Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ukma!xanth!lll-winken!uunet!portal!cup.portal.com!bcase
From: bcase@cup.portal.com (Brian bcase Case)
Newsgroups: comp.arch
Subject: Re: i860 and compilers
Message-ID: <16081@cup.portal.com>
Date: 21 Mar 89 20:23:07 GMT
References: <4524@pt.cs.cmu.edu>
Organization: The Portal System (TM)
Lines: 32

>Part of the problem is the (poor) idea of having to catch the hot,
>smoking data as it flys out of the pipe.  Self-draining pipelines would
>have been easier on both the compilers and the interrupt handling.  The
>pipeline mode's startup and shutdown transients, which trash things,
>are also undesirable. 

Yes, I thought the idea of have to catch "the hot, smoking data" (I
*love* that characterization!) was a dumb one too, until I realized
(i.e. was told) why it is done that way:  This way, the FP register
file is much more useful.  With "normal" interlocking, a full FP pipe
would lock nearly *all* the FP registers.  With the way the i860 does
it, the registers that will be destinations can be used while the
results that will eventually go there are being computed.  This makes
sense, assuming that one can have only 32 FP registers (16 dp registers).
That might be where the mistake was made....

>The data-chaining features sound intractable.  Perhaps another
>dual-instruction mode would have been better - one instruction to the
>FADD unit, the other to FMUL.

Yep, it's a bitch.  Again, the constraint was probably somewhat
related to implementation technology.  Again, I think they should
have given up 4K bytes of the data cache in order to permit them to
implement a cleaner *architecture* instead of sacrificing architecture
to *implementation*.  But then, what do I know.

>Perhaps Intel should have dropped the floating point unit, and put in a
>second integer ALU instead.  It would have goosed the Dhrystones, no?

I love it!  *Now* you're talking!  I mean, what's more important,
FP or having the longest drystone histogram bar in your marketing
"literature"! :-) :-) :-)