[COFF] UNIVAC and other character sets (was: v7 K&R C)

[grog at lemis.com (Greg 'groggy' Lehey)]

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 at gmail.com>
> wrote:
>
>> On 5/16/20, Steffen Nurpmeso <steffen at sdaoden.eu> wrote:
>>>
>>> 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,
>
> Was this perhaps a typo for 9 4-bit BCD digits?

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.

> 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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