Fujitsu MHS2020AT, Product Manual

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Operations
6-12 C141-E171-03EN
6.4 Read-ahead Cache
Read-ahead Cache is the function for automatically reading data blocks upon
completion of the read command in order to read data from disk media and save
data block on a data buffer.
If a subsequent command requests reading of the read-ahead data, data on the data
buffer can be transferred without accessing the disk media. As the result, faster
data access becomes possible for the host.
6.4.1 Data buffer structure
This device contains a data buffer of 2 MB. This buffer is divided into two areas:
one area is used for MPU work, and the other is used as a read cache for another
command. (See Figure 6.7.)
For MPU work For R/W command
2048 KB (2097152 bytes)
0.5 KB
(512 bytes)
1984 KB
(2031616 bytes)
63.5 KB
(65024 bytes)
For MPU work
Figure 6.7 Data buffer structure
The read-ahead operation is performed while the READ SECTOR (S) or READ
MULTIPLE or READ DMA command is in progress. Read-ahead data is stored
in the read cache part of the buffer.
6.4.2 Caching operation
The caching operation is performed only when the commands listed below are
received. If any of the following data are stored on the data buffer, the data is
sent to the host system.
•
All of the sector data that this command processes.
•
A part of the sector data including the start sector, that this command
processes.
If part of the data to be processed is stored on the data buffer, the remaining data
is read from disk media and sent to the host system.