NAME
    CMD_READ -- normal I/O entry point

FUNCTION
    CMD_READ is a standard command for a single audio channel (io_Unit).
    If the allocation key (ioa_AllocKey) is correct, CMD_READ returns a
    pointer (io_Data) to the I/O block currently writing (CMD_WRITE) on
    the selected channel; otherwise, CMD_READ returns an error
    (ADIOERR_NOALLOCATION).  If there is no write in progress, CMD_READ
    returns zero.  CMD_READ is synchronous and only replies (mn_ReplyPort)
    if the quick bit (IOF_QUICK) is clear.

INPUTS
    mn_ReplyPort- pointer to message port that receives I/O request after
                  if the quick flag (IOF_QUICK) is clear
    io_Device   - pointer to device node, must be set by (or copied from
                  I/O block set by) opendevice function
    io_Unit     - bit map of channel to read (bit 0 thru 3 corresponds to
                  channel 0 thru 3), if more then one bit is set lowest
                  bit number channel read
    io_Command  - command number for CMD_READ
    io_Flags    - flags, must be cleared if not used:
                  IOF_QUICK - (CLEAR) reply I/O request
    ioa_AllocKey- allocation key, must be set by (or copied from I/O block
                  set by) opendevice function or adcmd_allocate command

OUTPUTS
    io_Unit     - bit map of channel successfully read (bit 0 thru 3
                  corresponds to channel 0 thru 3)
    io_Error    - error number:
                  0                    - no error
                  ADIOERR_NOALLOCATION - allocation key (ioa_AllocKey)
                                         does not match key for channel
    ioa_Data    - pointer to I/O block for current write, zero if none is
                  progress