Compatibility: Win 3.x
Expects: AX    1685H
BX    VM_ID (virtual machine ID)
CX    flags:  bit 0: 1=wait until interrupts are enabled
bit 1: 1=wait until critical section is released
DX:SI 32-bit priority boost setting (DX=high word, SI=low word)
ES:DI callback function address
Returns: AX    return code:  0000H (and CF is clear) in successful
0001H invalid VM_ID
0002H invalid priority boost
0003H invalid flags
Info: This causes Windows to switch to a specified virtual machine and
then call a specified address.

This is used by network support software or software which needs
to perform certain functions while executing in a specific
virtual machine.

Please see the Windows Device Driver Kit (DDK) for full

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INT 2fH 1685H: Switch VMs and CallBack