INT 31 P - DPMI 0.9+ - SEGMENT TO DESCRIPTOR
        AX = 0002h
        BX = real mode segment
Return: CF clear if successful
            AX = selector corresponding to real mode segment (64K limit)
        CF set on error
            AX = error code (DPMI 1.0+) (8011h) (see AX=0000h)
Notes:  multiple calls for the same real mode segment return the same selector
        the returned descriptor can never be modified or freed
        not supported by MS Windows 3.0 in Standard mode