INT 31 P - DPMI 0.9+ - CALL REAL MODE PROCEDURE WITH FAR RETURN FRAME
        AX = 0301h
        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 real mode procedure must exit with a FAR return
        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
SeeAlso: AX=0302h,INT 21/AX=250Eh,INT 21/AH=E1h"OS/286"