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From: matloff@bizet.Berkeley.EDU (Norman Matloff)
Newsgroups: comp.arch
Subject: Re: undoing autoincrement
Message-ID: <22017@agate.BERKELEY.EDU>
Date: 25 Mar 89 00:03:09 GMT
References: <21971@agate.BERKELEY.EDU> <27799@apple.Apple.COM>
Sender: usenet@agate.BERKELEY.EDU
Reply-To: matloff@heather.ucdavis.edu (Norm Matloff)
Organization: EECS, UC Davis
Lines: 15

In article <27799@apple.Apple.COM> baum@apple.UUCP (Allen Baum) writes:

>Well, suppose that I have an ADD instruction following a conditional branch,
>and the ADD is evaluated before the branch decision. Do I have to undo the
>ADD? Whats the difference? Am I missing something here?

Are you assuming a "multiple resource" CPU, so that instructions might
execute out of order?  [I was assuming a simple linear pipe, e.g.
instr. fetch/decode/operand fetch/execute.]  Anyway, in your case
you again would have to do more work in undoing an ADD which has
autoincrement than a plain ADD without autoincrement, and thus the
autoinc wouldn't be "almost free" in this case either.  Or am *I*
missing something [a real possibility :-) ]?

   Norm