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3.2 Look-Ahead Cache Feature
C141-C013 133
3.2.2 Caching parameters
The HDD supports the MODE SELECT parameters (caching parameters: page
code = 8) for controlling the cache feature. See Chapter 5 concerning details of
the MODE SELECT parameters.
3.2.3 Look-Ahead operation, Look-Ahead volume
1) Excluding the conditions in 3) and 4) for the amount of data specified as the
minimum pre-fetch volume, data are read for look-ahead irrespective of track
boundaries or cylinder boundaries. Commands which are already in the
queue or commands which are newly received while a look-ahead operation
is in progress, are executed after the look-ahead operation is completed.
However, if the new command is a READ or READ EXTENDED command,
and it is a command in which the first specified logical data block is a
sequential access data block, the command is executed without the look-
ahead operation being halted. When a newly received command is a
command which is not the object of caching, that command is executed
immediately in parallel with a look-ahead operation.
Also, in the case of a command which disables all data which are objects of
caching (see Section 3.2.1 (3)), the look-ahead operation is halted and that
command is executed immediately.
2) When commands exist in the queue, the look-ahead operation is halted at the
point when reading of the specified amount of data at the minimum pre-fetch
volume is completed, then execution of the new command starts. When there
are no commands that need to be newly executed, the data look-ahead
operation is continued, even after the volume of data specified as the
minimum amount of pre-fetch data have been read, until one of the following
conditions, 1), 3) or 4), occurs.
− When "1" is specified in the DISC bit, if look-ahead of data equal in
volume to a cache segment is completed.
− When "0" is specified in the DISC bit, if look-ahead of all the data is
completed up to the track boundary or the cylinder boundary.
3) During a data look-ahead operation, when any error occurs, the data look-
ahead operation is terminated at that point (retry is not executed).
4) When receiving HARD RESET primitive, or when receiving ABORT TASK
(TASK IU 0x01), ABORT TASK SET (TASK IU 0x02), or CLEAR TASK
SET (TASK IU 0x04), the look-ahead operation is terminated at that point
and all the look-ahead data stored in the data buffer are invalidated.
5) During a look-ahead operation, if "sector slip processing" is applied, or
defective sectors exist in the same cylinder to which "alternate processing" is
applicable, the look-ahead operation is continued without interruption.