Re: Byte Addressability And Beyond

According to Lawrence D'Oliveiro  <ldo@nz.invalid>:

Byte addressing was invented by IBM for the System/360, introduced in
1964. At least as I understand it. Up to that time, and indeed for a long
time after, machines had a “word length” which was the smallest
addressable unit of memory. This could have a range of sizes, e.g.
>
   12 -- DEC PDP-5/8
   18 -- DEC PDP-1/4/7/9
   36 -- DEC PDP-6/10
   60 -- CDC 6000-series
   64 -- Cray

Commercial machines were character or digit addressed, as was at least
one scientfic computer, the IBM 1620.

The IBM 650 had 10 digit words, with characters stored as digit pairs.
The 702 and 705 were decimal character addressable. Instructions were
5 characters but data could be arbitrary length and location.  The very
popular 1401 was also character addressed with variable length data.

Why was byte addressing invented? I think it was for easy handling of
strings and other binary data. But why stop there?

It was to be reasonably efficient both for character business data and
word scientific data. Since the words had to be aligned, it was easy
to handle them as a single unit in parallel on machines with internal
data paths wider than 8 bits, all the models bigger than 360/30.

I guess the idea of
going all the way down to bit-level addressing was considered a bit
extreme?

STRETCH had bit addressing. It added a great deal of complication for
very little benefit. In the relatively rare situations where you want
to handle bit fields, shifting and masking is good enough without
slowing everything else down.

One side-effect of byte addressing has been the “endian wars”: the
inconsistency, between different machine architectures, ...

Until the PDP-11, all byte addressed machines were bigendian. Despite
a lot of looking, I have never found an explanation of why DEC made
the PDP-11 littlendian. I'm reasonably sure they were aware that it
was reversed from the 360, but they never said why.

Please do me a favor and DO NOT guess why they did it -- we have
already had lots and lots of guesses and we have no way to tell
whether any of the guesses are right.

--
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John Levine, johnl@taugh.com, Primary Perpetrator of "The Internet for Dummies",
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