INT 31 P - DPMI 0.9+ - CALL REAL MODE PROCEDURE WITH IRET FRAME
        AX = 0302h
        BH = flags
            bit 0: reset the interrupt controller and A20 line (DPMI 0.9)
                   reserved, must be 0 (DPMI 1.0+)
            others must be 0
        CX = number of words to copy from protected mode to real mode stack
        ES:DI / ES:EDI = selector:offset of real mode call structure
                        (see INT 31/AX=0300h)
Return: CF clear if successful
            real mode call structure modified (all fields except SS:SP, CS:IP
              filled with return values from real mode interrupt)
        CF set on error
            AX = error code (DPMI 1.0+) (8012h,8013h,8014h,8021h)(see AX=0000h)
        protected mode stack unchanged
Notes:  16-bit programs use ES:DI as pointer, 32-bit programs use ES:EDI
        the flags in the call structure are pushed on the real mode stack to
          form an interrupt stack frame, and the trace and interrupt flags are
          clear on entry to the handler
        the real mode procedure must exit with an IRET
        DPMI will provide a small (30 words) real mode stack if SS:SP is zero
        the real mode handler must return with the stack in the same state as
          it was on being called