alt.hn

2/23/2026 at 8:05:15 PM

UNIX99, a UNIX-like OS for the TI-99/4A

 https://forums.atariage.com/topic/380883-unix99-a-unix-like-os-for-the-ti-994a/

by marcodiego

2/23/2026 at 9:11:45 PM

@UsagiElectric on YouTube has a series of videos on building a homebrew around the TMS9900 processor. Would be cool if a unix-like OS could be used on something like that, though sounds like this project is specifically targeting the TI-99/4A system.

The TI-99/4A was the first computer I owned as a teenager. I had used TRS-80s and Apple ][ at school. I eventually bought the expansion box and a couple of accessory cards (floppy disk drive, memory and RS232). It all went in the e-waste dumpster about 20 years ago during a move.

by SoftTalker

2/23/2026 at 10:27:10 PM

TI-99/4A was my first computer as well. I still have two of them, and they still work as well as they did in the '80s. I graduated to an Apple ][GS which I still have as well, although it needs some TLC before attempting to boot it so as not to let out the magic smoke.

by mikestaas

2/23/2026 at 9:18:09 PM

I had one in grade school. Taught me the value of backups early in life. Spent all night typing in a game from a magazine. Started it without saving to tape first. It was so loud! Panicked and restarted the machine. Sadness ensued.

Replaced it with a C128-D. Didn’t get my first intel until I bought a 386 after graduating high school. Good times.

by ectospheno

2/23/2026 at 9:20:32 PM

Same here - parents bought one for me in 1982, IIRC. By 1984 I had moved to Atari XL, but I'll always have a soft spot for the TI-99/4A, Extended Basic cartridge, speech synthesizer, cassette drive, etc.

My sister and I used to co-type programs from "Compute!". The times were so much simpler then..

by hn_acc1

2/23/2026 at 9:30:03 PM

The TI99/4A was my first computer when I was 7 or 8. Unfortunately, no cassette drive. As soon as I shut it off, my basic program was gone!

by icedchai

2/23/2026 at 9:44:30 PM

The TI99/4A was also my first computer. I was about 5, and I didn’t really seriously try writing programs until I was about 8. Fortunately, since my father bought this for work, we had a large collection of peripherals, including the floppy disk drive. Unfortunately I learned the hard way why my father stopped using it: peripheral expansion bus devices were exquisitely sensitive to static shocks. I remember reeling in horror after watching hours of work just disappear from the disk drive. I suppose this was probably a good lesson to learn at an early age!

by raddan

2/23/2026 at 9:56:36 PM

Can you drop this yt channel name?

by sunanda35

2/23/2026 at 9:58:50 PM

you only get 3 guesses

by Brian_K_White

2/23/2026 at 10:27:19 PM

For some reason I was thinking it was that $99 dollar Sinclare from the 80s which had the most unusable keyboard on earth.

by arnonejoe

2/23/2026 at 8:45:35 PM

This is the main updated comment with the user guide and download

https://forums.atariage.com/topic/380883-unix99-a-unix-like-...

by haunter

2/23/2026 at 9:23:52 PM

Thanks. I wasn't looking forward to browsing all those pages in the hopes of finding the source. Did they never put it up on GitHub?

by bink

2/23/2026 at 9:15:03 PM

HOLY COW. Thank you for this. I LOVE the Ti99/4a, its one of the first computers I ever used. I've got one up and running at home now currently and can't wait to try this.

by nonamenoslogan

2/23/2026 at 9:47:27 PM

Btw, there is a lovely third party replacement for the TI99/4a video chip that lets you output VGA. It’s a major life improvement if you are seriously using it. I Dremeled my case but you can route the ribbon cable to avoid it if you’d prefer not to modify anything. Happy to send you a link if you don’t already know about it.

by raddan

2/23/2026 at 10:23:04 PM

Thanks! I've got mine running in to a 9" Pelco PVM and it works well but yea, its tiny. I'd love to plumb in VGA and use a 15" flat panel. Would love a link.

by nonamenoslogan

2/23/2026 at 9:43:10 PM

It's the first computer I ever programmed, I was twelve years old then. <3

by raphar

2/23/2026 at 8:48:31 PM

Wow. The TI-99 is such a perfect fit for this too given the chip was designed for multi-user computing in a way other home computer chips weren’t.

