ARCHIVED: What are CISC and RISC technologies, and how do they compare?

CISC and RISC (Complex and Reduced Instruction Set Computer, respectively) are dominant processor architecture paradigms. Computers of the two types are differentiated by the nature of the data processing instruction sets interpreted by their central processing units (CPUs). They both have advantages and drawbacks, which are detailed below.

To improve performance, CISC systems try to reduce the number of instructions programs must call. To do this, they have large sets of microcode instructions that cover a broad range of tasks. A single microcode instruction, in turn, when translated in the CPU, may become several tasks the processor performs. As a consequence, instructions are of variable length and often require more than one clock cycle to complete.

RISC systems, on the other hand, seek to improve performance by reducing the number of clock cycles required to perform tasks. They have small sets of simplified instructions, doing away with microcode altogether in most cases. While this means that tasks require more instructions, instructions are all of the same length and usually require only one clock cycle to complete. Because of this, RISC systems are capable of processing instructions in parallel in a process called pipelining. The CPU works on more than one instruction at once by starting the second instruction before it completes the first one. This greatly increases throughput and makes RISC systems substantially faster than their CISC counterparts. RISC systems do have a few disadvantages, however. Notably, because programs usually have more instructions, compilers and applications written in assembly language are more difficult to build for RISC systems.

Examples of CISC processors include the 680x0 and the VAX. RISC processors include the PowerPC, MIPS, SPARC, and the Alpha. Recent processors in the x86 (e.g., IA-32) family are often described as CISC, but in reality they are hybrids of the two architectures.

For more information about RISC and CISC, read Apple Computer's Introduction to RISC Technology.