CG> All screen output that goes through DOS or the BIOS uses INT 29h the
  > FAST PUTCHAR service and can be easily trapped by the appropriate ISR.

GB> If a program calls BIOS routines directly, Int 29h is not involved.
  > In addition, it's possible (but not very likely) that the user has
  > installed a CON device driver that doesn't support Int 29h, in
  > which case DOS won't call Int 29h at all.

I stand corrected. However the INT 29h method is the only method I know
of trapping screen output and it does allow you to run a DOS command
line in a window as well as programs that use the standard output device.
But, as I said, it's no use with programs that write directly to video
memory.

Maybe someone else has a better idea?

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 * 1st 2.00o #323 * I humbly beg your indulgent pardon.