Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!mailrus!cornell!batcomputer!dumesny
From: dumesny@batcomputer.tn.cornell.edu (Alain Dumesny)
Newsgroups: comp.arch
Subject: Re: A simple question on RISC
Message-ID: <6729@batcomputer.tn.cornell.edu>
Date: 1 Nov 88 02:34:47 GMT
References: <6544@xanth.cs.odu.edu>
Reply-To: dumesny@tcgould.tn.cornell.edu (Alain Dumesny)
Organization: Cornell Theory Center, Cornell University, Ithaca NY
Lines: 45

In article <6544@xanth.cs.odu.edu> aiko@cs.odu.edu (John K Hayes) writes:
>Can someone explain, in general, what RISC technology is?  Starting, perhaps,
>with what RISC stands for.  I'm not exactly an architecture person.

RISC stands for Reduced Instruction Set Computer, which are processor like the
M88000.  On the other side are what are called the CISC (Complex Instruction
Set Computer) which are processors like the M680x0 and 80x86 (yuck).
The main difference between RISC and CISC is that RISC processors only include
the most basic (and necessary) intructions.  Therefore when you send it an
intruction, the instruction won't have to be "decoded" (by decoded I mean that
it won't go through a lookup table of micro-code), which realizes a big 
improvement in performance.  On CISC processors you have intructions that are
rather complicate (i.e. they do a lot of thing) and they need to go to the
look-up table to be "decoded".

A simple (if you program in assembly language) example might be using a JMP
(jump) to subroutine.   On a CISC chip you just say JMP address, and the
processor does the rest for you.  On a RISC chip you normally won't have this 
kind of intructions because they can be broken down into basics on.  Like 
saving what ever register you need on the stack (including the return address
which is in the PC) and load the new PC (program counter) with the new add.

Also RISC chips tends to have a lot more registers build in (made possible 
because RISC chips require a lot less component, and therefore the extra space
can be used for fast memory (registers)).

Advantages of RISC are that they are much faster (on the number of instructions
per cycle) and cheaper.  However IN GENERAL you need more instructions for
a RISC chip for a given program than you need for a CISC chip for the same
program (maybe about 30%).  But you still have faster processing, which 
explaines for the "recent" rush for RISC technology (which actually was there
before CISC).

You might wonder why insn't everybody using RISC then ? right.
Well you have to take into consideration the udge investment in software, and
also the fact that RISC chips require smarter compiler when you program in
a high level language.

Anyway, I hope this helped you instead of confusing you !  :-)

----------------
Alain Dumesny
Cornell University
awzy@crnlvax5.bitnet

No flame if I am wrong, I am learning this tuff now !  :-)