INT 2fH 1686H: Detect INT 31h DPMI Services

                                                      Compatibility: Win 3.x 
 Expects: AX    1686H
 Returns: AX    0000h = INT 31H (DPMI) services are available
                else  = not available
    Info: TSRs and device drivers can use this to detect whether INT 31H
          (DPMI) protected-mode services are available.

          Protected mode services are used to allocate and manage memory
          while the CPU is in protected mode.

          See DOS Protected Mode Interface for related information.

    Note: The DPMI spec describes this fn differently.  It says INT 2fH
          AX=1686H is a way to determine the current CPU operating mode.
          If AX=0000H on return, then you can assume protected mode.
          Otherwise, the CPU is in either Real Mode or Virtual 86 Mode.

          I'm not sure who wins the INT 2fH API war.

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