The History of CISC

Modern computers all use RISC chips, not CISC.

CISC stands for Complex Instruction Set Computer. The history of CISC is closely tied with that of RISC, which is Reduced Instruction Set Computing. CISC is older than RISC, and the relative success of each type of instruction set relies on the cost of hardware.

Complex Instructions

CISC systems date to the early 1960s and the early development of computers. IBM created a range of computers based around a set of standards called System/360. This was the first CISC machine. Earlier computers expected programmers to issue low-level instructions to the computer. This mindset was inherited from the early days of programmable machines that derive from the Jacquard loom. Computer instructions broke down every action the computer had to perform. The System/360 method created a series of commands that stood for a series of instructions. That was what made them complex instructions, although the term CISC was not applied at this point.

RISC

The term CISC only came into being when an alternative architecture, RISC, was invented. “CISC” didn't need to be defined until there was a comparable system to talk about. RISC was another invention of IBM. In the mid-1970s, IBM technicians noticed that most computer programs were dominated by a small number of simple instructions and the wide vocabulary of complex instructions was rarely used. This resulted in Reduced Instruction Set Computing, which wasn't implemented in commercially available systems until the mid-1980s.

Decline

RISC rapidly overtook CISC as the preferred system for new computers. Research switched to RISC instructions sets and methods of optimizing compilers. Compilers convert text programs into instructions for a computer and it is the interpretation methods programmed into compilers that determined the amount of memory each program command takes and the type of memory that a program occupies. This is a crucial factor in the debate between CISC and RISC.

Reason

CISC reduces the amount of fetches needed to execute a program. Program instructions are held in memory until the processor is ready for the next chunk. Complex instructions take longer to execute, but they take up less space in memory. RISC instructions execute quicker by tie up memory. CISC was cheaper; RISC was faster. The reduced price of computer memory and the rapid expansion in technology to make on-board memory much bigger, removed CISC's price advantage and made RISC a universally preferred technology.

References

• Star Fighter 3000: RISC: The Processor Architecture of the Future
• Andrew Allison: A Brief History of RISC (circa 2001)
• ARM: Instruction Set Architecture
• Stanford University: What is RISC
• Amiga History Guide: RISC/CISC
• Lycos Retriever: Complex Instruction Set Computer: Cisc Cpu