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    ADCMD_ALLOCATE -- allocate a set of audio channels

    ADCMD_ALLOCATE is a command that allocates multiple audio channels.
    ADCMD_ALLOCATE takes an array of possible channel combinations
    (ioa_Data) and an allocation precedence (ln_Pri) and tries to allocate
    one of the combinations of channels.

    If the channel combination array is zero length (ioa_Length), the
    allocation succeeds; otherwise, ADCMD_ALLOCATE checks each
    combination, one at a time, in the specified order, to find one
    combination that does not require ADCMD_ALLOCATE to steal allocated

    If it must steal allocated channels, it uses the channel combination
    that steals the lowest precedence channels.

    ADCMD_ALLOCATE cannot steal a channel of equal or greater precedence
    than the allocation precedence (ln_Pri).

    If it fails to allocate any channel combination and the no-wait flag
    (ADIOF_NOWAIT) is set ADCMD_ALLOCATE returns a zero in the unit field
    of the I/O request (io_Unit) and an error (IOERR_ALLOCFAILED).  If the
    no-wait flag is clear, it places the I/O request in a list that tries
    to allocate again whenever ADCMD_FREE frees channels or ADCMD_SETPREC
    lowers the channels' precedences.

    If the allocation is successful, ADCMD_ALLOCATE checks if any channels
    are locked (ADCMD_LOCK) and if so, replies (ReplyMsg) the lock I/O
    request with an error (ADIOERR_CHANNELSTOLEN). Then it places the
    allocation I/O request in a list waiting for the locked channels to be
    freed.  When all the allocated channels are un-locked, ADCMD_ALLOCATE:
      . resets (CMD_RESET) the allocated channels,
      . generates a new allocation key (ioa_AllocKey), if it is zero,
      . copies the allocation key into each of the allocated channels
      . copies the allocation precedence into each of the allocated
        channels, and
      . copies the channel bit map into the unit field of the I/O request.

    If channels are allocated with a non-zero allocation key,
    ADCMD_ALLOCATE allocates with that same key; otherwise, it generates a
    new and unique key.

    ADCMD_ALLOCATE is synchronous:
      . if the allocation succeeds and there are no locked channels to be
        stolen, or
      . if the allocation fails and the no-wait flag is set.

    In either case, ADCMD_ALLOCATE only replies (mn_ReplyPort) if the
    quick flag (IOF_QUICK) is clear; otherwise, the allocation is
    asynchronous, so it clears the quick flag and replies the I/O request
    after the allocation is finished.  If channels are stolen, all audio
    device commands return an error (IOERR_NOALLOCATION) when the former
    user tries to use them again.  Do not use ADCMD_ALLOCATE in interrupt

    If you decide to store directly to the audio hardware registers, you
    must either lock the channels you've allocated, or set the precedence
    to maximum (ADALLOC_MAXPREC) to prevent the channels from being

    Under all circumstances, unless channels are stolen, you must free
    (ADCMD_FREE) all allocated channels when you are finished using them.

    ln_Pri      - allocation precedence (-128 thru 127)
    mn_ReplyPort- pointer to message port that receives I/O request after
                  the allocation completes is asynchronous or quick flag
                  (ADIOF_QUICK) is set
    io_Device   - pointer to device node, must be set by (or copied from
                  I/O block set by) OpenDevice function
    io_Command  - command number for ADCMD_ALLOCATE
    io_Flags    - flags, must be cleared if not used:
                  IOF_QUICK   - (CLEAR) reply I/O request
                                (SET) only reply I/O request only if
                                      asynchronous (see above text)
                  ADIOF_NOWAIT- (CLEAR) if allocation fails, wait till is
                                (SET) if allocation fails, return error
    ioa_AllocKey- allocation key, zero to generate new key; otherwise,
                  it must be set by (or copied from I/O block set by)
                  OpenDevice function or previous ADCMD_ALLOCATE command
    ioa_Data    - pointer to channel combination options (byte array, bits
                  0 thru 3 correspond to channels 0 thru 3)
    ioa_Length  - length of the channel combination option array
                  (0 thru 16, 0 always succeeds)

    io_Unit     - bit map of successfully allocated channels (bits 0 thru
                  3 correspond to channels 0 thru 3)
    io_Flags    - IOF_QUICK flag cleared if asynchronous (see above text)
    io_Error    - error number:
                  0                   - no error
                  ADIOERR_ALLOCFAILED - allocation failed
    ioa_AllocKey- allocation key, set to a unique number if passed a zero
                  and command succeeds

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