Texas Instruments TMS9900

TMS9900 in ceramic housing

TMS9900 in a plastic case as used in the TI-99 home computers

A The one TI TMS9900

The TMS 9900 was a 1976 imagined 16-bit processor by Texas Instruments (TI), which mainly by the use in the home computer of the TI-99 is known series. It was one of the first 16-bit microprocessors and was designed as a one-chip version of the TI-990 series of minicomputers .

A special feature of the TMS9900 is that the registers are located in part of the external RAM . An internal register called a workspace pointer (WP) points to an area of 16 16-bit words (the workspace) that can be used like registers. This enables different sets of registers to be changed quickly, which is particularly helpful for multitasking applications and interrupt processing, since it avoids saving the registers when changing context.

Furthermore, the TMS9900 has an unusual command in its command set which is designated as 'X'. Here, an instruction is executed which is located at an address in the memory specified by one of the registers. In contrast to the usual jump commands , the program is then processed further directly after the 'X' command and not continued after the jump address. In combination with a disassembler , which determines the address of the next command to be executed by determining the code length and jump commands, a single-step debugger can be set up. The Millers Graphics Explorer on the TI-99 / 4A is an example .