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Western Digital Hard Disk Drive OEM Specification
Execution of these commands involves no data transfer.
10.4
DMA Data In commands and DMA Data Out commands
These commands are:
Download Microcode DMA
Read DMA
Read DMA Ext
Read Log DMA Ext
Read Stream DMA Ext
Trusted Receive DMA
Trusted Send DMA
Write DMA
Write DMA Ext
Write DMA FUA Ext
Write Log DMA Ext
Write Stream DMA Ext
Execution of this class of command includes the transfer of one or more blocks of data between the device and the
host using DMA transfer.
10.5
First-party DMA commands
These commands are:
Read FPDMA Queued
Write FPDMA Queued
Execution of this class of commands includes command queuing and the transfer of one or more blocks of data
between the device and the host. The protocol is described in the section 13.6 “Native Command Queuing” of “Serial
ATA Revision 3.2
”.
Host knowledge of I/O priority may be transmitted to the device as part of the command. There are two priority
classes for NCQ command as high priority, the host is requesting a better quality of service for that command than
the commands issued with normal priority.
The classes are forms of soft priority. The device may choose to complete a normal priority command before an
outstanding high priority command, although preference is given to the high priority commands. The priority class is
indicated in bit 7 (Priority Information) in the Sector Count register for NCQ commands (READ FPDMA QUEUED
and WRITE FPDMA QUEUED). This bit can indicate either the normal priority or high priority class. If a command is
marked by the host as high priority, the device attempts to provide better quality of service for the command. The
device may not process all high priority requests before satisfying normal priority requests.
