from
The Free On-line Dictionary of Computing (8 July 2008)
TMS 9900
<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 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 switch}es.
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)