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From: clc5q@hemlock.cs.Virginia.EDU (Clark L. Coleman)
Newsgroups: comp.arch
Subject: Re: VAX EDIV remainder (was new instructions)
Message-ID: <1991May28.212003.24361@murdoch.acc.Virginia.EDU>
Date: 28 May 91 21:20:03 GMT
References: <1991May21.191034.25980@murdoch.acc.Virginia.EDU> <25874@as0c.sei.cmu.edu> <13587@dog.ee.lbl.gov>
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Organization: University of Virginia Computer Science Department
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In article <13587@dog.ee.lbl.gov> torek@elf.ee.lbl.gov (Chris Torek) writes:
>
>According to my `VAX instruction timings (with FPA)', the original sequence

   [timing estimates deleted]

Actually, the timing disadvantage for EDIV is worse than the estimates. My
timings were based on a version of the assembly code that did not do the
longword-swap on the two halves of the quadword dividend (it used R0/R1
instead of R1/R0 pair, you might say) and it was producing overflow, which
is faster than producing the right answer. Hence, the "cc" compiler is
quite justified in NOT using the EDIV instruction.

Unfortunately, as is often the case, a point was lost through use of a very
bad example.

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|||  clc5q@virginia.edu (Clark L. Coleman)