TMS 9900

TMS 9900

(processor)One of the first true 16-bit microprocessors,released by Texas Instruments in June 1976 (the first areprobably National Semiconductor IMP-16 or AMD-2901 bit slice processors in 16-bit configuration). It was designedas a single chip version of the TI 990 minicomputerseries, much like the Intersil 6100 was a single chipPDP-8, and the Fairchild 9440 and Data General mN601were both one chip versions of Data General's Nova.Unlike the IMS 6100, however, the TMS 9900 had a mature, wellthought out design.

It had a 15-bit address space and two internal 16 bitregisters. One unique feature was that all user registerswere actually kept in memory - this included stack pointersand the program counter. A single workspace registerpointed to the 16 register set in RAM, so when asubroutine was entered or an interrupt was processed, onlythe single workspace register had to be changed - unlike someCPUs which required dozens or more register saves beforeacknowledging a context switch.

This was feasible at the time because RAM was often fasterthan the CPUs. A few modern designs, such as the INMOStransputer, use this same design using caches or rotating buffers, for the same reason of faster context switches.Other chips of the time, such as the 650x series had asimilar philosophy, using index registers, but the TMS 9900went the farthest in this direction.

That wasn't the only positive feature of the chip. It hadgood interrupt handling features and very good instructionset. Serial I/O was available through address lines. Intypical comparisons with the Intel 8086, the TMS9900 hadsmaller and faster programs. The only disadvantage was thesmall address space and need for fast RAM.

Despite very poor support from Texas Instruments, the TMS 9900had the potential at one point to surpass the Intel 8086 inpopularity.