All due to TI’s desire to use the same chip standards across all their machines big and small, IIRC.

by MBCook

2/23/2026 at 8:57:01 PM

While the CPU is a better fit than the 8 bit contemporaries, the 16kb of working memory is going to be a struggle.

by jandrese

2/23/2026 at 9:15:46 PM

It's cool because the registers are all in RAM, with a "workspace pointer" on the CPU pointing at where they are. This is slow, but a context switch is just changing that pointer.

by SoftTalker

2/23/2026 at 10:22:42 PM

Well, it has 256 bytes of RAM which is basically a really big register file, and everything else goes in the 16kb of "video RAM" which you can read and write by poking at I/O registers. So it is not easy to program.

It's arguably the only 8-bit computer which has a really different architecture from the others. You could otherwise imagine pulling the SID chip off a C-64 and putting it on a TRS-80 Color Computer etc.

Sharing the main RAM with video was a weak point in computers of that time period because the video system stole many of the memory access cycles. Some recent retrocomputers that revisit that period like

https://www.c64-wiki.com/wiki/Commander_X16

have a full-size memory bank and a video RAM memory bank which is accessed through a port which can be pretty efficient because you can auto-incremement the address register and just write 1 byte to the port to write 1 byte to video RAM and repeat.

by PaulHoule

2/23/2026 at 9:33:47 PM

Yep, but it lacks a MMU so memory protection and paging are going to require a lot of work. I think the only reason this is feasible at all is they're running the OS out of a ROM cartridge.

by jandrese

2/23/2026 at 9:43:32 PM

The PDP machines that Unix was developed on had MMUs, which they needed because the 16 bit processors couldn't address the multi-megabyte address space the hardware supported.

I'm pretty sure the Centurion doesn't run Unix.

by jandrese

2/23/2026 at 10:46:08 PM

The PDP-10 had an MMU similar to a modern MMU with page tables and such, the PDP-11 had an 8-segment-of-8kb MMU like what the TRS-80 Color Computer 3 had except the PDP-11 had a real supervisor mode and if a user mode program tried to change the MMU configuration it would fault.

by PaulHoule

2/23/2026 at 9:37:20 PM

Did the minicomputers of the time have MMUs?

I thought UsagiElectric showed a case where his Centurion didn’t, but I may be misremembering.

by MBCook

2/23/2026 at 10:35:26 PM

In fact the 9900 itself was used in an entire line of minicomputers that included a hardware memory map.

The 9900 is a single chip implementation of the CPU board in the TI 990. They even created a dedicated memory mapper chip to go along with the product line, though it is significantly different than the one in the minicomputer line.

(edit: the 990 was first built in the early 70s, memory mappers are quite old conceptually)

(edit 2: in fact the necessity of using a memory mapper is what killed the platform, and was one of the things that made the IBM PC team decline the 9900.)

Unfortunately I don't think that there is a reasonable way to perform real hardware-level memory protection with that chip alone. I'm working on a project documenting the genesis of the 99000 chips, which include a privilege bit in the status register, from the minicomputer line.

Essay forthcoming, and probably an OS. Maybe a year...

by McGlockenshire

2/23/2026 at 9:13:00 PM

Yeah it really was an interesting choice on their part. Makes sense as a move for TI. Not the target market.

by MBCook

2/23/2026 at 9:26:01 PM

I learned to program on this exact hardware in the early 80s as a small child. It uses BASIC. It's hard drive was modem tones recorded to an analog audio tape. Its monitor was an analog TV. There was no mouse. The keyboard was built into the computer itself.

by hunterpayne

2/23/2026 at 9:24:10 PM

So assuming one wanted to buy a used one of these (I had timex sinclairs around this time) how would one display the composite video nowdays?

by UncleOxidant

2/23/2026 at 10:24:56 PM

I'm using a Pelco 9" PVM that had a former life as a security camera monitor. Suprisingly good video for composite, but alas its not very large.

by nonamenoslogan

2/23/2026 at 9:36:00 PM

A USB video capture device or a converter box. There are devices sold specifically to interface these old machines with modern displays. One of the more famous ones is the RetroTINK.

by jandrese

2/23/2026 at 8:42:44 PM

The joy of computing still lives in the age of AI...

by glimshe

2/23/2026 at 9:03:28 PM

Imagine a Beowulf cluster of these.

by bananamogul

2/23/2026 at 8:43:14 PM

WoW!

by Zardoz84

2/23/2026 at 9:14:46 PM

Does it run PARSEC? Nice shot captain!

by buildsjets