Fujitsu MHX2250BT, Product Manual

Page: 270 / 296

Operations
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.
(1) Commands that are targets of caching
The commands that are targets of caching are as follows:
• READ SECTOR (S) (EXT)
• READ MULTIPLE (EXT)
• READ DMA (EXT)
•
READ FP DMA QUEUED
However, if the caching function is prohibited by the SET FEATURES command,
the caching operation is not performed.
(2) Data that is a target of caching
The data that is a target of caching are as follows:
1) Read-ahead data that is read from disk media and saved to the data buffer
upon completion of execution of a command that is a target of caching.
2) Pre-read data that is read from disk media and saved to the data buffer before
execution of a command that is a target of caching.
3) Data required by a command that is a target of caching and has been sent to
the host system one. If the sector data requested by the host has not been
completely stored in the read cache portion of the buffer, this data does not
become a target of caching. Also, If sequential hits occur continuously, the
caching-target data required by the host becomes invalid because that data is
overwrited by new data.
6-14 C141-E254