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11.13 Write Cache function
Write cache is a performance enhancement whereby the device reports completion of the write command
(Write Sectors and Write Multiple) to the host as soon as the device has received all of the data in its
buffer. The device assumes responsibility to write the data subsequently onto the disk.
! Even though writing data after completed acknowledgment of a write command, soft reset, or hard
reset does not affect its operation, power off terminates writing operation immediately and unwritten
data is lost.
! Flush cache, Soft reset, Standby, Standby Immediate, and Sleep are executed after the completion of
writing to disk media on enabling write cache function. The host system can confirm the completion of
write cache operation by issuing flush cache command, Soft reset, Standby command, Standby
Immediate command, or Sleep command, and confirming the completion of the issued command.
11.14 Delayed Write function (vendor specific)
Delayed Write function is a power saving enhancement whereby the device delays the actual data writing
into the media. When the device is in the power saving mode and the Write command (Write Sectors,
Write Multiple, or Write DMA) comes from the host, the transferred data is not written into the media
immediately, it is only stored into the cache buffer. When the cache buffer becomes full or reaches the
predefined size or if any command except the Write command is issued, the operation to write the data
from the cache buffer into the media is begun.
Power consumption can be reduced by Delayed Write. When Write commands come with a long interval,
the device must exit from the power saving mode and enter into the power saving mode again without
Delayed Write function. If Delayed Write is enabled, such power saving mode transition times can be re-
duced. As a result, the additional energy for power saving mode transition can be saved and the average
power consumption of the device can be reduced.
However, the time elapsed from the completion of the Write command to the media write completion will
be extended with Delayed Write function. If the power for the device is turned off during this time, the data
which has not been written to the media is lost. Therefore, a command listed in the Write Cache Function
section is issued before the power off to confirm whole cached data has been written into the media.
For the sake of safety, Delayed Write function is disabled at Power On Default. The Enable Delayed Write
command is issued to the device to enable Delayed Write function at every Power On Reset or Hard
Reset. The actions of each reset are shown in Figure 57 on page 74.
11.14.1 Enable/Disable Delayed Write command
Command code FAh with Feature register 07h enables Delayed Write function.
Command code FAh with Feature register 87h disables Delayed Write function.
Ensure that, even if Delayed Write function is enabled by this command, Delayed Write does not work
when Write Cache function is disabled.
Hitachi Travelstar 60GH & 40GN hard disk drive specifications
98
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