Hitachi hard disk drive specifications
106
10.4
DMA Data In commands and DMA Data Out commands
These commands are:
Read DMA
Read DMA Ext
Read Stream DMA
Write DMA
Write DMA Ext
Write DMA FUA Ext
Write Stream DMA
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 4.2
“Native Command Queuing” of
“Serial ATA II: Extensions to Serial ATA 1.0a”.
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 shallbe 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 shall attempt to provide better quality of
service for the command. It is not required that devices process all high priority requests before satisfying
normal priority requests.
