Source : Free On-Line Dictionary of Computing

TMS 9900
     
        One of the first true 16-bit {microprocessor}s, released by
        {Texas Instruments} in June 1976 (the first are probably
        {National Semiconductor} {IMP-16} or {AMD-2901} {bit slice
        processor}s 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
        {register}s.  One unique feature was that all user {register}s
        were actually kept in memory - this included {stack pointer}s
        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
        {CPU}s 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 {CPU}s.  A few modern designs, such as the {INMOS}
        {transputer}, use this same design using {cache}s or {rotating
        buffer}s, 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 register}s, 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)