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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.arch
Subject: Re: Keeping other stuff with addresses
Date: Tue, 03 Dec 2024 17:43:15 -0800
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anton@mips.complang.tuwien.ac.at (Anton Ertl) writes:
[...]
> The technique of putting stuff in unused bits of an address has its
> drawbacks, but it also has benefits, in particular type information is
> often stored there (even on architectures that do not ignore any
> bits).  Of course AMD and Intel have the bad examples of S/360 and
> 68000 in mind, and did not want to have anything to do with that
> during the first two decades of AMD64.
[...]

One example of this that I haven't seen mentioned here is used by the C
and C++ compilers for Cray vector machines (I've used the T90 and SV1).

These systems had a 64-bit word size, and were heavily optimized
for floating-point operations.  Hardware addresses referred to 64-bit
words.  Compatibility with other systems required support for 8-bit
bytes (CHAR_BIT==8).  This was implemented in generated code by using
the high-order 3 bits of an address as a byte offset.

The fact that all byte addressing was implemented in software meant
that, for example, string manipulation was remarkably slow -- but it
worked.

(I'm using the past tense because I don't know whether any of these
systems are still in use.)

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */