RISC Architecture

RISC Architecture

RISC stands for Reduced Instruction Set Computer. PA-RISC is the name for Hewlett-Packard's standard hardware that runs both the MPE/iX and HP-UX operating systems. The Classic HP 3000, DEC VAX, and IBM 360 all used CISC processors: Complex Instruction Set Computer. The instructions that programmers use on those machines are not the real hardware instructions. Each complex instruction is implemented by a hidden microprogram written in the real instructions.

On RISC computers there are no microprograms. Machine instructions are implemented directly in hardware. Any task too complex for the hardware to execute in a single cycle is done by executing a series of basic instructions, either as in-line code or by calling a subroutine.

Extensive research into patterns of computer usage reveals that general-purpose computers spend up to 80% of their time executing simple instructions such as load, store, and branch. The more complex instructions are used infrequently. On architectures with large, complex instruction sets, the simple, often executed instructions incur a performance penalty by the overhead of additional instruction decoding, the use of microcode, and the longer cycle time resulting from increased functionality.-----Hewlett-Packard's "Precision Architecture and Instruction Reference" manual.