INT 31 P - DPMI 0.9+ - ALLOCATE REAL MODE CALLBACK ADDRESS
        AX = 0303h
        DS:SI / DS:ESI = selector:offset of procedure to call
        ES:DI / ES:EDI = selector:offset of real mode call structure
                        (see AX=0300h)
Return: CF clear if successful
            CX:DX = segment:offset of real mode call address
        CF set on error
            AX = error code (DPMI 1.0+) (8015h) (see AX=0000h)
Notes:  the real mode call structure is static, causing reentrancy problems;
          its contents are only valid at the time of a callback
        the called procedure must modify the real mode CS:IP before returning
        values are returned to real mode by modifying the real mode call struc
        DPMI hosts must provide at least 16 callbacks per client
SeeAlso: AX=0C00h

Values callback procedure is called with:
        DS:SI / DS:ESI = selector:offset of real mode SS:SP
        ES:DI / ES:EDI = selector:offset of real mode call structure
        SS:SP / SS:ESP = locked protected mode API stack
        interrupts disabled
Return: (with IRET)
        ES:DI / ES:EDI = selector:offset of real mode call structure to restore