Search in: Word
Vietnamese keyboard: Off
Virtual keyboard: Show
Computing (FOLDOC) dictionary
TMS 9900
Jump to user comments
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
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
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.