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C156-E142-02EN 4 - 15
4.4.5 Cache function
The ODD supports the read cache and MO write cache.
The read cache consists of the read ahead cache and the LRU cache that reads write data remaining
in the data buffer.
The read ahead cache allows for data transfer at a near effective transfer speed during continuous
read operation without causing any delay owing to rotation latency. LRU allows for data transfer
with no mechanical operation if write and read operations are executed frequently on the same
sector.
The MO write cache writes data in batch mode during continuous write operation, thus reducing
positioning operations depending on the buffer size and recovering the write throughput.
4.4.5.1 Data buffer
The ODD uses part of the buffer area as work memory for control firmware and the remaining area
as a buffer. The data buffer consists of plural segments and contains write data of plural segments.
4.4.5.2 Read cache
The ODD enables the read ahead cache and LRU cache when the cache function is enabled. When
the read cache is enabled, the ODD contiguously prereads the specified sector, then prereading the
next sector when receiving a READ (10) or READ (12) command (called Read command in this
document). While prereading the next sector, the ODD transfers the data in the sector specified by
the command.
When data in the sector specified by the host hits preread data, the ODD transfers data directly
from the buffer without any mechanical operation.
Preread stops under the following conditions:
! Access to the alternate sector
! Read error and retry
! Logical zone boundary of media
! When a read command is received for a sector not continuous from the sector specified by the
previous read command
! When a command other than Read is received
! When the data buffer is full
! Media ejection instruction using the Eject switch
! Hardware reset, software reset, DEVICE RESET command is received, or power-off