30 Mar
2018
30 Mar
'18
8:09 p.m.
From: Sytse van Slooten
digging through the documentation of KL-11 and DL-11 I
did find
references to generating a break (bit 0 in the XCSR). But not on how it
would be received. ... How did a DL-11 like interface signal the
reception of a break?
As JohnW says, framing error. FWIW, the UART chips used back then actually
produce a 'framing error' output, which is sent straight into that bit in the
RCSR.
And how did the operating systems and software deal
with it? Was it
actually used at all?
Different systems used it for different things.
Unix V6 used 'break' on dial-up lines as the signal to switch speeds when you
first connected up - it would try 110, then 150, then 300. (Later this got
extended, I expect - too lazy to check.)
I see the hacked PWB1 Unix at MIT used it to send an interrupt:
if (c & FRERR) {
signal(tp->t_pgrp, SIGINT);
return;
}
That's as far as my knowledge extends, others may know of more uses.
Noel