Martin_H wrote:
I have a TI-99/4a in my retrocomputer collection, and the TMS9900 is a decent microprocessor, especially when paired with the TMS9918. It really was 16 bits in the 8 bit era, and its ISA feels more like a minicomputer than a typical microprocessor of the era. TI also had interesting ideas to extend memory by giving support chips their own RAM or ROM, so its address space felt less cramped.
That is because the TMS9900 is a microprocessor implementation of the TI 990 minicomputer.
I am still undecided just how fast the 9900 could be in a "proper" design.
https://ia801205.us.archive.org/14/item ... l_1977.pdfAppendix A goes into excruciating detail about the time taken by each instruction. It speaks in terms of machine cycles. A machine cycle consists of two clock cycles. A common clock rate for the 9900 is 3 MHz.
Martin_H wrote:
Unfortunately it got a bad rep due to TI's poor decisions with the TI-99/4a. I've heard the TI-99/4a was supposed to be a game console, hence the tiny amount of RAM on the CPU bus. But the home computer boom made TI quickly rework it into a personal computer, while using the TMS9918's video memory to store BASIC programs. The results were less than stellar and gave the machine a bad rep.
TI designed the 99/4 for a version of the 9900 with an 8-bit bus interface, much like the Intel 8088 is to the 8086. But the processor was late forcing hasty modifications made to use the 16-bit processor in the 99/4. Each memory access was broken down into two 8-bit accesses by external hardware.
Martin_H wrote:
But if you are using a program out of a ROM, or have expanded RAM, the computer compares favorably with even the C64 which came years later. Slightly better decisions from TI could have produced a real winner.
TI has plans to make a large share of the money from royalties selling program cartridges rather than machine sales. When some developers balked, later versions of the machine refused to run "unauthorized" cartridges.