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PRF size (was: The Third Wish)

From Stefan Monnier <monnier@iro.umontreal.ca>
Newsgroups comp.arch
Subject PRF size (was: The Third Wish)
Date 2025-07-17 16:34 -0400
Organization A noiseless patient Spider
Message-ID <jwvbjpiin55.fsf-monnier+comp.arch@gnu.org> (permalink)
References (16 earlier) <1059reg$1652n$1@dont-email.me> <c776c7556431b66ea75f87f243082d9d@www.novabbs.org> <105bdtd$1g3td$1@dont-email.me> <105bidj$qjc0$1@dont-email.me> <798ca7045ef0b91b3cc24c8c1aa5d2b7@www.novabbs.org>

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> of forwarding. K9's PRF was 160-odd registers, instead of an arbitrary
> 40-for integer, 40-for FP, 40-for SIMD, and 40-for memory. It is the
> same reason you can put any kind of data in the data cache !!! The
> unified renamer ran out of registers a LOT less often than partitioned
> renamer.

IIUC, access to a size-40 register file is about 1/2 a cycle to access
while a size-160 regfile will probably be a full cycle.
I assume this is part of what makes the pipeline longer, and it also
makes the forwarding network more complex since there are more values
that can benefit from forwarding (i.e. where forwarding is needed to
avoid having to wait for write+read in the register file).
But in return for that, fewer values get read from the regfile, so you
need fewer read ports, right?

How do they deal with the massive number of PRF writes per cycle?
Do they try to "kill" those writes that can be determined to be useless
(because the remaining reads are serviced via the forwarding network instead)?
Do they use banking?


        Stefan

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                PRF size (was: The Third Wish) Stefan Monnier <monnier@iro.umontreal.ca> - 2025-07-17 16:34 -0400
                Re: PRF size mitchalsup@aol.com (MitchAlsup1) - 2025-07-17 21:38 +0000
                Re: PRF size EricP <ThatWouldBeTelling@thevillage.com> - 2025-07-20 11:47 -0400
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