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An instruction set architecture (ISA) is an abstract model of a computer, also referred to as computer architecture.A realization of an ISA is called an implementation.An ISA permits multiple implementations that may vary in performance, physical size, and monetary cost (among other things); because the ISA serves as the interface between software and hardware.
An early (retroactively) RISC-labeled processor (IBM 801 – IBM's Watson Research Center, mid-1970s) was a tightly pipelined simple machine originally intended to be used as an internal microcode kernel, or engine, in CISC designs, [citation needed] but also became the processor that introduced the RISC idea to a somewhat larger audience.
[36] [37] Some RISC processors such as the PowerPC have instruction sets as large as the CISC IBM System/370, for example; conversely, the DEC PDP-8—clearly a CISC CPU because many of its instructions involve multiple memory accesses—has only 8 basic instructions and a few extended instructions. [38]
A RISC instruction set normally has a fixed instruction length, whereas a typical CISC instruction set has instructions of widely varying length. However, as RISC computers normally require more and often longer instructions to implement a given task, they inherently make less optimal use of bus bandwidth and cache memories.
Note that in classic RISC, all instructions have the same length. (This is one thing that separates RISC from CISC [1]). In the original RISC designs, the size of an instruction is 4 bytes, so always add 4 to the instruction address, but don't use PC + 4 for the case of a taken branch, jump, or exception (see delayed branches, below).
MIPS (Microprocessor without Interlocked Pipelined Stages) [1] is a family of reduced instruction set computer (RISC) instruction set architectures (ISA) [2]: A-1 [3]: 19 developed by MIPS Computer Systems, now MIPS Technologies, based in the United States.
The reason given is: Many CISC processors now do instruction fetch and decode in hardware, and execute most if not all instructions in hardware, and both RISC and CISC processors execute several operations per clock cycle. Please help update this article to reflect recent events or newly available information. (December 2023)
Alpha (original name Alpha AXP) is a 64-bit reduced instruction set computer (RISC) instruction set architecture (ISA) developed by Digital Equipment Corporation (DEC). Alpha was designed to replace 32-bit VAX complex instruction set computers (CISC) and to be a highly competitive RISC processor for Unix workstations and similar markets.