Compatibility: DPMI 0.9+
Expects: AX    0901H
                                                                  
Returns: CF    NC (clear; always successful)
AL    previous setting: 00H = virtual interrupts were disabled
01H = virtual interrupts were enabled
                                                                  
Info: Enables the virtual interrupt flag and returns the previous state
of that flag.

AH is not modified by the call, so you can return to the previous
state immediately by calling INT 31H.

If you don't need to know the previous state, you may simply use
the STI opcode.

See Also: DPMI API
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INT 31H 0901H: Query and Disable Virtual Interrupt State