Compatibility: Win 3.x
Expects: AX    1683H
Returns: BX    virtual machine identifier
Info: DOS programs and device drivers can use this service to learn
which virtual machine (e.g., DOS session) is running.  This is
handy if your TSR maintains separate data or execution contexts
for each VM or DOS session.

Windows typically runs in VM 0001H (but that's not guaranteed in
the future).  Also, Windows may need to recycle VM IDs, so you
are supposed to monitor for VM starts and terminations (though it
is not obvious how to do so).

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INT 2fH 1683H: Get Current Virtual Machine ID