A Lost IBM PC/at Model? Analyzing a Newfound Old Bios

https://int10h.org/blog/2025/11/lost-ibm-at-model-bios-analysis/

115•TMWNN•3mo ago

Comments

mrlonglong•3mo ago

Excellent write-up.

viler•3mo ago

Appreciated, thanks!

drfuchs•3mo ago

Any chance it was for the "IBM Personal Computer AT/370" that nobody remembers (perhaps because nobody used)?

viler•3mo ago

That was one option I thought of at first (mentioned in the first section), but the info I found indicated that the /370 models used the same firmware as the "plain" 5170s - if there were any BIOS extensions, they were probably somewhere on the add-on cards. The AT/370 also had 512K of on board RAM, while this BIOS seems to indicate 640K.

m463•3mo ago

I remember that. I think it ran VM/SP or whatever it must have been called.

I recall the 370 part was on a card.

ForOldHack•3mo ago

3 cards. CPU/Memory and communications cards.

TMWNN•3mo ago

Article discusses and dismisses that possibility

ForOldHack•3mo ago

Details: The IBM AT/370 used standard bios on the motherboard, and the two 68k custom cards had their own bioses. The 68ks were very heavily modified by one of the motorola engineers.

Its the second version of the AT Bios that was disgusting was verion 2, that ran on 6mhz 286s and prevented you from swapping the crystal for a 16Mhz/8Mhz speed up. The first version had bugs, and the third version was for the 8Mhz machines. ( still a few bugs ).

This is the AT/370:

https://en.wikipedia.org/wiki/PC-based_IBM_mainframe-compati...

https://www.cpushack.com/2013/03/22/cpu-of-the-day-ibm-micro...

https://anycpu.org/forum/viewtopic.php?f=22&t=350

There was one additional model of the IBM AT: THE IBM XT/286: An AT class mother board in an XT sized case.

https://www.dosdays.co.uk/computers/IBM%20PC-XT-286%20(5162)...

drfuchs•3mo ago

Oops. Anyway, I remember attending a talk by one of the IBM engineers back when they first released the XT/370. He said that they looked at all possible ways to integrate their production line as a kind of secondary track off of one of the main production lines for the PC/XT, but the most economical option ended up being a separate facility that would receive normal pallets of regularly boxed, end-user XTs from the main factory, unbox them, make the mods, and pack them back into XT/370-labeled boxes for shipping.

kjs3•3mo ago

Plenty of people remember, and used, them. Just not people who tend to hang out here. I knew several IBM VM dev types who had them as light dev/remote mainframe access machines, usually at home. They were popular enough there was a followon product: the PC/390 which was the same idea, more advanced processor, based on a PS/2 microchannel platform (and, AFAIK, OS/2).

You want really obscure? Unisys had the same idea with the "Micro-A", which was a PC running OS/2 with a coprocessor card with a single chip implementation of an A-series mainframe. I know of 2, possibly 3, still around.

eek2121•3mo ago

Back when I was a teenager, I would have absolutely gone down a rabbit hole like the author did. From "Upgrading and Repairing PCs" to reading all the technical manuals, usenet, etc. I definitely nerded out over this stuff! Glad to see folks still take an interest.

These days, I've an acquired brain injury. Between that an old age, it was a bit hard to read, but also, just a little bit familiar, so I enjoyed it.

Now I am expecting "256 color VGA programming in C" to resurface at some point! :D

Old hardware was always so much fun...

mrandish•3mo ago

256 colors? VGA?

Bah! You kids with your newfangled graphics modes. 320 x 200 CGA and 16 colors is more than enough. See the linked "8088 MPH" video for proof: https://trixter.oldskool.org/2015/04/07/8088-mph-we-break-al...

m463•3mo ago

> 320 x 200 CGA

with the opposite of smooth scrolling - video off during text scrolling. BLINK!

kind of like the crazy blinking ancestor of vsync off

vardump•3mo ago

320 x 200 CGA allows you to have only 4 colors at a time. Of course you can change the fixed palette on a certain scanline to have more colors. See for example California Games.

1313ed01•3mo ago

160x100 CGA low resolution could do 16 color (bit of a hack, but semi-official).

And then there were tricks to get more colors in higher resolutions as well, especially if you used the TV output instead of a CGA monitor.

https://int10h.org/blog/2015/04/cga-in-1024-colors-new-mode-...

vardump•3mo ago

> 160x100 CGA low resolution could do 16 color (bit of a hack, but semi-official).

That's a text mode, but ok, I think some games used it for graphics.

mrandish•3mo ago

Yes, I did consider writing "4 colors", but thought someone would say "but you could have 16 colors on-screen" citing the early 80s titles which did scanline palette tricks. So, I went with "16 colors" as it was technically the upper-bound of 'official' documented colors (excluding composite artifact, dithering and other cool tricks). I didn't really want to get into explaining more since my goal was just linking the "8088 MPH" demo for the GP and anyone who hadn't seen it. Also, I have no idea if a technically correct, definitive and complete statement about CGA's maximum possible colors in a single English-language sentence is even possible.

