TMS9900

Lore & Background

The TMS9900 was designed as a single-chip version of the TI 990 minicomputer series, similar to how the Intersil 6100 was a single-chip PDP-8 and the Fairchild 9440 was a single-chip Data General Nova. Unlike multi-chip 16-bit microprocessors such as the National Semiconductor IMP-16 or DEC LSI-11, the 9900 was a self-contained single-chip 16-bit microprocessor. Its minicomputer roots gave rise to architectural features uncommon in designs starting from a blank sheet, notably processor registers mapped into main memory. This allowed fast context switching by changing a single register, the Workspace Pointer, to point to a list of register values in memory, rather than saving all internal registers to memory or the stack. The downside was slower register access; in minicomputers with fast memory the effect was small, but in typical single-user microcomputers the tradeoff was less worthwhile. TI used the same architecture across divisions for corporate synergy: 'one company, one computer architecture'. In the late 1970s, Walden C. Rhines presented the TMS99110 (code-named 'Alpha') to an IBM group developing a personal computer, but IBM chose the Intel 8088 for the IBM PC. One factor was the lack of a roadmap for accessing more than 64 KB of logical memory; the 9900 family could expand to 16 MiB only by page-mapping, while the 8088 could address 1 MB through segments.

Reader's Guide

The TMS9900's architecture lived on into the 1990s as the Communications Processor in TI's TMS380 chipset for Token Ring networking (later Ethernet). By the mid-1980s, the microcomputer field was moving to 16-bit systems such as the Intel 8086 and newer 16/32-bit designs such as the Motorola 68000. With no obvious future for the chip, TI's Semiconductor division turned its attention to special-purpose 32-bit processors: the TMS320 (introduced in 1983) and the TMS340 graphics processor. The TI 990 minicomputer was so successful that TI ended plans to use the TMS9900 in higher-end personal computers that would compete with the minicomputer, only producing the low-end TI-99/4A. After discontinuing the 99/4A, the company's microprocessor division eventually switched focus to the TMS320 series. Microcomputer-on-chip implementations in 40-pin packages included the TMS9940, TMS9980/81, and TMS9995. The SBP9900 was a ruggedized version. The last generation was the 99000 series, created to be the CPU of the 990/10A in 1981; the TMS99105 and 110 were sold as catalog parts.

Did You Know?

• The TMS9900's registers are mapped into main memory, allowing fast context switching by changing a single Workspace Pointer register.
• The chip required +5 V, −5 V and +12 V power supplies and a four-phase clock with a maximum frequency of 3 MHz.
• The TMS9900 was packaged in a 64-pin, 0.9-inch wide DIP, unlike the more common 40-pin package.
• The TI-99/4 and TI-99/4A home computers using the TMS9900 ultimately sold about 2.8 million units.