[Robert Baruch] found a TMS9900 CPU from 1983 in a surplus store. If that name doesn’t ring a bell, the TMS9900 was an early 16-bit CPU from Texas Instruments. He found that, unlike modern CPUs, the chip took several voltages and a four-phase twelve-volt clock. He decided to fire it up and — of course — one thing led to another and he wound up with a system on a breadboard. You can see one of the videos he made about the machine below.
This CPU had some odd features, most notably that it stored its registers in off-chip memory and can switch contexts by changing where the registers reside. That was a novel idea when the memory and the CPU were similar in speed. In a modern computer, the memory is much slower than the CPU and this would be a major bottleneck for program execution. The only onboard registers were the program counter, the status register, and a pointer to the general-purpose registers in memory.
[Robert] doesn’t quite have a full system yet, but we bet he’ll get there. He built a four-digit display and did some simple control line decoding of the processor. He can watch the address bus and also manages to single step. The TMS9900 used dynamic logic, so you can’t just halt the clock. However, there is a way to do it with a state machine [Robert] built with some flip flops and an ATmega processor.
To overcome the fast memory problem, TI’s TI-99/4 home computer only had 128 words of memory directly addressable. Main program storage was cheaper and slower dynamic RAM (16 K bytes) that you could only access via the display controller. This meant performance suffered. If you had the optional floppy controller, you got extra RAM, but that was pricey in the day.
We recently saw a TI 99/4A doing service as a weather station. If [Robert] duplicates the TI-99/4 display system, maybe he can run this demo. We are waiting to see where [Robert] takes his surplus store find from here.