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INT 31 - DPMI 0.9 API - protected mode - SIMULATE REAL MODE INTERRUPT
        AX = 0300h
        BL = interrupt number
        BH = flags
            bit 0: reset the interrupt controller and A20 line
            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 below)
Return: CF set on error
        CF clear if successful
            real mode call structure modified (all fields except SS:SP, CS:IP
              filled with return values from real mode interrupt)
        protected mode stack unchanged
Notes:  16-bit programs use ES:DI as pointer, 32-bit programs use ES:EDI
        CS:IP in the real mode call structure is ignored for this call,
          instead, the indicated interrupt vector is used for the address
        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
        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

Format of real mode call structure:
Offset  Size    Description
 00h    DWORD   EDI
 04h    DWORD   ESI
 08h    DWORD   EBP
 0Ch    DWORD   reserved
 10h    DWORD   EBX
 14h    DWORD   EDX
 18h    DWORD   ECX
 1Ch    DWORD   EAX
 20h    WORD    flags
 22h    WORD    ES
 24h    WORD    DS
 26h    WORD    FS
 28h    WORD    GS
 2Ah    WORD    IP
 2Ch    WORD    CS
 2Eh    WORD    SP
 30h    WORD    SS