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From: bs@linus.UUCP (Robert D. Silverman)
Newsgroups: comp.arch,misc.wanted,comp.sources.wanted
Subject: Re: MIPS Assembler Procedure
Message-ID: <57725@linus.UUCP>
Date: 26 Jun 89 22:25:50 GMT
References: <57125@linus.UUCP> <1989Jun24.230056.27774@utzoo.uucp> <41957@bbn.COM>
Reply-To: bs@gauss.UUCP (Robert D. Silverman)
Organization: The MITRE Corporation, Bedford MA
Lines: 42

In article <41957@bbn.COM> slackey@BBN.COM (Stan Lackey) writes:
:In article <1989Jun24.230056.27774@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes:
:>In article <57125@linus.UUCP> bs@gauss.UUCP (Robert D. Silverman) writes:
:>>This, in my opinion is one of the major faults of RISC processors. They
:>>do not provide basic arithmetic instructions. 
:>
:>When the list of "basic" arithmetic instructions is pages long, one starts
:>to wonder how many of them are really "basic".  The instruction you ask
:>for -- divide double length by single length yielding single-length result --
:>is not exactly frequently needed.  Just how much silicon is it worth to make
:>it run faster than an implementation as a subroutine?
:
:The RISC suppliers would like us (users) to believe that RISC is
:ALWAYS faster.  I can't help thinking back to when Patterson's first
:RISC papers came out.  He managed to tune the code on his RISC until
:all of the benchmarks ran faster than the VAX 11/780, except for one,
:which happened to exercise one of the microcoded character string
:instructions a lot.
:
:The moral of the story: If your application needs mul and div
:performance, get a micro with hardware support for them.  If that
:means using a CISC, so be it.
:
:BTW:  divide-double-by-single-yielding-single is the one the 68000
:included.  Is there a chicken-and-egg situation here?
 
No, to the last question. Such an instruction is required to compute
AB/C or AB MOD C exactly, when AB can overflow and C > A,B. 

The problem with CISC is that no one is likely to come out with a 40MIP
CISC in the near future [e.g. i860]. I simply contend that operations
like multiplication and division should be fundamental to ANY computer.
I also include add, subtract and add/subtract with carry. How else
would you compute AB/C or AB % C exactly?

Note that I've also seen some pretty arcane instructions on so called
RISC processors. The i860 even has a custom purpose piece of real estate
on the chip to support special graphics operations. However, it can't
multiply or divide!!!! This is RISC?

Bob Silverman
:-Stan