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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.
[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).
Before the RISC philosophy became prominent, many computer architects tried to bridge the so-called semantic gap, i.e., to design instruction sets that directly support high-level programming constructs such as procedure calls, loop control, and complex addressing modes, allowing data structure and array accesses to be combined into single instructions.
If a CPU has an NX bit, it is more likely to be viewed as being a complex instruction set computer (CISC) or reduced instruction set computer (RISC). MISC chips typically lack hardware memory protection of any kind, unless there is an application specific reason to have the feature. If a CPU has a microcode subsystem, that excludes it from ...
RISC microprocessors like these were the first to have superscalar execution, because RISC architectures free transistors and die area which can be used to include multiple execution units and the traditional uniformity of the instruction set favors superscalar dispatch (this was why RISC designs were faster than CISC designs through the 1980s ...
RISC vs. CISC: CISC instruction sets often have variable instruction lengths, [8] often have a larger number of possible instructions that can be used, and each instruction could take differing amounts of time. RISC instruction sets attempt to limit the variability in each of these: instruction sets are usually constant in length, with few ...