Not quite right on the history. The 990 started life as a business computer built for Ramada Inn as a reservation system. So large PWBs with a bunch of DIP integrated circuits. That computer was commercialized as the 990. Most of them were rack-mounted, had a row of 16 LEDs and a row of 16 buttons below them. So the box in the photo isn't any of the 990s I knew and loved - It might be one of the Ramada-vintage boxes.
The TMS9900 microprocessor was designed with the same instruction set as the 990 minicomputer, rather than vice-versa. It had a symmetrical 16-register file rather than an accumulator-based architecture, which was useful for the real-time applications I was building. A complete interrupt context switch was one instruction which internally performed 3 16-bit writes to memory. The down side was the register file was actually main memory words, so register operations were memory read/write - No cacheing in the earlier 990 models.
The later 990 models had an extended 20-bit address. One of those supported a development team of maybe 20 programmers on 20 timeshared terminals (not batch), building a collision-avoidance system for the US Federal Aviation Administration. At a time when the DEC, Data General, et.al. programmers were using glass TTY command lines, the 990 DX10 UI filled the 80/24 diaplay screens. Text, not pizels, but 2-D UIs and cursor moves in the text editor, with none of the vi bletchery.
The 99/4 and 99/4A home computers did use the TMS9900 with its 16-bit data bus externally narrowed to 8 bits. The TMS9980 with an 8-bit data bus was introduced later, but not used in the home computer. Another part, the TMS9945, if I remember correctly, was spec'd out but never made it to production - It included a UV-erasable program memory. The 20-bit address of the later 990 minicomputers was also spec'd out in the TMS99000 microprocessor, which I think did have some caching. I think some of those did get deployed. TI had a somewhat-successful commercial systems business. This was sold to Hewlett-Packard who apparently saw value in the customer list, but the computers themselves and the microprocessor line died out.
Biting the hand that feeds IT
