TI-990

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 uniquely innovative features, and was easier to program than its predecessors.

The TI-990 had a unique concept that 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.

Programmers liked the TI-990 design because it had a fairly orthogonal instruction set which allowed a programmer to separately memorize all of the operations and the methods of accessing operands.

The basic instruction formats allowed for one, two and three word instructions. The model 990/12 CPU allowed for a four word instruction with the extended mode operations.

(R is a general register, 0 to 15.)

Several registers had special usages that reserve their use, the register and their usages are:

The 990/4, 990/5, 990/9 instruction sets consisted of 69 instructions, the 990/10 had 72 instructions, the 990/10A had 77 instructions and the 990/12 had 144 instructions. The instructions are divided into types that have similar characteristics.

The first part of the word specifies the operation to be performed, the remaining two parts provide information for locating the operands.

The first part of the word specifies the operation to be performed, the second part is a relative offset to where to go, for JMP instructions, or the relative offset for CRU bit addressing.

One part of the word specifies the operation, the second part provides the register, the third part provides information for locating the second operand.

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