INT 2fH 1685H: Switch VMs and CallBack

                                                      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
          information.

See Also: INT 2fH 16xxH: Enh386 Windows Services & Notifications
          INT 2fH: Multiplex Interrupt
          DOS Functions
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