Texas Instruments TMS9900
| Designer | Texas Instruments |
|---|---|
| Bits | 16-bit |
| Introduced | 1976 |
| Design | CISC |
| Endianness | Big |
| Registers | |
| PC, WP, ST | |
| General purpose | 16 × 16-bit in external RAM |
Introduced in June 1976, the TMS9900 was one of the first commercially available, single-chip 16-bit microprocessors. The TMS9900 found its most widespread use in the TI-99/4 and TI-99/4A home computers.
The TMS9900 was designed as a single chip version of the TI 990 minicomputer series, much like the Intersil 6100 was a single chip PDP-8 (12 bit), and the Fairchild 9440 and Data General mN601 were both one-chip versions of Data General's Nova. Unlike other 16-bit microprocessors such as the National Semiconductor IMP-16 or DEC LSI-11, some of which predated the TMS9900, the latter was a single-chip, self-contained 16-bit microprocessor.
The TMS9900 has three internal 16-bit registers — Program counter (PC), Status register (ST), and Workspace Pointer register (WP). The WP register points to a base address in external RAM where the processor's 16 general purpose user registers (each 16 bits wide) are kept. This architecture allows for quick context switching; e.g. when a subroutine is entered, only the single workspace register needs to be changed instead of requiring registers to be saved individually.
Addresses refer to bytes with big endian ordering convention. The TMS9900 is a classic 16 bit machine with an address space of 216 bytes (65,536 bytes or 32,768 words).
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