Int 31 Fn 0303  P - DPMI 0.9+ - Allocate Real Mode Callback Address        [E]

   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

See Also: 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