Mikael Kjellstr”m wrote in a message to Mike Bilow:

 > Scrolling is handled inside the presentation device drivers.  There is a
 > good reason for this, since many video boards these days have the
 > capability of accelerating common operations such as scrolling in
 > hardware.  Unfortunately, a side effect of this is that device drivers
 > are never pre-empted by the scheduler without their explicit yield, so
 > scrolling operates by default at apparent high priority.

 MK> But why?  If I for an example run a DOS-program and then
 MK> enters "dir c:\ /s" the program is STOPPED and isn't
 MK> accepting any characters from the modem = character loss.  I
 MK> thing that it's a bad solotion.

Oh no, this is very different.  The modem is serviced by an interrupt
handler, so characters are not missed.  Interrupts continue to be serviced
in these conditions.  Also, the video driver is going to return after each
write call, usually after each line of text.  As a result, although the
system might be very jerky while scrolling, the process that is doing the
scrolling will be a candidate for preemption after each line of text.
 
-- Mike


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