10 Nov
2015
10 Nov
'15
5:11 a.m.
"Jeremy C. Reed" <reed(a)reedmedia.net> writes:
By the way, I don't have experience with the CR3
control register, and
don't really understand what it means from the hardware perspective.
If I understand correctly, it's not hardware, it's part of the kernel
tty driver. There's a two bit field in the stty flags, for selecting a
delay mode. The purpose of the delay was to allow the terminal time to
process it (i.e. to physically move the carriage, for a printing
terminal) - the kernel would wait before sending more characters.
As can be seen in tty.c (link below), CR1 caused a delay of 5 units, and
CR2 a delay of 10 units. CR3 had no meaning in the standard kernel, and
so some custom version of the kernel could have interpreted it to have
some other meaning.
http://minnie.tuhs.org/cgi-bin/utree.pl?file=V7/usr/sys/dev/tty.c
It's not clear where this was actually done. The 3BSD kernel used
CR3 to indicate that (if I read it correctly) a number of padding
characters depending on the column position should be inserted.
P.S. Somewhat confusingly, searching for "CR3" finds a page about
"control registers" of the _intel 386_, of which CR3 has something to do
with "paging" in the virtual memory sense of the word. This has nothing
to do with the topic under discussion.