19 May
2020
19 May
'20
4 a.m.
On Monday, 18 May 2020 at 12:11:00 -0400, Dan Cross wrote:
On Sun, May 17, 2020 at 12:24 PM Paul Winalski
<paul.winalski(a)gmail.com>
wrote:
No, I think it was intended to be 6 text characters. My recollection
is that the older "scientific" machines didn't do BCD. Certainly the
older UNIVAC 1100s and 490s didn't.
On 5/16/20, Steffen Nurpmeso
<steffen(a)sdaoden.eu> wrote:
Was this perhaps a typo for 9 4-bit BCD digits?
Why was there no byte or "mem" type?
These days machine architecture has settled on the 8-bit byte as the
unit for addressing, but it wasn't always the case. The PDP-10
addressed memory in 36-bit units. The character manipulating
instructions could deal with a variety of different byte lengths: you
could store six 6-bit BCD characters per machine word, 6x6-bit data would certainly hold BAUDOT data, and I
thought the
Univac/CDC machines supported a 6-bit character set? Does this live
on in the Unisys 1100-series machines? I see some reference to
FIELDATA online.
Most machines of the day used a 6 bit character encoding. You're
right with Fieldata, but I don't know if it's still used. Are the
1100s still in use? They date back 60 years.
My guesswork from here on:
To bring it back slightly to Unix, when Mary Ann and I
were playing
around with First Edition on the emulated PDP-7 at LCM+L during the
Unix50 event last USENIX, I have a vague recollection that the B
routine for reading a character from stdin was either `getchar` or
`getc`. I had some impression that this did some magic necessary to
extract a character from half of an 18-bit word (maybe it just
zeroed the upper half of a word or something).
That would be extremely inefficient on such a small machine. This was
ASCII, right? I'd guess that the PDP-7, like the PDP-8, was intended
to use 6 bit characters, and that was probably the reason why it was
18 bits rather than 16 bits.
If I had to guess, I imagine that the coincidence
between
"character" and "byte" in C is a quirk of this history, as opposed
to any special hidden meaning regarding textual vs binary data,
particularly since Unix makes no real distinction between the two:
files are just unstructured bags of bytes, they're called 'char'
because that was just the way things had always been.
I was going to say "C doesn't talk about bytes", but I was wrong. But
from K&R, 1st edition, page 126:
The size is given in unspecified units called "bytes," which are the
same size as a char.
Greg
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