February 5th, 2015 ~ by admin

TI TMS9900/SBP9900: Accidental Success

TI TMS9900JL - 1978

TI TMS9900JL – 1978

In June 1976 TI released the TMS9900 16-bit processor.  This was one of the very first 16-bit single chip processor designs, though it took a while to catch on.  This is no fault of its own, but rather TI’s failure to market it as such.  The 9900 is a single chip implementation of the TI 990 series mini-computers.  It was meant to be a low end product and thus was not particularly well supported by TI, who did not want to cut into the higher margins of their mini-computer line.    By the late 1970’s TI began to see the possibilities of the 9900 as a general purpose processor and began supporting it with development systems, support chips, and better documentation.  If TI had marketed and supported the 9900 from its release the microprocessor market very much may have turned out a bit different.  A large portion of Intel’s success (with the 808x) was not due to a good design, but rather good support and availability.

The original TMS9900 was a 3100 gate (approx 8000 transistors) NMOS design running at up to 3MHz.  It required a 4-phase clock and 3 power supplies (5V, 12V, -5V).  It had a very orthogonal instruction set that was very memory focused, making it rather easy to program.  General purpose registers were stored off chip, with only a PC, Workspace Register (which pointed to wherever the general registers would be) and a Status Register on chip.  This made context switching fairly quick and easy.  A context switch required saving only 2-3 registers. The 9900 was packaged in a, then uncommon, and expensive, 64 pin DIP.  This allowed the full 15-bits of address and 16-bits of data bus to be available.

TI had a trick up their sleeve for the 9900 line…

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