In our computer systems lecture we were introduced to the MIPS processor. It was

Question

In our computer systems lecture we were introduced to the MIPS processor. It was (re)developed over the course of the term and has in fact been quite easy to understand. It uses a RISC design, that is its elementary commands are regularly encoded and there are only few of them in order to keep the wires simple.

It was mentioned that CISC follows a different philosophy. I looked briefly at the x86 instruction set and was shocked. I can not image how anyone would want to build a processor that uses so complex a command set!

So I figure there have to be good arguments why large portions of the processor market use CISC architectures. What are they?

Explanation / Answer

The x86 instruction set is a bit of a special case. I think that Motorola's 68K and DEC's VAX are somewhat better examples of CISC. In the days of a lot of assembly-language code, people thought that a very regular, very inclusive ISA was better: I believe they called the difference between assembly code and the way people thought the "Semantic Gap". Theoretically, you wanted an instruction set that matched the way you thought.

The other big design driver for CISC seems to be "orthogonality": every instruction would work with every addressing mode (register, absolute address, relative offset, etc etc). You can see the bogey man of orthogonality show up in API design in Distributed Computing Environment (DCE) and in CORBA. That idea isn't limited to instruction set design.

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