The PDP8 series of computers, manufactured by Digital Equipment Corporation (DEC), is considered the first successful 'mini-computer' to be produced. The first model was announced in 1965, with several models following and the PDP8/e was announced in 1970, along with the PDP8/f and PDP8/m which used the same processor cards, but a smaller system box.

The PDP8/e is one of the last models in the PDP8 line. It is a single address, fixed word length computer with a single accumulator and two's complement, 12 bit arithmetic. The memory (and machine) cycle is 1.2 microseconds (1.4 for auto indexing). The basic PDP8 has 4096 words of 12 bit memory, organised as 32 pages of 128 words each. The optional extended memory address and time share control (KME-8) adds additional logic to allow 32K words of memory to be addressed as 8 fields.

A memory reference instruction consists of a 3 bit opcode, 1 bit direct or indirect mode, 1 bit page zero or current page mode and 7 bits for the page address. Page 0 handles addresses 0010-0017 as a special case for auto-index registers.

The instructions are divided into 8 groups :-

Operators Console

Switches left to right are :-

  • Key switch. Power OFF, ON and panel lock (disable all switches except data)

  • SW. Switch asserts OMNIBUS SW line. Used by some peripherals such as the MI8-E bootstrap loader.

  • Switch Register bits (0-11). Note that DEC has the most significant bit numbered 0.

  • ADDR LOAD. Loads the contents of the switch register into the CPMA (central processor memory address) register. This is used as the address for memory examines/deposits and for processor starts.

  • EXTD ADDR LOAD. Loads the extended address bits (KM8-E option), bits 6-8 into the instruction field and bits 9-11 into the data field.

  • CLEAR. Generate an initialise pulse, and load zeros into the AC and link registers as well as clearing all flag bits in I/O registers.

  • CONT. Starts the processor running at the address currently in the CPMA register.

  • EXAM. Loads the contents of the memory location pointed to by the CPMA register into the MD (memory data) register, and increments the CPMA register. Note that the display switch (described latter) must have MD selected.

  • HALT. Halt the processor on the next instruction.

  • SING STEP. When down, the processor will execute one instruction after each depression of the continue switch.

  • DEP. Lifting this switch will deposit the data in the switch register into the memory location pointed to by the CPMA register. The register is then incremented.
The lights are as follows (left to right, top to bottom) :-

  • EMA - extended memory address bits
  • Memory address display (0-11)
  • RUN - processor running
  • Data display, selected by mode switch

The display modes are:-

  • STATE - The current processor state, bits 0-11 are :- Fetch, Defer, Execute, IRQ 0-2, MD DIR, BREAK DATA CONT, SW, PAUSE, BREAK IN and BREAK CYCLE
  • STATUS - The current processor status, bits 0-11 are :- Link set, greater than flag set, interrupt request, interrupt inhibit, interrupt enabled, user mode, 3 bit instruction field and 3 bit data field.
  • AC - accumulator register
  • MD - memory data bus
  • MQ - multiplier quotient
  • BUS - OMNIBUS data lines

System Box

The system box is made up of a power supply (black box on the left) and one or two omnibus backplanes containing 20 board slots. Boards may be mounted anywhere in the system. In the system pictured, the boards are as follows :-

  • M8330, M8310 and M8300 - CPU boards
  • M837 - Memory extension and time share control
  • M865 - 20ma, 110 baud TTY control
  • M8655 - RS232, 9600 baud TTY control
  • M869 and M885 - Point plot display system
  • Home made real-time clock (card with white handle)
  • M840 - High speed paper tape reader and punch
  • G227, G619 and G104 - Core memory system, 4 k words (green handles)
  • G227, G619 and G104 - Core memory system, 4 k words (green handles)
  • M832 - Bus loads
  • Home made opto isolator interface board (horizontal on point plot power supply box).

System Components

The power supply uses linear regulators, rather than the more common switch mode power supplies used today. As a result, it has several, very large filter capacitors, and a large SCR for the 5 volt crowbar overvoltage protection. The supply provided :-

  • +5 volts @ 20 amps
  • +15 volts @ 1 amp
  • -15 volts @ 8 amps
  • +8 volts unregulated for display lamps

The processor consists of 4 boards. There are 'over the top' connector blocks for some of the signal interconnections. The boards are:-

  • KCA-EA - Programmers console (front panel)
  • M3800 - Major states register
  • M3810 - Major registers control module
  • M8330 - Timing generator module
  • M849 - RF shield

The core memory (4K by 12 bits) uses 3 boards :-

  • G227 - XY driver and current source module
  • G619 - Core memory stack
  • G104 - Sense/Inhibit module

Finally, a M865 interfaces to an ASR-33 teletype using 20ma loop, and the last board in the system in a M832 Bus loads module.

WEB site devoted to PDP-8's

Yet Another PDP-8 Web Site

Highgate's PDP-8 Page (has circuits)

Please address comments to John Holden at:- johnh@psych.usyd.edu.au

Copyright © 2000 by John Holden