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SCROLOCK.DOC
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1986-09-14
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This routine is intended to make the scroll lock key
(uppermost, rightmost key) function as its name implies.
That is, stop the screen from scrolling.
BIOS normally performs this function in response to
control-numlock, for some mysterious reason, but the result
is neither elegant, convenient, nor esthetically pleasing.
This routine works by installing a resident module
that intercepts interrupts for video output.
Once this module is installed, any request for BIOS video
output comes here first. This module first passes the request
on to BIOS, then it checks the status of the scroll lock
toggle (which is maintained but ignored by BIOS). If scroll
lock is on, this routine will loop until it goes off.
Scroll locking action may be turned on and off by re-running
this module.
Two items of interest should be noted:
1) This routine halts output by stopping the machine
from doing anything at all except wait for scroll lock
to go away. Hence, hitting scroll lock causes the
machine to effectively grind to a halt (the same is
true, however, about control-numlock).
2) The machine does not, however, grind to a halt until
it tries to output something. So you can hit scroll
lock, and your machine will continue to run until
it tries to type something.
3) This module was originally driven by the keyboard
interrupt, and hence was guaranteed to pick up on
a keystroke, but it would occasionally stop the machine
when video output was disabled (thus reducing its
usefulness). This version is much less likely to do
that, but it will not stop output which is being
generated without using BIOS. Such output, however,
generally does not scroll.