INT 31H 0301H: Simulate Real Mode FAR CALL

                                                    Compatibility: DPMI 0.9+ 
 Expects: AX    0301H
          BH    flags: must be 00H
          CX    number of words to copy to the real-mode stack
          ES:DI (or ES:EDI) address of a DpmiRegRec structure.
 Returns: CF    NC (clear) if successful; else AX = DPMI Error Code
          ES:DI (or ES:EDI) structure contains returned data
    Info: Simulates a FAR CALL to a real-mode procedure.  The called
          procedure must return via RETF.

          The DPMI host switches to real mode, loads the registers,
          performs a FAR CALL to the CS:IP in the register structure,
          switches back to protected mode, copies the returned registers
          into the caller's structure and returns the status.

    ES:DI (or ES:EDI, when in 32-bit mode) points to a DpmiRegRec structure
          that sets up the registers on entry and will contain the modified
          register values upon return.

          The address of the far procedure must be in the wRegCS and wRegIP
          fields of the DpmiRegRec structure.

       CX is used when you are expected to push data onto the stack before
          calling int procedure.  When CX is non-zero, the host copies that
          many 2-byte words from the protected mode onto the real-mode
          stack before invoking the interrupt.

See Also: DPMI API
          INT 2fH: Multiplex Interrupt
          DOS Functions