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Simple-Data-Processor-8

A very simple PDP-8 simulator (even for embedded systems).

This is just a little fun project, the result of one day reading and hacking about a classic masterpiece of computer architecture: A very simple PDP-8 simulator.

The best way to really understand a system design is (at least for me) to write an emulator. And doing so for the PDP-8 is surprisingly simple. The instruction set architecture is designed to be implemented with discrete DTL logic and highly optimized to be as simple as possible, but still surprisingly efficient and powerful. A beautiful example of well-done optimization, almost a piece of art!

It turns out the the simulator core compiles to only 1 to 1.25kB of code for popular MCUs like the AVR or the STM8.

Simulator features

Supported CPU features:

  • regular commands, Group 1 and 2 OPR commands incl. microcoding
  • 4kWords of core memory
  • auto-preincrement registers 0010-0017
  • simplyfied I/O: All IOT commands are directed to stdin and stdout
  • read paper tape images in BIN and RIM format

Missing features:

  • Group 3 OPR commands (multiply, division)
  • OSR command (Read the front-panel switches to AC) is ignored

Severe simplifications:

  • No accurate timing. It runs as fast as the host CPU is capable of.

Compilation

$ make
cc    -c -o sdp8.o sdp8.c
cc    -c -o sim.o sim.c
cc    -c -o memory.o memory.c
cc    -c -o io.o io.c
cc    -c -o readtape.o readtape.c
cc   sdp8.o sim.o memory.o io.o readtape.o   -o sdp8

Usage

sdp8 filename [startadr]

Paper tape images can be in BIN or RIM format. The optional start address can be in decimal, octal (with leading '0') or hex (with leading '0x'). If no address is given, the execution starts at address 0.

Example:

./sdp8 examples/arraycopy/arraycopy.bin 0200

Code size

compiling for different MCUs (only the simulator core, without paper tape reader and debug output):

  • AVR (avr-gcc -O2): 1333 bytes
  • STM8 (sdcc 3.9.1 #11242): 1054 bytes
  • ARM:

Introduction into PDP-8 programming

Must-reads:

  • Wikipedia article with very detailed but compact information about the instruction set and system architecture
  • Introduction to programming PDP-8 Family Computers: An excellent introduction into assembler programming in general. It covers all the basics including the conversion of floating point numbers. This book should be part of the standard curriculum to all computer schools.

Useful stuff:

Software

MACRO8: The simh packages (see below) include macro8x, an assembler for PDP-8 code. Unfortunatly, the simulator can't read the .bin files generated by the assembler.

Images of original software for the PDP-8 (incl. FOCAL and OS8): http://so-much-stuff.com/pdp8/papertape/papertape.php

Similar projects

There are so many PDP-8 simulators out there. And now I understand why: The architecture is so simple. So clean. So beautiful. And there is so much to learn from it. Highly recommanded educational and fun project for every programmer.

[//]: # vim: tw=78