TMS9900

The TMS9900 was a 16-bit microprocessor developed by Texas Instruments (TI) and released in 1976. It was notable for being one of the first commercially available single-chip 16-bit microprocessors. The TMS9900 differed significantly from contemporary microprocessors like the Intel 8080 and Motorola 6800 in its architecture.

A key architectural feature of the TMS9900 was its use of a memory-to-memory architecture for its registers. Unlike most CPUs that stored register data in dedicated hardware registers on the chip itself, the TMS9900 used main memory to store its register set. This approach, while allowing for a flexible number of registers and easy context switching, came at the cost of performance due to the slower access times of external memory compared to on-chip registers. The "workspace pointer" (WP) register held the address of the starting location in memory that served as the first of the 16 registers.

The TMS9900 had a relatively small instruction set compared to some of its competitors. Instructions typically operated on memory locations directly, reflecting the register-in-memory approach.

While the TMS9900 itself did not achieve widespread adoption in personal computers, it found use in some industrial control systems and the TI-99/4 and TI-99/4A home computers. The high cost of the TMS9900 and the support chips, as well as its relatively slower performance compared to competing 8-bit processors at the time, contributed to its limited success in the broader consumer market. The architecture also presented programming challenges due to the memory-resident register scheme. Despite its limitations, the TMS9900 was an important step in the development of 16-bit microprocessors and influenced subsequent designs. Its lineage continued with later processors like the TMS9995 and the TMS320 family of digital signal processors.