From: adap@edmund.cs.andrews.edu (Edsel Adap)
Subject: Re: CISC and RISC
Organization: Andrews University
Date: Wed, 16 Dec 1992 02:28:24 GMT

In article <Bz8FD1.Dxt@ns1.nodak.edu> dewald@plains.NoDak.edu (Eric Dewald) writes:
>Why should computer makers go from CISC to RISC?
>

RISC - Reduced Instruction Set Computer
CISC - Complex Instruction Set Computer

The reason for many companies going RISC is because of its great speed
advantages over CISC.  This is accomplished in many ways.  In CISC, (ie a
Motorolla 68000) the instruction set is what is called a macro instruction.
These macro instruction are a collection of micro instructions that do what
the macro instructions is supposed to do.  In RISC instead of a macro
instruction, there is a minimal amount of instructions, and a minimal amount
of addressing modes.  More efficiency is obtained by working with the micro
code because there are things that the macro instructions do in order to
generalize the instructions for various purposes.  Micro code is to Macro
code as assembler is to a high level language.  The Micro code provides more
efficiency.

Another Advantage of RISC is the abundance of Register memory.  CISC
commonly contains few registers (The 68000 has 8 data registers and 8
address registers) while RISC processors contain anywhere from 32 registers
to a few hundred registers (It is not uncommon to have RISC processors that
have 512 registers).  Since Register memory is about 10 times faster than
main memory, most of the instructions are register to register, therefore
increasing efficiency.

In RISC chips the data path cycle time is minimized by logically reducing
the number of steps it takes for an instruction to be executed and by
physically shortening the distances between components in the chip (the
large number of registers play a role in this).

There is also a significant amount of parallellism and pipelining in RISC
chips.  As soon as an instruction is started another instruction may be
started even before the completion of the previous instruction.  Output of
one instructions is passed on to another Unit in the chip for further
processing while the current unit executing the instruction fetches a new
instruction to process.  On many RISC chips as many as 4 instructions can be
completed in one clock cylce as a result of this.

>I was under the impression that CISC meant complicated instruction set
>and RISC meant reduced instruction set. So wouldn't that mean larger
>code size on a RISC machine compared to CISC.
>
>What are the advantages of CISC and RISC?

CISC I think is easier to program.  RISC is difficult as you have to worry
about timing your instructions properly.  If you issued a memory write and
do a read from the same location immediately after the write instruction,
the data you may have wanted to read may not be there yet!  So you may have
to issue the write a few steps earlier than you would normally on a CISC
chip.

However, the CISC is slow compared to RISC.

>Thank you in advance,
>Eric Dewald
>dewald@plains.NoDak.edu

I hope I have been of help.

Edsel
--
"That girl is one of the least benightedly unintelligent organic life
forms it has been my profound lack of pleasure not to be able to avoid
meeting."
________________________________________________________________________________
|    Procrastination is my sin          | Edsel Adap
|    It brings me naught but sorrow     | Computer Science / Mathematics Major
|    I know that I should stop it       | Andrews University
|    In fact I will -- Tomorrow!        |
|                         -Gloria Pitzer| adap@cs.andrews.edu
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