mov ax, 4003h ; Enter Critical Section int 2fh
Enter Critical Section notifies the virtual-display device that the VM application has entered a critical section and cannot respond to Save Video Register State (Interrupt 2Fh Function 4005h).
A VM application calls this function when it has started critical section processing.
This function has no parameters.
After the VM application enters the critical section, the virtual-display device postpones calling Save Video Register State for up to 1000ms or until the VM application calls Exit Critical Section (Interrupt 2Fh Function 4004h), whichever comes first.
If time elapses without the VM application calling Exit Critical Section, the virtual-display device reprograms the video hardware anyway and, when its operation is complete, calls Notify Foreground Switch (Interrupt 2Fh Function 4002h) in an attempt to reinitialize the application properly.
See also Interrupt 2Fh Function 4002h Notify Foreground Switch
See also Interrupt 2Fh Function 4004h Exit Critical Section
See also Interrupt 2Fh Function 4005h Save Video Register State