Native Command Queuing (NCQ) is a command protocol in Serial ATA that can be implemented only on native Serial ATA hard drives. It allows multiple commands to be outstanding within a drive at the same time. Drives that support NCQ have an internal queue where outstanding commands can be dynamically rescheduled or re-ordered, along with the necessary tracking mechanisms for outstanding and completed portions of the workload. NCQ also has a mechanism that allows the host to issue additional commands to the drive while the drive is seeking data for another command.
NCQ allows the drive to set up the direct memory access (DMA) operation for a data transfer without host software intervention. This is also called first-party DMA - it means that the device is capable of complex sequences of operations without CPU intervention. The drive itself knows the current angular and rotational position of the drive head. The drive then selects the next data transfer to minimize both seek and rotational latencies.
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