CuriousMarc taught a 1976 MMD-1 trainer board to pass for an Altair and run Microsoft's first BASIC

Chris at a 1970s trainer computer with paper tape and a teletype, watching it come alive — Illustration: LockerGnome

The MMD-1 came from Jon Titus, the designer “of Mark-8 fame,” and first ran in Radio Electronics in May 1976 as the Dyna Micro, according to CuriousMarc’s own documentation page. E&L sold it as a kit or fully built. It came out a year after the Altair 8800, used the same Intel 8080, and is, in Marc’s words, “arguably the first single board computer.”

Getting one took two tries. “The first unit, which was in great cosmetic condition, was lost by USPS. So I bought a second one, which unfortunately was in terrible shape,” he writes. Chips were missing or in the wrong sockets, the case was rusty, and “there wasn’t even a processor on it when it arrived.” Three months later the post office found the first one.

So the wreck got the full treatment. Marc stripped it to bare components and found that car paint polish was what finally brought the dull PCB back. That rebuilt unit, now “the better looking of the two, by far,” is the one he turned into a mini-Altair.

Memory came first. He used a modern 32K FRAM chip on a small module his friend TubeTimeUS made so it plugs straight into the breadboard and can overlap the original ROM area “to boot into them, without having to burn a new PROM.” The FRAM’s address latching raced the slow 8080 bus, so he “added a few TTL gates of shame.” The curiousmarc.com page credits the board to “TubeTime, aka Eric Schlaepfer,” and says the final version doesn’t need the extra gates.

The serial port was the clever part. BASIC expects the Altair’s own interface, so Marc built one from a 6850 ACIA and a Harris HD-4702 rate generator. “It would emulate, very faithfully, the odd Altair interface that Bill Gates had expected, since it was made with the same chips.”

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Then BASIC fought back. Its boot code reads the Altair’s front-panel switches “not just at one, but at two places,” and the loader checks its own checksums, so every patch broke the load. “I decided to disable the checksum code for expediency. That led to much hacking of Bill’s masterpiece to get a version that finally loaded properly. Sorry Mr. Gates.”

After the long tape load on a TI Silent terminal came “the famous “MEMORY SIZE?”” prompt. Marc worked through programs from 101 BASIC Games, including a BANNER printout reading JON TITUS RULES, and later got Titus and David Larsen on a Zoom call for the series finale. One of his two units also carries an unidentified chip in ROM 1. He dumped it, guesses it might be a serial ROM monitor, and asks anyone who reverse engineers it to tell him.

Why it matters

The breadboard was the expansion bus. On an Altair, memory and serial IO each take a card in the backplane. On the MMD-1, Marc did both in the breadboard area that came on the board, which is roughly the difference between adding a room to the house and putting a desk in the corner.

Matching chips beats matching specs. He didn’t write a driver to make some other serial chip look like an Altair’s. He used the same parts the Altair used, so Gates’ code saw what it expected without being told.

Microsoft’s first code assumed the machine it ran on. BASIC read specific front-panel switches at two spots in its boot and protected its own loader with checksums. Code written for one exact box tends to carry a lot of assumptions about that box, and here they all surfaced at once.

Steal this

When old software refuses to run, copy the original hardware’s chips before you write an adapter. Marc’s serial port worked because it was built from the parts BASIC was written against. This is like buying the same brand of filter the furnace manual names instead of one that “should fit.”
Before patching a program that checksums itself, find and disable the check first, and write down where it is. Otherwise every edit fails the load and you can’t tell your bug from the checksum. Kind of like turning off the smoke detector before you test the toaster, then putting a sticky note on it so it goes back on.
Buying a second, worse unit as a parts donor pays off even when the first one turns up. Marc borrowed the good unit’s processor to wake up the wreck in a day. It’s the spare key you’re glad you made the week you lock yourself out.

The playlist around this run is eight parts long and about a year old, and the hackaday.io write-up is brand new, built for a retro computing contest. Plenty of 1976 trainer boards are still sitting in drawers with breadboards on them. How many of them are a couple of chips away from running BASIC too?

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CuriousMarc taught a 1976 MMD-1 trainer board to pass for an Altair and run Microsoft's first BASIC

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