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PDP-8

The PDP-8 was the first sucessful commercial minicomputer, produced by DEC in the 1960s. It was a 12-bit computer with 4096 words of memory. It had 8 instructions, and one register, the accumulator. It operated at a clock rate of 1MHz, and took (I think) 10 clocks for each instruction, so that it ran at 0.1MIP. It was the first widely-sold computer in the DEC PDP series of computers (the PDP-5 was not originally intended to be a general-purpose computer).

The instruction set had only eight instructions:

000 - AND - and operand with AC.
001 - TAD - add operand to <L,AC> (a 13 bit value).
010 - ISZ - increment operand and skip if result is zero.
011 - DCA - deposit AC in memory and clear AC.
100 - JMS - jump to subroutine.
101 - JMP - jump.
110 - IOT - input/output transfer.
111 - OPR - microcoded operations.

A wide variety of operations are available through the OPR microcoded instructions. In general, the operations within each Group can be combined by oring the bit patterns for the desired operations into a single instruction. If none of the bits are set, the result is the NOP instruction.

Group 1 operations:
CLA - clear AC
CLL - clear the L bit
CMA - ones complement AC
CML - complement L bit
IAC - increment <L,AC>
RAR - rotate <L,AC> right
RAL - rotate <L,AC> left
RTR - rotate <L,AC> right twice
RTL - rotate <L,AC> left twice
Group 2 operations:
SMA - skip on AC < 0 (or group)
SZA - skip on AC = 0 (or group)
SNL - skip on L /= 0 (or group)
SKP - skip unconditionally
SPA - skip on AC >= 0 (and group)
SNA - skip on AC /= 0 (and group)
SZL - skip on L = 0 (and group)
CLA - clear AC
OSR - or switches with AC
HLT - halt

The newsgroup alt.sys.pdp8 (news:alt.sys.pdp8) exists to discuss the PDP-8 computer.

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