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From: firth@sei.cmu.edu (Robert Firth)
Newsgroups: comp.arch
Subject: Re: Branch prediction in the 532
Message-ID: <1123@aw.sei.cmu.edu>
Date: Fri, 24-Apr-87 09:43:10 EST
Article-I.D.: aw.1123
Posted: Fri Apr 24 09:43:10 1987
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References: <324@dumbo.UUCP> <165100008@uiucdcsb> <18537@ucbvax.BERKELEY.EDU>
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Keywords: branch prediction, branch target buffers, BTB

In article <18537@ucbvax.BERKELEY.EDU> shebanow@ji.Berkeley.EDU.UUCP (Mike Shebanow) writes:
...
>Based on the results published, the best prediction algorithms use run-time
>information. As developed by IBM, a branch target buffer is kept in the
>machine (BTB). The BTB is an associative table (direct mapped, n-way set
>associative, fully associative, whatever) which uses the address of the
>BRANCH as an index. Associated with the branch address are two pieces
>of information (usually):
>
>	- target address of the branch (where to go if branch taken -- OR,
>	    perhaps the first few instructions at the target address (eg.,
>	    AM29000))
>	- some prediction information (see below).

Mike, could you please elaborate on this, because after 2 days I find
myself no nearer to understanding it.  The purpose of the "prediction"
information seems to be to allow the processor to prefetch whichever
of the branch continuations (successor or destination) is more likely
to be needed.

But if you have a branch cache, surely BOTH continuations are available
already - one in the normal pipeline and the other in the cache.  So
who cares which is needed? - and why bother to predict it?