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6.5 Read-Ahead Cache
C141-E050-02EN 6-15
• READ SECTOR (S)
• READ MULTIPLE
• READ DMA
When caching operation is disabled by the SET FEATURES command, no
caching operation is performed.
(2) Data that are object of caching operation
Follow data are object of caching operation.
1) Read-ahead data read from the medium to the data buffer after completion of
the command that are object of caching operation.
2) Data transferred to the host system once by requesting with the command that
are object of caching operation (except for the cache invalid data by some
reasons).
3) Remaining data in the data buffer (for write command) transferred from the
host system by the command that writes data onto the disk medium, such as
the WRITE SECTOR (S), WRITE DMA, WRITE MULTIPLE.
Followings are definition of in case that the write data is treated as a cache data.
However, since the hit check at issurance of read command is performed to the
data buffer for read command prioritily, caching write data is limited to the case
that the hit check is missed at the data buffer for read command.
• When all data requested by the read command are stored in the data buffer for
write command (hit all), the device transfers data from the data buffer for
write command. At this time, the read-ahead operation to the data subsequent
to the requested data is not performed.
• Even if a part of data requested by the read command are stored in the data
buffer for write command (hit partially), all data are read from the disk
medium without transferring from the data buffer for write command.
(3) Invalidating caching data
Caching data in the data buffer is invalidated in the following case.
1) Following command is issued to the same data block as caching data.
− WRITE SECTOR(S)
− WRITE DMA
− WRITE MULTIPLE
2) Command other than following commands is issued (all caching data are
invalidated)
− READ SECTOR (S)
− READ DMA