Developing a windowed OS for a homebrew Am29000 computer

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Developing a windowed OS for a homebrew Am29000 computer

by Oscar Toledo G. Jul/30/2026

Once upon a time in the mysterious lands of Mexico, starting in Fall 1996 and through Spring 1997, I coded a windowed operating system in 32-bit machine code, fit it into a floppy, and jumped in excitement as it boot up.

Of course, I knew it was a great thing because you couldn't boot Windows 3.1 or GEM from a floppy disk, but I was very far from knowing how so cool it was. Unfortunately, not a thing I could give away for everyone to see, as it was written for a homebrew computer based on the Am29000 processor.

Let’s go back to my memories, some notes, and the ride of discovering again what I forgotten.

The fastest processor in the world

If you've been following my series of articles on transputer, I was feeling very satisfied with my operating system, and I had reached the limits of the system. My father on the other side was busy developing a new computer system based on a new 32-bit processor. Well, in 1994 it was only new for us. The AMD Am29000 processor came out in 1987, also announced as the fastest processor in the world (do you see a meme here?) and it could process one instruction every cycle, so indeed it was fast.

The Am29000 processor and its successor the Am29050, were used in several Postscript language add-ons for printers, video NuBus cards for Quadra Macs, arcade games like F-15 Strike Eagle and Battle of the Solar System (both by Microprose), and I read in the Wikipedia it is also inside avionics in dual-processor boards for redundancy.

This speed was based on a ton of assumptions, like having speedy enough RAM or ROM, or having very high-speed TTL chips. And all three were still very expensive in the early 1990s. Besides some bus design decisions for the Am29k family like a separate data and instruction bus made it really cumbersome to interface it. At the end, doing anything useful with the Am29k required DRAM memory, and it was slow, and then all the advantage was lost.

However, my father was confident in static RAM memory and he sourced dozens of 128KB static RAM chips with 8-bit bus. Surface mount started to making an appearance and several of these were built with four 32K RAM chips mounted in nice miniature green boards.

So the first Am29000 computer made by my father ran at 6 mhz, and it had the whooping memory capacity of 512 KB of RAM, along 128 KB of EPROM (using two 16-bit chips). It had ISA connectors, and the video was a Cirrus Logic GD5429 card, along a standard parallel/serial/floppy-disk controller card (multiport card), and reused the keyboard chip from an old PC to interface AT keyboards. The computer was called G11.

It was a team work, he did some initial programming to start the processor, I translated my Z280 code for initializing the Cirrus Logic VGA, made the keyboard decoder, and the disk routines. Once this was stable, I went ahead to work in the monitor program for listing the memory, saving data, and as the instruction set was complicated enough, I also integrated a micro-disassembler and a micro-assembler. This means we could code with real mnemonics, although no labels could be used, also moving code was a complete pain.

We needed to return to the Z280 machine to program the EPROMs, but everything was going really well. I stumbled in the dark reality that I needed to port everything from the Z280 machine to this machine, it was like 200 programs between applications, utilities, and games. Besides the Z280 programs were basically outdated, I couldn't replicate the same.

I wanted a G11V1 picture, but there were no digital cameras before 1999 and I don't know where is the machine. So this is a picture of a G11V2 computer, you can see four 512KBx8 memories and it has already PCI bus (not present in the G11V1).

Intermission

As the G11 computer was finished, my father took over it and started to investigate how every processor special register worked or why it had certain values, and writing documentation about it. I was somewhat startled, because I believed the processor was already documented in its databook, and I wasn't interested in undocumented behavior, but anyway, I ended typing all his notes and printing them.

I could concentrate on the transputer now that it was available for my use, and I switched ocasionally to the G11 computer when it was free in order to enhance the floppy disk operating system, I started coding some games, a text editor that handled our new Canon BJC-600 bubblejet printer, and even a MSX emulator. This was the first time I could see Gradius running, I was hooked.

I was recreating my Z280 programs, and I felt like I was going backwards when I had already an advanced operating system in the transputer. In the meanwhile, Windows 3.1 looked more useful with...