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Concept
TI-990
Summary
The TI-990 was a series of 16-bit minicomputers sold by Texas Instruments (TI) in the 1970s and 1980s. The TI-990 was a replacement for TI's earlier minicomputer systems, the TI-960 and the TI-980. It had several unique features, and was easier to program than its predecessors.
Among its core concepts was the ability to easily support multiprogramming using a software-switchable set of processor registers that allowed it to perform rapid context switches between programs. This was enabled through the use of register values stored in main memory that could be swapped by changing a single pointer.
TI later implemented the TI-990 in a single-chip implementation, the TMS9900, among the first 16-bit microprocessors. Intended for use in low-end models of the TI-990, it retained the 990's memory model and main memory register system. This design was ultimately much more widely used in the TI-99/4A, where details of its minicomputer-style memory model presented significant disadvantages.
On the TI-990, registers are stored in memory and are referred to through a hard register called the Workspace Pointer. The concept behind the workspace is that main memory was based on the new semiconductor RAM chips that TI had developed and ran at the same speed as the CPU. This meant that it didn't matter if the "registers" were real registers in the CPU or represented in memory. When the Workspace Pointer is loaded with a memory address, that address is the origin of the "registers."
There are three hard registers in the 990; the Workspace Pointer (WP), the Program Counter (PC) and the Status register (ST). A context switch entailed the saving and restoring of only the hard registers.
The TI-990 had a facility to allow extended operations through the use of plug in hardware. If the hardware is not present the CPU traps to allow software to perform the function. The operation code (XOP) allowed for 15 attached devices on a system. Although, device 15 is reserved, by convention, to be used as the systems call entry for user programs to request systems services.
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