Datasheet

Data Buffer Management
3-12 C141-C008
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 LIP (y, x), or when any INIT issues a TARGET RESET
(FCP TASK MANAGEMENT), 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.