4 Sep
2017
4 Sep
'17
7:44 p.m.
On 9/3/17, Steve Johnson <scj(a)yaccman.com> wrote:
I think the reason for this is pretty obvious. At the
time -- for
many of the paper terminals, line feed simply rotated the platen, and
the print head stayed at the same column position. Carriage return
returned the print head to the first column without advancing the
paper (remember overstriking?). But there were also some terminals
that both advanced the paper and returned the print head (I'm hazy
about this, but I think the IBM Selectric was one...).
IIRC, you're right about this. I'm pretty sure I remember the IBM
2741 and friends (the Selectric terminals) operating this way.
I do remember one guy who wrote his program to output LF/CR instead of
CR/LF. The teletype driver would do a LF and then space out to the
previous column and then do a carriage return. Pretty painful at 300
baud!
At 110 baud, the model 33 Teletype took up to two character times to
return the typeball to the left margin when executing a CR. You
needed to follow CR with two non-printing characters to avoid
erroneously striking a character in the middle of the line. With
CR/LF, the LF served as one of these non-printables. CR/LF/NUL was
the common sequence used to return the carriage and advance the
platen. If you did LF/CR you'd need LF/CR/NUL/NUL, thus wasting one
character time. So just about everyone implemented newline as CR/LF
rather than LF/CR.
Doug might remember better than I but I suspect some terminal did the
combined CR/LF only, so that's why Unix adopted it.
I suspect UNIX used LF as EOL for two reasons: [1] as just mentioned,
nearly everyone did CR/LF vs LF/CR, and [2] breaking lines on the LF
allowed for overstriking.
-Paul W.