PDP8e

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 :-

AND (0XXX) - logical AND of accumulator and memory referenced by XXX.

TAD (1XXX) - Two's complement add of memory referenced by XXX to the accumulator.

ISZ (2XXX) - Increment and skip if zero. Memory location referenced by XXX is incremented, and the next instruction is skipped if the result was zero.

DCA (3XXX) - Deposit and clear accumulator. The contents of the AC is stored at the location referenced by XXX and the AC is cleared.

JMS (4XXX) - Jump to subroutine. The current value of the program counter is stored in location referenced by XXX, and program execution starts at the following location. Note that this is strictly non-reentrant.

JMP (5XXX) - Unconditional jump to location referenced by XXX

IOT (6XXY) - I/O controller boards (and some internal cpu registers) are accessed by these instructions. XX is the device code, and Y is the operation to be performed.

OPS (7XXX) - These are the microinstructions, arranged in three groups. The groups are executed at different phases of the CPU clock and so several steps can be coded into a single instruction. For example, 7200 clears the AC and 7001 increments the AC. Combining both as 7201 sets the accumulator to 0001.

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