PDL POINTER is checked on entry to every function, and at a few other necessary places. If there is insufficient room left within the PDL BUFFER for a maximum size frame, some of the PDL BUFFER is stored into main memory to make room. If there is also insufficient space left within the virtual memory allocated to the PDL, a PDL-OVERFLOW error is signalled. Similarly, the function exit code decides whether to pull some stack back in from main memory.