But after wasting... oh, nearly ~800ms on this internal debate, I realized I'd already cleverly chosen to not link directly to the 8088 MPH video but instead a site containing both the video and links to explications revealing the myriad brilliant tricks and unnatural acts behind 8088 MPH. And from that rabbit hole, one may learn more than any mortal should know about CGA graphics. Especially apropos since the author of the OP was instrumental in creating 8088 MPH (although the OP's post is about BIOS things).

reaperducer•3mo ago

Bah! You kids with your newfangled graphics modes. 320 x 200 CGA and 16 colors is more than enough.

Bah! You kids with your newfangled 320x200x16 CGA cards.

We had 720x384x2 Hercules cards and we liked it!

sema4hacker•3mo ago

My OCD tendencies would have made me label the one chip ..ODD.. instead of .ODD... just for a little more symmetry.

notherhack•3mo ago

The blog refuses connections from VPNs. https://archive.is/Ef75R

viler•3mo ago

Strange... if it does, that's not my doing - it seems to work fine with the VPN I use, but I'll look into it. Thanks for the heads up.

breakingcups•3mo ago

I love this sort of digital archeology

Show HN: MCP to get latest dependency package and tool versions

The better you get at something, the harder it becomes to do

Show HN: WP Float – Archive WordPress blogs to free static hosting

Show HN: I Hacked My Family's Meal Planning with an App

Sony BMG copy protection rootkit scandal

The Future of Systems

NASA now allowing astronauts to bring their smartphones on space missions

Claude Code Is the Inflection Point

Show HN: MicroClaw – Agentic AI Assistant for Telegram, Built in Rust

Show HN: Omni-BLAS – 4x faster matrix multiplication via Monte Carlo sampling

The AI-Ready Software Developer: Conclusion – Same Game, Different Dice

AI Agent Automates Google Stock Analysis from Financial Reports

Voxtral Realtime 4B Pure C Implementation

I Was Trapped in Chinese Mafia Crypto Slavery [video]

U.S. CBP Reported Employee Arrests (FY2020 – FYTD)

Show HN: I built a free UCP checker – see if AI agents can find your store

Show HN: SVGV – A Real-Time Vector Video Format for Budget Hardware

Study of 150 developers shows AI generated code no harder to maintain long term

Spotify now requires premium accounts for developer mode API access

When Albert Einstein Moved to Princeton

Agents.md as a Dark Signal

System time, clocks, and their syncing in macOS

McCLIM and 7GUIs – Part 1: The Counter

So whats the next word, then? Almost-no-math intro to transformer models

Ed Zitron: The Hater's Guide to Microsoft

UK infants ill after drinking contaminated baby formula of Nestle and Danone

Show HN: Android-based audio player for seniors – Homer Audio Player

Starter Template for Ory Kratos

LLMs are powerful, but enterprises are deterministic by nature

Make your iPad 3 a touchscreen for your computer

Show HN: MCP to get latest dependency package and tool versions

The better you get at something, the harder it becomes to do

Show HN: WP Float – Archive WordPress blogs to free static hosting

Show HN: I Hacked My Family's Meal Planning with an App

Sony BMG copy protection rootkit scandal

The Future of Systems

NASA now allowing astronauts to bring their smartphones on space missions

Claude Code Is the Inflection Point

Show HN: MicroClaw – Agentic AI Assistant for Telegram, Built in Rust

Show HN: Omni-BLAS – 4x faster matrix multiplication via Monte Carlo sampling

The AI-Ready Software Developer: Conclusion – Same Game, Different Dice

AI Agent Automates Google Stock Analysis from Financial Reports

Voxtral Realtime 4B Pure C Implementation

I Was Trapped in Chinese Mafia Crypto Slavery [video]

U.S. CBP Reported Employee Arrests (FY2020 – FYTD)

Show HN: I built a free UCP checker – see if AI agents can find your store

Show HN: SVGV – A Real-Time Vector Video Format for Budget Hardware

Study of 150 developers shows AI generated code no harder to maintain long term

Spotify now requires premium accounts for developer mode API access

When Albert Einstein Moved to Princeton

Agents.md as a Dark Signal

System time, clocks, and their syncing in macOS

McCLIM and 7GUIs – Part 1: The Counter

So whats the next word, then? Almost-no-math intro to transformer models

Ed Zitron: The Hater's Guide to Microsoft

UK infants ill after drinking contaminated baby formula of Nestle and Danone

Show HN: Android-based audio player for seniors – Homer Audio Player

Starter Template for Ory Kratos

LLMs are powerful, but enterprises are deterministic by nature

Make your iPad 3 a touchscreen for your computer

A Lost IBM PC/at Model? Analyzing a Newfound Old Bios

Comments