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6-14 C141-E042-01EN
6.5 Read-Ahead Cache
After read command which involes read data from the disk medium is completed,
the read-ahead cache function reads the subsequent data blocks automatically and
stores the data to the data buffer.
When the next command requests to read the read-ahead data, the data can be
transferred from the data buffer without accessing the disk medium. The host can
thus access data at higher speed.
6.5.1 Data buffer configuration
The drive has a 128-KB data buffer. The buffer is used by divided into three
parts; for read commands, for write commands, and for MPU work (see Figure
6.9).
for read commands for write commands for MPU work
128 MB (131,072 bytes)
65,536 byte (128 sector)
49,152 byte (96 sector)
16,384 byte
Figure 6.9 Data buffer configuration
The read-ahead operation is performed at execution of the READ SECTOR(S),
READ MULTIPLE, or READ DMA command, and read-ahead data is stored in
the buffer for read commands. However, the lead sector specified in the read
command is continued to the last sector specified in the previous read command,
the read-ahead operation is not performed.
6.5.2 Caching operation
Caching operation is performed only at issurance of the following commands.
The device transfers data from the data buffer to the host system at issurance of
following command if following data exist in the data buffer.
•
All sectors to be processed by the command
•
A part of data including load sector to be processed by the command
When a part of data to be processed exist in the data buffer, remaining data are
read from the medium and are transferred to the host system.
(1) Commands that are object of caching operation
Follow commands are object of caching operation.