From: wbe@bbn.com (Winston Edmond)
Newsgroups: alt.folklore.computers
Subject: Re: Control characters
Message-ID: <62097@bbn.BBN.COM>
Date: 15 Jan 91 05:44:48 GMT
References: <1991Jan3.005253.7069@sq.sq.com> <2753@charon.cwi.nl> <13747@milton.u.washington.edu>
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In article <2753@charon.cwi.nl> dik@cwi.nl (Dik T. Winter) writes:
>OK, now the base question, where derives the use of ^A etc.  My first exposure
>to this usage was in the seventies when I aquired access to our Unix system.
>And checking through the manuals I have here at home (quite a lot) I find
>that *none* of them make use of the circumflex to mark a control character!
>But then, I do not need Unix manuals :-).  The range is from DEC-10 to
>Cray.  So I suppose it derives either from Unix or its predecessor (MULTICS),
>or perhaps from BCPL (but I suppose not the last).  Was it already in version
>6 of Unix?

In article <13747@milton.u.washington.edu> Mark Crispin writes:
>The first DEC operating systems for the PDP-6 (the predecessor to the
>PDP-10 and the DEC-20) used uparrow (the predecessor of circumflex) to
>mark control.  This is in 1964 documentation.
>
>I don't know if any PDP-5, PDP-4, or PDP-1 software used this
>convention.  I doubt anything on the PDP-5 ever did.  I don't think
>DEC ever wrote an OS for the PDP-1, but they may have for the PDP-4.

The PDP-1 used FLEXO code, a six-bit character set in which up-shift and
down-shift were characters (like old 5-bit teletypes), and no, I didn't see
^X in any documentation or program for it.

"Control" characters existed in many character sets, including Flexo, but I
only recall seeing them described as ^X in connection with ASCII, which had
carefully arranged things so that all the character codes were used for
something.  Other character sets did not have the regularity in structure
that would make such a convention useful.

My recollection is that an earlier version of representing control characters
was to write the capital letter with a circumflex over it.  There was no
circumflex in ASCII at the time, so "<up-arrow><character>" was used as the
closest, convenient ASCII representation.  Up-arrow was later replaced by
circumflex (^), which helped, but it was both clearer and easier to write two
characters, ^X, than to bother doing the backspace and overstrike, so the old
convention stuck.  There were also other conventions used to indicate control
characters, such as enclosing the character in a circle, each of which used
typesetting tricks to distinguish control-whatever from whatever, but all of
these conventions lost favor because they couldn't be represented in ASCII
documentation.
 -WBE