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INT 31 - DPMI 0.9 API - protected mode - ALLOCATE REAL MODE CALL-BACK 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 set on error
        CF clear if successful
            CX:DX = segment:offset of real mode call address
Notes:  the real mode call structure is static, causing reentrancy problems
        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 should provide for at least 16 call-backs per task
SeeAlso: AX=0304h

Values call-back 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 omde API stack
        interrupts disabled
Return: (with IRET)
        ES:DI / ES:EDI = selector:offset of real mode call structure to restore