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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.
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.
Competition between RISC and conventional CISC approaches was also the subject of theoretical analysis in the early 1980s, leading, for example, to the iron law of processor performance. RISC-V prototype chip (2013)
A single RISC instruction typically performs only a single operation, such as an "add" of registers or a "load" from a memory location into a register. 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 ...
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).
RISC-V [b] (pronounced "risk-five" [2]: 1 ) is an open standard instruction set architecture (ISA) based on established reduced instruction set computer (RISC) principles. . The project began in 2010 at the University of California, Berkeley, transferred to the RISC-V Foundation in 2015, and on to RISC-V International, a Swiss non-profit entity, in November 20
Some CISC processors include instructions that can take a very long time to execute. Such variations interfere with both interrupt latency and, what is far more important in modern systems, pipelining. When designing a new processor, a hardwired control RISC has the following advantages over microcoded CISC:
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 ...