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CISC - Complex Instruction Set Computer

By: Preethi Ramkumar

Meaning of CISC – “Complex Instruction Set Computer”, is a processor where each instruction can perform several low-level operations such as memory access, arithmetic operations or address calculations. The term was coined in contrast to Reduced Instruction Set Computer. Before the first (RISC) processors were designed, many computer architects were trying to bridge the "processor" - to design instruction sets to support high-level languages by providing "high-level" instructions such as procedure call and return, loop instructions such as "decrement and branch if non-zero" and complex addressing modes to allow data structure and array accesses to be compiled into single instructions.

While these architectures achieved their aim of allowing high-level language constructs to be expressed in fewer instructions, it was observed that they did not always result in improved performance. For example, on one processor it was discovered that it was possible to improve the performance by NOT using the procedure call instruction but using a sequence of simpler instructions instead. Furthermore, the more complex the instruction set, the greater the overhead of decoding an instruction, both in execution time and silicon area. This is particularly true for processors which used microcode to decode the (macro) instruction. It is easier to debug a complex instruction set implemented in microcode than one whose decoding is "hard-wired" in silicon. Examples of CISC processors are the Motorola 680x0 family and the Intel 80186 through Intel 486 and Pentium.

Other Related Definitions:

“…Complex Instruction Set Computer) Pronounced "sisk." The traditional architecture of a computer which uses microcode to execute very comprehensive instructions. Instructions may be variable in length and use all addressing modes, requiring complex circuitry to decode them. Contrast with RISC. ” []

“…Complex Instruction Set Computing. A microprocessor that uses complex instructions at the assembly language level. The instructions can be very powerful, allowing for complicated and flexible ways of calculating elements. CISC microprocessor work faster than RISC (Reduced Instruction Set Computing) microprocessors on complex instructions but are slower with simple tasks and instructions. ” [Auditmypc]

“…The term "CISC" (complex instruction set computer or computing) refers to computers designed with a full set of computer instructions that were intended to provide needed capabilities in the most efficient way. Later, it was discovered that, by reducing the full set to only the most frequently used instructions, the computer would get more work done in a shorter amount of time for most applications. Since this was called reduced instruction set computing (RISC), there was now a need to have something to call full-set instruction computers - thus, the term CISC. ” [ WHATIS.COM]

Related Links:

Linktionary - CISC (Complex Instruction Set Computer)
Auditmypc - – Acronym of Complex Instruction Set Computing.
Utoronto - High Level Instructions
Visionengineer - Pentium 4: Most Computers Use CISC Processors
Sunderland - The development of CISC
Ucsb - The Case for Reduced Instruction Set Computer
Amigau - CISC Vs. RISC
Helpwithpcs - Processor Basics - A basic guide to the processor
Karbosguide - Data and instructions
Abig - What are CISC and RISC technologies, and how do they compare?
Eserver.bell - Processor Types and Architectures
Physinfo - The Emergence of CISC

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Books About:

Ones and Zeros: Understanding Boolean Algebra, Digital Circuits, and The Logic of Sets by John R. Gregg
The Dlx Instruction Set Architecture Handbook by Philip M. Sailer, David R. Kaeli

See Also:

Other CISC Related Resources


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