NAME
       OpenDevice - open the audio device

   SYNOPSIS
       error = OpenDevice("audio.device", unitNumber, iORequest, flags);

   FUNCTION
       The OpenDevice routine grants access to the audio device.  It takes an
       I/O audio request block (iORequest) and if it can successfully open
       the audio device, it loads the device pointer (io_Device) and the
       allocation key (ioa_AllocKey); otherwise, it returns an error
       (IOERR_OPENFAIL).  OpenDevice increments the open count keeping the
       device from being expunged (Expunge).  If the length (ioa_Length) is
       non-zero, OpenDevice tries to allocate (ADCMD_ALLOCATE) audio channels
       from a array of channel combination options (ioa_Data). If the
       allocation succeeds, the allocated channel combination is loaded into
       the unit field (ioa_Unit); otherwise, OpenDevice returns an error
       (ADIOERR_ALLOCFAILED).  OpenDevice does not wait for allocation to
       succeed and closes (CloseDevice) the audio device if it fails.  To
       allocate channels, OpenDevice also requires a properly initialized
       reply port (mn_ReplyPort) with an allocated signal bit.

   INPUTS
       unitNumber- not used
       iORequest - pointer to audio request block (struct IOAudio)
               ln_Pri      - allocation precedence (-128 thru 127), only
                             necessary for allocation (non-zero length)
               mn_ReplyPort- pointer to message port for allocation, only
                             necessary for allocation (non-zero length)
               ioa_AllocKey- allocation key; zero to generate new key.
                             Otherwise, it must be set by (or copied from I/O
                             block that is set by) previous OpenDevice
                             function or ADCMD_ALLOCATE command (non-zero
                             length)
               ioa_Data    - pointer to channel combination options (byte
                             array, bits 0 thru 3 correspond to channels 0
                             thru 3), only necessary for allocation (non-zero
                             length)
               ioa_Length  - length of the channel combination option array
                             (0 thru 16), zero for no allocation
       flags     - not used

   OUTPUTS
       iORequest - pointer to audio request block (struct IOAudio)
               io_Device   - pointer to device node if OpenDevice succeeds,
                             otherwise -1
               io_Unit     - bit map of successfully allocated channels (bits
                             0 thru 3 correspond to channels 0 thru 3)
               io_Error    - error number:
                             0                   - no error
                             IOERR_OPENFAIL      - open failed
                             ADIOERR_ALLOCFAILED - allocation failed, no open
               ioa_AllocKey- allocation key, set to a unique number if passed
                             a zero and OpenDevice succeeds
       error     - copy of io_Error