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.

Slide Rule Universe

Slide Rule Universe has everything you every wanted to know about slide rules, even how to build your own… You did want to know about slide rules, didn’t you?

An amazing variety of slide rules are on sale in the catalog - fancy calculating the trajectory of a 105mm Howitzer shell, or computing flow parameters for hydraulic valves, then they have a slide rule for you.

Personally I’m still looking for a good set of Napier’s Bones - never seem to turn up on eBay for some reason.

Binary Adder Made of Wood and Marbles

Here’s a wooden binary adder that uses marbles for the bits. Be sure to watch the video demonstration - you only realise how clever the design is when you see it in action, especially as the carry bits cascade through the digits.

There’s plenty more to see on the rest of the site, including Marble Machine 2, an ingenious piece of kinetic art, and photography using a flatbed scanner.

K'NEX Computer

Lego, relays, wood and marbles… it was only a matter of time before somebody came up with a K’NEX computer. This 4-bit computer, ten feet tall and built of K’NEX, works in a similar way to other ball-based computers. Check out the links on the right of the page for some useful explanations.

How to build NAND gates from Relays

Here's a nice simulation of various logic gates, adders, flipflops and multiplexers built from relays. Click on the inputs on the left to animate the circuits.

Homebrew Relay Computer

Following on from yesterday’s homebrew CPU link, here’s another stunning homebrew machine. But none of that fancy new 74 series stuff for Harry Porter, of Portland State University’s Computer Science Department - oh, no - he’s built his machine from 415 four-pole double throw relays.

Follow the ‘more information’ link and check out the movies and MP3 - they are worth it just for the sound of the machine in action.

Now where did I put that old box of Audions…

Design your own CPU

Every time I happen across the 74HCTXXX pin-out page in the Maplin catalogue I get a deep longing to design my own computer from the NAND gates up. Nostalgic dreams of front panels full of flashing lights and rows of toggle switches, of operating systems that fit into Kilobytes rather than Gigabytes swirl before my eyes.

Rather than just dream, Bill Buzbee has designed and built a superbly engineered home brew CPU from about 200 74-series TTL chips. Once you’ve marveled at the details of Magic-1, check out the Links page and the Homebuilt CPU ring.