WTW101@PSUVM.BITNET (Bill Warner) (06/29/89)
When writing a small TSR application, I discovered an strange situation (to
me, anyway): Whenever my program intercepted the Int 20h vector (the {now
obsolete} program exit service), the DOS 'CLS' command clears the screen and
border colors to black. If Int 20h is not intercepted, or if my program
restores the vector to its previous contents, the command works as
expected - simply clearing the screen, leaving the background and border
alone.
I have tried changing the vector both thru DOS and manually, with the same
results in both cases. (I have also noticed this effect with other TSRs
who intercept that vector - it doesn't seem to be just me.) I am running
IBM PC-DOS 3.30.
Does anyone have any idea why this is happening? Why does 'CLS' depend on
Int 20h? After all, it is an internal DOS command.
(I haven't had the time yet to trace thru COMMAND.COM for CLS, but I may
eventually have to if merely to satisfy my curiosity!)
Any ideas about this would be greatly appreciated!
Thanks in advance,
Bill