Path: utzoo!mnetor!uunet!husc6!bloom-beacon!gatech!purdue!i.cc.purdue.edu!k.cc.purdue.edu!l.cc.purdue.edu!cik
From: cik@l.cc.purdue.edu (Herman Rubin)
Newsgroups: comp.arch
Subject: Re: RISC  a short answer??
Message-ID: <770@l.cc.purdue.edu>
Date: 5 May 88 10:53:49 GMT
References: <1036@nusdhub.UUCP> <1988May3.224604.2252@utzoo.uucp> <383@m3.mfci.UUCP>
Organization: Purdue University Statistics Department
Lines: 27
Summary: But what if there are many slow ones?


The philosophy of RISC seems to be, to quote one of the above articles, that
one shoul not worry about the "slow" instructions to speed up 70% of the 
instructions.  If the 30% slow instructions now run at 1/3 the speed of the
fast ones, this would give an overall time of 1.7/instruction.  Now if the 
time of the slow instructions was doubled, and the fast instructions run
infinitely fast, the overall time would be 2/instruction.

It seems that, instead, there is great merit in a VCISC design, in which
useful instructions are included to decrease the number of instructions needed
in a program.  This is especially important if the current instructions require
branches, which are difficult to speed up in many cases.  In many cases, the
instruction complexity can be handled in a tag field, which does not have to
be decoded in the initial assignment to a computing unit.  An example of this
is how to handle the quotient and remainder for a division, depending on the
signs of the arguments.  This may even be important if both arguments are
positive; one may want the remainder non-positive, or of minimum magnitude.
Here software is expensive compared to hardware; if the hardware default is
not what is wanted, it may be necessary to use several instructions, including
conditional branches, to achieve the result.  If 10% of the instructions are
of such a type, I doubt that increasing the instruction time of the "fast"
instructions by 30% will compensate.  And why should there not be different
classes of instructions?
-- 
Herman Rubin, Dept. of Statistics, Purdue Univ., West Lafayette IN47907
Phone: (317)494-6054
hrubin@l.cc.purdue.edu (ARPA or UUCP) or hrubin@purccvm.bitnet