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In fact, over the years, RISC instruction sets have grown in size, and today many of them have a larger set of instructions than many CISC CPUs. [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 ...
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.
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).
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 ...
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)
Dhrystone may represent a result more meaningfully than MIPS (million instructions per second) because instruction count comparisons between different instruction sets (e.g. RISC vs. CISC) can confound simple comparisons. For example, the same high-level task may require many more instructions on a RISC machine, but might execute faster than a ...