Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!leah!itsgw!steinmetz!uunet!microsoft!w-colinp
From: w-colinp@microsoft.UUCP (Colin Plumb)
Newsgroups: comp.arch
Subject: Re: In defense of the VAX
Message-ID: <752@microsoft.UUCP>
Date: 26 Feb 89 23:22:37 GMT
References: <11037@tekecs.TEK.COM> <76700073@p.cs.uiuc.edu> <324@taniwha.UUCP> <668@m3.mfci.UUCP> <501@megatek.UUCP>
Reply-To: w-colinp@microsoft.uucp (Colin Plumb)
Organization: very little
Lines: 22

mark@corona.UUCP (Rocket J. Squirrel) wrote:
> * The last project of the DEC-10 group was a thing called "jupiter" which was
> to be a pipelined DEC-10. This would have been quite an effort in its own
> right. The DEC-10 had fixed-length instructions (which people in this group
> seem to find desireable) and they were very orthogonal. Unfortunately, the
> architecture encouraged the use of skips and instructions that modified a
> register and immediately branched based on the result.

H'm... while I have my concerns over compare-and-branch (it still seems to
serve MIPS very well), I think skips are a natural for deep pipelines.
It's frequently the case that you only need one or two instructions to
execute one side of an if statement, and a pipeline bubble is more expensive
than wasting those one or two cycles.  PDP-10 and Acorn ARM every-instruction-
conditional strategies are even better, as they don't waste cycles decoding
skip instructions, but they require valuable instruction bits and disagree
with the conditions-in-general-registers school.

But skip instructions are one thing I wish the 29000 had.
-- 
	-Colin (uunet!microsoft!w-colinp)

"Don't listen to me.  I never do."