Xref: utzoo comp.arch:8626 comp.sys.intel:732
Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!bloom-beacon!apple!oliveb!sun!dgh!dgh
From: dgh%dgh@Sun.COM (David Hough)
Newsgroups: comp.arch,comp.sys.intel
Subject: Re: i860 overview (long)
Summary: slow divide bad
Message-ID: <92634@sun.uucp>
Date: 6 Mar 89 19:22:25 GMT
References: <807@microsoft.UUCP>
Sender: news@sun.uucp
Lines: 30


> The frcp and frsqr insns give return approximate reciprocal and 1/square
  root "with absolute significand error < 2^-7".  Intel supplies routines
  for Newton-Raphson approximations that take 22 clocks (*almost* single
  precision) or 38 clocks (*almost* double precision), and the Intel i860
  library provides true IEEE divide.  [RISC design principles at work:
  divides are infrequent enough not to slow down/drop some other feature
  to provide divide hardware.]

Another RISC design principle that will be discovered by whoever trys to
build an engineering work station out of this chip:  you base your design
on few or small benchmarks at your peril.

Not every engineering computation can be reduced to linpack-style memory
intensive adds and multiplies.  Division and sqrt are important in a lot
of realistic applications - spice for starters.  And spice convergence
sometimes is a function of how clean the arithmetic is (at least for the
inappropriately popular mosamp2 benchmark).   A system with a TI 8847
running at the same clock should beat an i860 on a number of realistic
applications.

None of which should be construed to imply that the i860 won't be very 
good on the applications for which it was designed, such as graphics
processors.

David Hough

dhough@sun.com   
na.hough@na-net.stanford.edu
{ucbvax,decvax,decwrl,seismo}!sun!dhough