Some SCSI host adapters use DMA to access large blocks of data in memory.
Since the CPU does not have to deal with the individual DMA requests, data
transfers are faster than CPU-mediated transfers and allow the CPU to do
other useful work during a block transfer (assuming interrupts are
enabled).
The host adapter will use a specific DMA channel. This DMA channel will be
determined by the 66 function and requested from the kernel
with the 67 function. This function takes the DMA
channel number as its only parameter and returns zero if the DMA channel
was successfully allocated. Non-zero results may be interpreted as
follows:
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