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12
Cheetah 73FC Product Manual, Rev. C
to be written are already stored in the cache from a previous read or write command. If there are, the respec-
tive cache segments are cleared. The new data is cached for subsequent Read commands.
If the number of write data logical blocks exceed the size of the segment being written into, when the end of the
segment is reached, the data is written into the beginning of the same cache segment, overwriting the data that
was written there at the beginning of the operation; however, the drive does not overwrite data that has not yet
been written to the medium.
If write caching is enabled (WCE=1), then the drive may return Good status on a write command after the data
has been transferred into the cache, but before the data has been written to the medium. If an error occurs
while writing the data to the medium, and Good status has already been returned, a deferred error will be gen-
erated.
The Synchronize Cache command may be used to force the drive to write all cached write data to the medium.
Upon completion of a Synchronize Cache command, all data received from previous write commands will have
been written to the medium.
Table 16 shows the mode default settings for the drive.
4.5.2
Prefetch operation
If the Prefetch feature is enabled, data in contiguous logical blocks on the disc immediately beyond that which
was requested by a Read command are retrieved and stored in the buffer for immediate transfer from the buffer
to the host on subsequent Read commands that request those logical blocks (this is true even if cache opera-
tion is disabled). Though the prefetch operation uses the buffer as a cache, finding the requested data in the
buffer is a prefetch hit, not a cache operation hit.
To enable Prefetch, use Mode Select page 08h, byte 12, bit 5 (Disable Read Ahead - DRA bit). DRA bit = 0
enables prefetch.
Since data that is prefetched replaces data already in some buffer segments, the host can limit the amount of
prefetch data to optimize system performance. The Max Prefetch field (bytes 8 and 9) limits the amount of
prefetch. The drive does not use the Prefetch Ceiling field (bytes 10 and 11).
During a prefetch operation, the drive crosses a cylinder boundary to fetch more data only if Mode parameters
page 08h, byte 2, bit 4 is set to 1 (Discontinuity--DISC bit).
When prefetch (read look-ahead) is enabled (enabled by DRA = 0), it operates under the control of ARLA
(Adaptive Read Look-Ahead). If the host uses software interleave, ARLA enables prefetch of contiguous blocks
from the disc when it senses that a prefetch hit will likely occur, even if two consecutive read operations were
not for physically contiguous blocks of data (e.g. "software interleave"). ARLA disables prefetch when it decides
that a prefetch hit will not likely occur. If the host is not using software interleave, and if two sequential read
operations are not for contiguous blocks of data, ARLA disables prefetch, but as long as sequential read oper-
ations request contiguous blocks of data, ARLA keeps prefetch enabled.
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