Emulating the PDP-8

Mentioning the PDP-8 yesterday set me thinking about building a PDP-8 replica using modern day TTL. The PDP-8 had a particularly simple architecture with only a few registers and eight basic op-codes. The early machines in the family, like the ancient one I used at University, was build from individual transistors and core memory. Later on DEC switched to TTL chips and RAM, so to build a replica would certainly be possible.

There was also a PDP-8 on a chip - the Intersil 6100. We used to have some prototyping boards for this low-power chip. The memory was backed by a single AA battery - at the end of the day you could just pull the memory board out. Next morning you could plug it back in and carry on working. It was used for ultra-low power application like satellite on-board systems.

A wander around the Web turned up Henk Gooijen's great looking 6809-based hardware emulation, and links to a number of software emulations, most of which ended in an HTTP 404 page. However there were some gems - for example Tim Gorton's Java Applet PDP-8 emulator that interactively runs a PDP-8 Lisp.

There was also Brian Shelburne's MS-DOS PDP-8 emulator which comes with a built in assembler and a lot of fascinating documentation. Apparently it used as part of a course on computer architectures - which is a little surprising because, although its a simple, elegant architecture, it doesn't use a stack which is a little unusual these days...

After spending an evening reading those sites and playing with some of the emulators I had a very restless night - I kept waking up with bits of Java code for emulating PDP-8 instructions running through my head. I had been looking for a little Java programming project to try out Swing (its all Web development at work) and this seems to have chosen itself.

So my next few commutes are going to be with the PDP-8 Programmer's Reference and a big pad of paper.

Obsolete Technology

It was 1975 and a spotty 15-year old boy was intently studying a photograph in a glossy magazine. Late at night and under the bed clothes with a torch, I couldn't sleep for thinking about running my hands over that curvaceous teletype, flipping those toggle switches and thrilling to the little red lights flickering on and off...



The magazine in question was Scientific American and the object of my lust was the IMSAI 8080. I never did get my hands on one - though I did my fair share of toggling bootstrap loaders into similar machines (mainly a PDP-8 with core memory and rather worn out front-panel switches.)

The Obsolete Technology site provides a fascinating journey through the first 18 years of personal computing. With lots of photos, information, specifications and old adverts that bring the memories come flooding back. I was amazed to find that I had owned or extensively used thirteen of the featured machines… and spent hours daydreaming about a lot more besides.