Reduced instruction set computer (RISC)

• Authors: Richard R. Oehler
• Venue: Encyclopedia of Computer Science
• Year: 2003

Until 1975, computer architecture and, consequently, computer design and implementation had grown more complicated with each successive generation. Instruction sets were large and individual instructions were complicated. Some of these complications were done to insure compatibility across a family, some were done to be better targets for compilers (q.v.) and some were done for performance enhancements. The result was often an architecture and implementation that had a lot of rarely used instructions and mechanisms because the nature of the compiler changed or because the compiler didn't change when it should have. Many other instructions were seldom used because they covered a very specific case or the implementation penalty was too large. Compilers and their run-time environments did not use some instructions because they did not fit in very well with generated code. Some complex instructions had such poor implementations that it was better to program an equivalent sequence than to use such instructions. Often, the implementation relied on a "micro-engine" to implement these numerous and complex instructions. The micro-engine, a small processor within the CPU, had a micro instruction set. Microprograms or microcode written in these microinstructions interpreted the complex instructions.