Xref: utzoo comp.unix.misc:372 comp.unix.internals:710
Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!wuarchive!emory!ogicse!cvedc!gssc!timr
From: timr@gssc.UUCP (Tim Roberts)
Newsgroups: comp.unix.misc,comp.unix.internals
Subject: Re: Summary of Request for Comparison of Altos and NCR
Summary: What is RISC?
Message-ID: <6422@gssc.UUCP>
Date: 18 Oct 90 16:32:23 GMT
References: <11@ACT.UUCP> <YZG6DXD@xds13.ferranti.com> <CEDMAN.90Oct17082702@lynx.ps.uci.edu>
Organization: Graphic Software Systems, Inc. Beaverton, Or.
Lines: 48

In article <CEDMAN.90Oct17082702@lynx.ps.uci.edu> cedman@lynx.ps.uci.edu (Carl Edman) writes:
>
>Just a short and (IMHO funny) note: Do you know how many instructions
>a RISC (Reduced Instruction Set Coding) CPU by IBM has ? 186 !
>Yes, this is the number of the machine instructions for the new
>IBM 6000 series.

This is a common misconception which I would like to soapbox upon for a while.
Computer architecturalists will want to skip the remainder of this post.

"RISC", for Reduced Instruction Set Computer, does NOT imply a reduced number
of instructions (although that is a common side-effect).   RISC is a philosophy
which advocates:

	- Simple instructions
	- Most or all instructions complete in one clock cycle
	- Instruction parallelism
	- Instruction overlap as register usage permits
	- Branch lookahead
	- Large register sets
	- Quite sophisticated optimizing instruction-scheduling compilers

The Control Data 6600 and its successors would have to qualify as RISC
machines, although the term was not yet popular.  It is startling how similar
the architecture of Seymour's first baby is to, for example, the MIPS R2000,
R3000 and R6280 chip sets.

In contrast, CISC architectures tend to have:

	- Complex, specialized instructions
	- Instructions which require multiple cycles to complete
	- Difficult pipelining
	- Smaller register sets (the benefits of a large register set decrease
	  as your average instruction time increases)
	- Less sophisticated compilers

Neither architecture is inherently better or worse.  RISC hardware tends to
be built more cheaply, at the cost of more expensive software, since many of
the functions provided in the CISC hardware have to be implemented in software.

Someday, twenty years from now, high school students enrolled in Computer 
Architecture and its Place In History will laugh at our silly debate.

-- 
timr@gssc.gss.com	Tim N Roberts, CCP	Graphic Software Systems

I think Lotus should come out with a PS/1 spreadsheet.  
They could call it 0-1-2.