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tms 9900
(foldoc) | TMS 9900
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 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 {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.
(1994-11-30)
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