[TUHS] Command-line options

John Cowan cowan at mercury.ccil.org
Sat Mar 26 12:10:18 AEST 2016

Greg 'groggy' Lehey scripsit:

> > I'm going to throw in an aside at this point. PDP-7 Unix packed 2
> > characters per 18-bit word.
> Really?  Most machines of that era used 6 bit characters, so you could
> fit 3 in an 18 bit word.  I had hypothesized that that was the reason
> for word lengths in increments of 6 bits in those days.

So they did, but DEC machines in general were always ASCII.  The PDP-8
used both 8-bit characters (ASCII with the high bit set) and 6-bit
ones (stripped ASCII without control characters or lower-case ones).
The former were packed three 8-bit characters in two 12-bit words; the
latter, two characters in one 12-bit word.  The PDP-10 packed five 7-bit
characters into a 36-bit word with one bit left over, which was used
(if at all) for various purposes.  Not until the PDP-11 did the 8-bit
character get aligned with the machine word size.

> In passing, it's interesting to note that the initial @ or # were used
> to identify a command.  Other systems of the time did similar things.
> My guess is that the command prompt in Unix started life as a "here's
> your command leadin for free".

All DEC systems had command prompts of the Unix type, though not changeable:
the original prompt was simply ".".  In the DOS/BATCH-11 operating system,
however, the shell-level command prompt was $ and the intra-command prompt
was *, and you wrote batch files with explicit $ and * sigils, so that
a $-marked line implicitly indicated the end of input for the previous
command, a compact equivalent of here-scripts.

John Cowan          http://www.ccil.org/~cowan        cowan at ccil.org
To say that Bilbo's breath was taken away is no description at all.  There are
no words left to express his staggerment, since Men changed the language that
they learned of elves in the days when all the world was wonderful. --The Hobbit

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