INT 31 P - DPMI 0.9+ - ALLOCATE REAL MODE CALLBACK ADDRESS
        AX = 0303h
        DS:(E)SI = selector:offset of procedure to call
        ES:(E)DI = 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
        the limited DPMI host built into Phar Lap's 286|DOS-Extender v2.5 does
          not support this function
SeeAlso: AX=0304h,AX=0C00h

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

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