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One of the first true 16-bit microprocessors, released by

Texas Instruments in June 1976 (the first are probably

National Semiconductor IMP-16 or AMD-2901 bit sliceprocessors in 16-bit configuration). It was designed as a

single chip version of the TI 990 minicomputer series,

much like the Intersil 6100 was a single chip PDP-8, and

chip versions of Data General's Nova. Unlike the IMS

6100, however, the TMS 9900 had a mature, well thought out

design.

It had a 15-bit address space and two internal 16 bit

registers. One unique feature was that all user registers

were actually kept in memory - this included stack pointers

and the program counter. A single workspace register

pointed to the 16 register set in RAM, so when a

subroutine was entered or an interrupt was processed, only

the single workspace register had to be changed - unlike some

CPUs which required dozens or more register saves before

acknowledging a context switch.

This was feasible at the time because RAM was often faster

than the CPUs. A few modern designs, such as the INMOS

transputer, use this same design using caches or rotatingbuffers, for the same reason of faster context switches.

Other chips of the time, such as the 650x series had a

similar philosophy, using index registers, but the TMS 9900

went the farthest in this direction.

That wasn't the only positive feature of the chip. It had

good interrupt handling features and very good instruction

set. Serial I/O was available through address lines. In

typical comparisons with the Intel 8086, the TMS9900 had

smaller and faster programs. The only disadvantage was the

small address space and need for fast RAM.

Despite very poor support from Texas Instruments, the TMS 9900

had the potential at one point to surpass the Intel 8086 in

popularity.

(1994-11-30)