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From: hansen@mips.UUCP (Craig Hansen)
Newsgroups: comp.arch,comp.sys.nsc.32k
Subject: Branch prediction in the 532
Message-ID: <324@dumbo.UUCP>
Date: Tue, 21-Apr-87 02:21:29 EST
Article-I.D.: dumbo.324
Posted: Tue Apr 21 02:21:29 1987
Date-Received: Wed, 22-Apr-87 05:37:36 EST
Lines: 34
Xref: mnetor comp.arch:1037 comp.sys.nsc.32k:102


I've been reading two publications from National, one the infamous "Series
32000: NS32532 Competitive Comparison" that lists the eight separate patent
applications we've already heard about, the other titled "Series 32000:
NS32532 Technical Introduction."

The feature that most caught my eye was that of branch prediction,
for which I quote from the "Technical Introduction":

	The selection is based on the branch condition and
	direction; forward branches are predicted as taken,
	backward branches as non-taken.

Well, I don't see how the branch prediction can depend on the branch
condition, since waiting for the condition corresponds to no branch
prediction at all.  (Certainly novel and non-obvious...)

But when I was a young sprout, they always told me that backward branches
often represented loops and were most frequently taken. Forward branches
often represent conditional expressions, and are more ambivalent. As far as
I can see from the description, there is no prediction based on past
behaviour from previous invocations of the branch instruction.

If the description in National's literature is entirely accurate, the branch
prediction scheme used is seriously broken and should predict the wrong
answer more often than the right answer. Then again, if National hasn't seen
silicon yet, and uses the opportunity to fix it now, I may have just saved
them a few tens of thousands of dollars in wafer costs and hundreds in
otherwise lost revenue....
-- 
Craig Hansen
Manager, Architecture Development
MIPS Computer Systems, Inc.
...decwrl!mips!hansen