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From: John Levine <johnl@taugh.com>
Newsgroups: comp.arch
Subject: Re: alignment, Byte Addressability And Beyond
Date: Sun, 5 May 2024 18:57:14 -0000 (UTC)
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According to Robert Swindells  <rjs@fdy2.co.uk>:
>On Sat, 04 May 2024 09:11:27 GMT, Anton Ertl wrote:
>On a byte-addressed machine you can use some lower bits "for free" if
>the objects being addressed are always word-sized or larger. SPARC has
>specific instructions to make use of this.

Only if you can count on them being aligned. On S/360 they required
everything to be aligned, and one of the changes on S/370 was to allow
arbitrary data alignment for data addresses. They quickly found that
Fortran programs used COMMON and EQUIVALENCE to put 8 bit reals on 4
byte boundaries in strictly standard conforming programs. Oops. The
Fortran library caught the traps and fixed them up but with dreadful
performance.

If your storage management is disciplined enough that you know that everything
is aligned on natural boundaries, this trick still works, but if you're going
to have to mask out flag bits anyway, the argument for putting the flags in
the low bits isn't as strong.

-- 
Regards,
John Levine, johnl@taugh.com, Primary Perpetrator of "The Internet for Dummies",
Please consider the environment before reading this e-mail. https://jl.ly