The ADCMD_ALLOCATE command tries the first combination , 3, to see if channels 0 and 1 are not being used. If they are available, the 3 is copied into the io_Unit field and you get an allocation key for these channels in the ioa_AllocKey field. You copy the key into other I/O blocks for any other commands you may want to perform on these channels. If channels 0 and 1 are being used, ADCMD_ALLOCATE tries the other combinations in turn. If all the combinations are in use, ADCMD_ALLOCATE checks the precedence number of the users of the channels and finds the combination that requires it to steal the channel or channels of the lowest precedence. If all the combinations require stealing a channel or channels of equal or higher precedence, the ADCMD_ALLOCATE request fails. Precedence is in the ln_Pri field of the io_Message in the IOAudio block you pass to ADCMD_ALLOCATE; it has a value from -128 to 127.
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