The early historical past of private computing might have been considerably totally different if Texas Devices hadn’t botched its design of the TI-99/4A laptop. This machine was launched means again in 1979; but it got here outfitted with a TMS9900 CPU. This was no toy, just like the processors discovered on most private computer systems of the period. This was a strong 16-bit minicomputer CPU designed for multi-user programs, but the TI-99/4A was priced competitively within the residence laptop market.
So what occurred? Why is the TI-99/4A the butt of so many jokes? The processor was clearly not the issue. Sadly, nearly every thing else on this laptop was an issue. The 16-bit TMS9900 was wedged right into a machine that was in any other case 8-bit, via and thru. As such, it couldn’t absolutely reap the benefits of its highly effective 16-bit instruction set. Moreover, the system reminiscence was all hidden away behind the video chip. All reminiscence requests needed to undergo this chip, leading to some very lengthy entry occasions.
Separate playing cards connect to a backplane (📷: Usagi Electrical)
These elements compelled the TMS9900 to crawl when it was designed to run. Usagi Electrical needed to seek out out what this chip might do with out these handicaps, so he has been designing a customized laptop constructed round this processor. Initially, he plans to equip it with 2K phrases of ROM and 6K phrases of RAM — however it will possibly deal with much more, so additional upgrades could come sooner or later.
Finally, Usagi Electrical intends to run the Intercom 1000 programming language, which he makes use of on a Bendix G15 vacuum-tube laptop, as a result of why not? A pc of the TMS9900’s classic could be more likely to run BASIC, however generally you simply do issues as a result of it feels like enjoyable.
It is alive! (📷: Usagi Electrical)
Earlier than he will get there, there may be some groundwork to put down first. For starters, an information terminal will likely be required to work with the system. An acceptable (and really cool) Texas Devices information terminal was chosen, then Usagi Electrical began figuring out a UART protocol for speaking with it. A classic TR1602-B UART chip, at the side of a ROM chip that consolidates memory-mapped I/O logic right into a single IC, fashioned the core of the circuit.
After writing some software program in TMS9900 meeting language to regulate it, this circuit was sending a gradual stream of “Hellorld!” messages to the terminal. Success! However there may be nonetheless numerous work to do. At current, the UART circuit can solely ship information. Receiving will likely be much more work — after which there may be the little matter of porting Intercom 1000 to the pc. Keep tuned for the most recent updates as they arrive in!
