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From: arc@myriasb.UUCP (Alan Covington)
Newsgroups: net.lang.c
Subject: Re: Numeric comparisons
Message-ID: <487@myriasb.UUCP>
Date: Thu, 26-Sep-85 19:06:45 EDT
Article-I.D.: myriasb.487
Posted: Thu Sep 26 19:06:45 1985
Date-Received: Thu, 26-Sep-85 20:49:39 EDT
References: <797@dataio.UUCP> <726@terak.UUCP> <1657@umcp-cs.UUCP>
Organization: Myrias Research, Edmonton
Lines: 16

Chris Torek writes
>   I offer another example.  Take a 32 bit machine---the Vax---both for
>   simplicity and because the Vax does compares via subtraction.  But I
>   will use a byte instruction:

The VAX cmp and sub instructions set the condition codes differently.
Thus, the cmp is not just a "subtract" with the result thrown away.
An easy example is comparing 0x80000000 and 1 results in the N bit
of the condition codes being set, whereas subtracting 1 from 0x80000000
results in the overflow bit being set.  The VAX may use a subtract as
part of the cmp instruction, but it doesn't do signed comparison as
Chris claims.
>    A signed comparison is a subtract with the result
>    thrown away.

		Alan Covington  ...!alberta!myrias!arc