Everything about Texas Instruments Tms9900 totally explained
Introduced in
1976 and based on the
Texas Instruments 990 minicomputer CPU, the
TMS9900 was one of the first true
16-bit microprocessors (the first were 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 bus, a 16-bit
data bus, and three internal 16-bit
registers (
PC, WP, and
ST). One unique feature, though, was that all general purpose user registers were actually kept in external memory. A single workspace register (WP)
pointed to the 16 register set (each register being 16 bits wide) 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 Transputers, use this same design using caches or rotating buffers, for the same reason of improved context switches. Other chips of the time, such as the
65xx 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.
Like the MOS Technology
6502, the TMS9900 had an
indirect jump instruction, called 'X' (or eXecute). This instruction was used to
execute another instruction at an address pointed to by a
register.
The TMS9900 was used in the
TI-99/4 and
TI-99/4A home computers. Unfortunately, to reduce the production costs, TI chose to use in these systems just 128 16-bit words of RAM. The rest of the memory was 16kB of
8-bit DRAM that was accessible only through the video display controller, which crippled the performance of the TMS9900.
Despite the very poor support from Texas Instruments, the TMS9900 had the potential at one point to surpass the
8086 in popularity.
TI later developed the more powerful and more capable TMS99000, which was used as the CPU in the 990/10A minicomputer as a cost reduction. Unfortunately, by the time the 990/10A made it to market, the end of the minicomputer era was already in sight.
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