PDP-8

The PDP-8 was the first successful 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 10 clocks for each instruction, so that it ran at 0.1MIPS. 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 newsgroup alt.sys.pdp8 exists to discuss the PDP-8 computer.

The instruction set had only eight instructions:

000 - AND - and operand with AC.

001 - TAD - add operand to (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

RAR - rotate right

RAL - rotate left

RTR - rotate right twice

RTL - rotate 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

Here is an example of a complete PDP-8 assembly language program, written for the PAL-III assembler.

/ adapted from example in Digital PDP-8 Handbook Series, Introduction to Programming, p5-12
   *200 / set assembly origin (load address)
   hello,  cla cll
           tls         / tls to set printer flag.
           tad charac  / set up index register
           dca ir1     / for getting characters.
           tad m6      / set up counter for
           dca count   / typing characters.
   next,   tad i ir1   / get a character.
           jms type    / type it.
           isz count   / done yet?
           jmp next    / no: type another.
           hlt

   type,   0           / type subroutine
           tsf
           jmp .-1
           tls
           cla
           jmp i type
   charac, .           / used as initial value of ir1
           310 / H
           305 / E
           314 / L
           314 / L
           317 / O
           254 / ,
           240 /
           327 / W
           317 / O
           322 / R
           314 / L
           304 / D
           241 / !
   m6,     -15
   count,  0
   ir1 = 10
   $
   

External links:

• The alt.sys.pdp8 FAQ
• http://www.cs.uiowa.edu/~jones/pdp8/
• http://www.cs.uiowa.edu/~jones/pdp8/man/
• http://www.faqs.org/faqs/dec-faq/pdp8-models/preamble.html
• pdp8.net has a running PDP8 that anyone can control through a Java applet, plus a webcam to show the results
• Bernhard Baehr's slick PDP-8/E Simulator for Macintosh