Complex Instruction Set Computing (CISC) — which is pronounced "sisk" — is a type of computer architecture in which the Central Processing Unit (CPU) supports hundreds of instructions. This is opposed to Reduced Instruction Set Computers (RISC), which support fewer instructions. What this means is that computers that support CISC, which includes the majority of personal computers on the market, are able to accomplish a wide variety of computing tasks, making them excellent general purpose computers.
The advent of CISC computing in the 1970s and 1980s allowed computers to operate and perform complex instruction sets. This, in turn, allowed for denser code to be written for these computers. In other words, less code was able to get more performance out of the computer as a result of the complex instruction set architecture. The instruction sets needed to operate CISC computers became more compact, creating smaller programs and saving in both computer memory and computer storage.
Another benefit of the complex programming changes allowed by the use of CISC computer architecture was improved programming efficiency. In the early days of computing, all instructions had to be broken down to assembly language. With CISC computing, instruction sets were added to the computer that allowed them to understand and process higher-level languages, which were more intuitive. This meant that programmers no longer had to break the code down to make it usable.
The downside to this highly evolved computer architecture was that the instruction sets were required to grow more and more complex to handle the increasingly intricate needs of computers. Newer operating systems allowed and encouraged parallel processing and multi-tasking. The more complicated the operating systems and programming languages became, the more work was required of the CPU and instruction sets to decode and execute them.
One of the offshoots of this growing complexity was the introduction of RISC processors. These evolved out of a need for a computer to do a smaller variety of tasks, but to do them very quickly and efficiently. There was no need for a purely business computer, for example, to have the capability to display and manipulate high-level graphics and sounds required by a computer that needed to be able to play games as well as process spreadsheets.
Computing has continued to progress, and the speed and power of machines has increased. This has, in some ways, made the line between CISC and RISC computing become less meaningful. With the newer processors able to execute more instructions in an overlapped, or pipelined, fashion and handle more parallelism efficiently, however, the CISC processors continue to maintain dominance in the computing industry.