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From: "Stephen Fuld" <SFuld@alumni.cmu.edu.invalid>
Newsgroups: comp.arch
Subject: Re: why bits, Byte Addressability And Beyond
Date: Tue, 7 May 2024 06:35:53 -0000 (UTC)
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MitchAlsup1 wrote:

> John Levine wrote:
> 
> > According to John Savard  <quadibloc@servername.invalid>:
> > > On Mon, 6 May 2024 02:54:11 -0000 (UTC), John Levine
> > > <johnl@taugh.com> wrote:
> > > 
> > > > Why do you think bit addressing will be
> > > > faster than shifting and masking? ...
> 
> > > So just because a processor has a 64-bit bus to memory doesn't
> > > mean it has to implement fetching a single byte from memory by
> > > doing a shift and mask operation in a 64-bit register. Instead,
> > > each byte of the bus could have a direct wired path to the low
> > > 8-bits of the internal data bus feeding the registers.
> 
> > I was more thinking about storing bit fields, where you probably
> > have to fetch the whole word or cache line or whatever, shift the
> > new field into it, and then store it back. You already have to do
> > something like that for byte stores but bit addressing makes it 8
> > times as hairy.
> 
> Which is no different than ECC, BTW...
> 
> Could someone invent a bit field ISA that was as efficient as a byte
> accessible architecture:: probably.
> 
> Could this bit accessible architecture outperform a byte ISA on
> typical codes:: doubtful. Two reasons:: 1) more delay in the LD/ST
> pipeline, 2) most programs use as little bit-fielding as possible
> (not as much as practical) !!!


Some time ago, I proposed an additional instruction, a load varient
that allowed you to address bit fields.  Would it be slower than a
"normal" byte oriented load?  Almost certainly.  But would it be faster
than doing all the shifts, masks, word crossing calculations, etc. via
extra instructions?  Again, almost certainly.  So you keep the benefits
of byte oriented loads most of the time, but have "reasonable" access
to bit fields when you need them, faster than without the
extrainstructions.  Hopefully the best of both worlds.




-- 
 - Stephen Fuld 
(e-mail address disguised to prevent spam)