Compatibility: Win 3.x
Expects: AX    4003H
Returns: none
Info: This signals to the VDD that the VM-aware application is entering
a critical section, during which time the video registers should
not be reprogrammed.  It cause the VDD to delay that action for
up to 1 second.

You should use INT 2fH 4004H (exit critical section) within
1000ms (1 second) of making this call.  It 1000ms elapse without
the VDD getting an INT 2fh 40004H call, it goes ahead and
reprograms the video registers anyway.

- -

INT 2fH 4003H: Enter Critical Section