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Copyright © 2000 Toshiba corporation. All rights reserved.
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10.14 Cache Memory
10.14.1 Cache Operations
(1) READ CACHE OPERATION
Receiving a read command, the data in the buffer memory are sent to the host without access to the disk
media as long as the object data reside in the buffer memory and the conditions for the drive’s read cache
operation are fulfilled.
If any of the conditions of the read cache operation is not fulfilled, the drive carries out read data operation
and the object data for the read command is read from the media and kept in the buffer and then the data is
transferred from the buffer to the host.
The following data required by the read command may continuously be read by the buffer under the drive’s
read ahead cache operation until the buffer available for read cache is full or the new command is received.
(2) WRITE CACHE OPERATION
Receiving a write command, the drive continuously receives the write data from the host until all data are
transferred or the buffer available for write cache is full, whether the data are written on the media or not. If all
data for the command are received, the drive reports completion of the command by negating BSY bit and
issuing INTERRUPT.
If the command which follows the write cache command is also a write command for succeeding block
address, the drive receives write data from host without waiting for the previously received data to be written
on the media. And the drive reports completion of the command when the buffer receives all the data.
If the command which follows the write cache command is not a write command, or the write command
which writes to non-succeeding block address, the drive waits for all the previously received write cache
command to be written on the media before executing the next command.
During a write cache operation, DASP (LED) signal line is kept “on” until all the data in the write buffer are
written on the media.
10.14.2 Notes for write cache
(1) Loss of data in write buffer
If write cache is enabled, hard reset or soft reset does not cause data loss . But power off immediate after
completion of the command may cause data loss, because actual writing of the data onto the media is not
completed at this moment. Therefore, it is recommended that any other command except write or read
command is executed and completion of the command is confirmed before powering off the drive. Stand-by
command can be helpful for this purpose.
(2) Error report
When write cache is enabled, any unrecoverable error encountered after the report of completion of a
command shall only be reported by the next command. Actual writing of the data onto the media may not be
completed at this moment. In this case, READY bit is negated to show that the error has occurred during
the write cache operation previously executed.
Address validity check is performed with actual media access . The error may be reported during the
execution of a command or after completion of a write cache command if the address the data has tried to
access is non-existent.
10.15 Automatic Write Reallocation
If the drive has difficulty in executing normal write operation due to unrecoverable errors such as ID NOT
FOUND, the sectors those show some errors may be reallocated automatically to continue normal operation
and secure the write data. This operation is helpful especially in write cache, when the completion of the
command is reported before actual writing to media. During write operation including this AWRE function,
DASP signal is kept on . This operation takes 20 seconds maximum to be completed, therefore, the
time-out period should be set longer than this value. If the next command is a write command, the data of
the first block will be transferred without any delay.