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From: dave@onfcanim.UUCP (Dave Martindale)
Newsgroups: comp.unix.questions
Subject: Re: desktop super computers
Message-ID: <15014@onfcanim.UUCP>
Date: Fri, 14-Nov-86 02:30:16 EST
Article-I.D.: onfcanim.15014
Posted: Fri Nov 14 02:30:16 1986
Date-Received: Fri, 14-Nov-86 06:44:17 EST
References: <4631@brl-smoke.ARPA> <15800022@datacube> <15010@onfcanim.UUCP> <765@mips.UUCP>
Reply-To: dave@onfcanim.UUCP (Dave Martindale)
Organization: National Film Board / Office national du film, Montreal
Lines: 43
Summary: 

In article <765@mips.UUCP> hansen@mips.UUCP (Craig Hansen) writes:
>
>> My experience with the Weitek 1164/1165 floating point chips, which I
>> believe is what is used by Sun's fast floating point board, suggest
>> that they are slightly slower than the 11/780's FPA in both single and
>> double precision.  [ more stuff deleted ]
>
>	This information is totally wrong.  The writer of the above
>	may be confused with the earlier (and inferior) Sky FPA board.
>	First of all, the Weitek 1164/1165 perform add, subtract, and multiply
>	faster than the 11/780's FPA when run at 16 MHz; the operations
>	are faster than 1 usec by a healthy margin. The chip
>	set directly performs integer/float conversions and short/long
>	(or single/double) conversions. The operations are implemented
>	in accordance with the IEEE standard, including support for
>	IEEE directed rounding modes; so 500.0/10.0 == 5.0 exactly.

I'm not confused by a Sky board.  I have had some experience with the
named Weitek chips as used in a Silicon Graphics IRIS 2400T (16MHz 68020),
not a Sun.  So I'm talking about the same hardware, but not the same
support software.

I did some very simple benchmarking when the new IRIS boards arrived,
and found that running real code, the IRIS was just slightly slower
than the 780.  I did get the impression that SGI was using software
for some functions (float/int conversion, for example) which could
have had much to do with slowing down performance.  The IRIS FPA was
out over a year ago, before Sun I believe, so the software may have
been put together in somewhat of a hurry, and may have improved since
then - I haven't had time to check.

The 500/10 problem was real enough - it caused printf to print out 500
as "4:0".  SGI fixed it by using software instead of hardware for that
division, so I just assumed that the hardware wasn't capable of doing
it right.

Anyway, I was just reporting what I'd experienced; it seems that it may
not be true anymore, or may not apply to the Sun.

Still, my original question stands:  How is "4 Mips" measured?  And what
is floating point performance really like?

	Dave Martindale