Don’t Hold Me Back
The TI-99/4A’s powerful 16-bit CPU was crippled by an 8-bit system design; now Usagi Electric is building a custom computer to unleash it.
The early history of personal computing could have been significantly different if Texas Instruments hadn’t botched its design of the TI-99/4A computer. This machine was released way back in 1979; yet it came equipped with a TMS9900 CPU. This was no toy, like the processors found on most personal computers of the era. This was a powerful 16-bit minicomputer CPU designed for multi-user systems, yet the TI-99/4A was priced competitively in the home computer market.
So what happened? Why is the TI-99/4A the butt of so many jokes? The processor was clearly not the problem. Unfortunately, just about everything else in this computer was a problem. The 16-bit TMS9900 was wedged into a machine that was otherwise 8-bit, through and through. As such, it could not fully take advantage of its powerful 16-bit instruction set. Furthermore, the system memory was all hidden away behind the video chip. All memory requests had to go through this chip, resulting in some very long access times.
These factors forced the TMS9900 to crawl when it was designed to run. Usagi Electric wanted to find out what this chip could do without these handicaps, so he has been designing a custom computer built around this processor. Initially, he plans to equip it with 2K words of ROM and 6K words of RAM — but it can handle a lot more, so further upgrades may come in the future.
Ultimately, Usagi Electric intends to run the Intercom 1000 programming language, which he uses on a Bendix G15 vacuum-tube computer, because why not? A computer of the TMS9900’s vintage would be much more likely to run BASIC, but sometimes you just do things because it sounds like fun.
Before he gets there, there is some groundwork to lay down first. For starters, a data terminal will be required to work with the system. An appropriate (and very cool) Texas Instruments data terminal was selected, then Usagi Electric started working out a UART protocol for communicating with it. A vintage TR1602-B UART chip, in conjunction with a ROM chip that consolidates memory-mapped I/O logic into a single IC, formed the core of the circuit.
After writing some software in TMS9900 assembly language to control it, this circuit was sending a steady stream of “Hellorld!” messages to the terminal. Success! But there is still a lot of work to do. At present, the UART circuit can only send data. Receiving will be a lot more work — and then there is the little matter of porting Intercom 1000 to the computer. Stay tuned for the latest updates as they come in!