<processor> 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 slice processors 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 the Fairchild 9440 and Data
General mN601 were both one 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
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 rotating buffers, 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.
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