Re: [TUHS] 2^n-bit operands (Was reviving a bit of WWB)

[John Cowan <cowan@ccil.org>]

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On Wed, Feb 3, 2021 at 9:55 AM M Douglas McIlroy <
m.douglas.mcilroy@dartmouth.edu> wrote:

With the 360 series, IBM fully committed to multiple operand sizes. DEC
> followed suit and C naturalized the idea into programmers' working
> vocabulary.
>

The steady expansion of character set sizes also had a great deal to do
with it.  The various 6-bit character sets were fine as long as the
industry was okay with English-only SHOUTING.  When that was outgrown,
7-bit ASCII and 8-bit EBCDIC on multiple-of-6 word sizes (as were found on
the big-endian DEC machines up to the PDP-10) were annoying to use.

On the 12-bit PDP-8, where I cut my teeth, ASCII was stored as
HHHHAAAAAAAA followed by LLLLBBBBBBBB, where the As represent the first
character, the Bs the second, and the Hs and Ls the third.  Padding was
done with NUL, which meant that, for example, the TTY driver simply filled
its read buffer with 0000AAAAAAAA 0000BBBBBBBB, which made rubout handling
much simpler.  Textual programs reading from it would already be set up to
ignore NULs.

On the 36-bit PDP-10, things were better: the sign bit was mostly ignored
and five 7-bit ASCII characters were packed into each word, again with NUL
padding.  (Line editors turned on the sign bit to indicate that this word
held an explicit ASCII line number.)



John Cowan          http://vrici.lojban.org/~cowan        cowan@ccil.org
Original line from The Warrior's Apprentice by Lois McMaster Bujold:
"Only on Barrayar would pulling a loaded needler start a stampede toward
one."
English-to-Russian-to-English mangling thereof: "Only on Barrayar you risk
to
lose support instead of finding it when you threat with the charged weapon."

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