Seagate Ultra160 Product Manual page 235

SCSI Interface Product Manual, Rev. B
cache according to size of segment, but rather by number of segments, this bit is used to enable or dis-
able ARLA (in non-Seagate equipment, this might be used to designate cache size).
Note. ARLA cannot be disabled in some Seagate drives using the ASAII code. See individual drive's
Product Manual, Volume 1.
[4] The ABORT PREFETCH (ABPF) bit, when set to one, with the DRA bit equal to zero, requests that the
SCSI device abort the prefetch upon selection. The ABPF set to one takes precedence over the Minimum
PREFETCH bytes. When set to zero, with the DRA bit equal to zero, the termination of any active prefetch
is dependent upon Caching Page bytes 4 through 11 and is operation and/or vendor-specific.
[5] The Caching Analysis Permitted (CAP) bit, when set to one, enables caching analysis. A zero indicates
caching analysis is disabled. Caching analysis results are placed in the SCSI logging information table,
(see Table 120). See individual drive's Product Manual, Volume 1, SCSI Bus Conditions and Miscella-
neous Features Supported table.
[6] The Discontinuity (DISC) bit, when set to one, requests that the SCSI device continue the prefetch across
time discontinuities, such as across cylinders or tracks up to the limits of the buffer, or segment, space
available for prefetch. When set to zero, the DISC requests that prefetches be truncated at time disconti-
nuities.
[7] The Size Enable (SIZE) bit, when set to one, indicates that the Cache Segment Size is to be used to con-
trol caching segmentation. When SIZE equals zero, the Initiator requests that the Number of Cache Seg-
ments is to be used to control caching segmentation. For Seagate drives covered by this manual, SIZE is
always zero.
[8] Write Cache Enable (WCE).
0: SCSI Write commands may not return status and completion message bytes until all data has been
written to the media.
1: SCSI Write commands may return status and completion message bytes as soon as all data has
been received from the host.
[9] Multiplication Factor (MF).
0: The Minimum PREFETCH and Maximum PREFETCH fields are interpreted as a number of logical
blocks.
1: Specifies that the target shall interpret the minimum and maximum prefetch fields to be specified in
terms of a scalar number which, when multiplied by the number of logical blocks to be transferred
for the current command, yields the number of logical blocks for each of the respective types of
prefetch.
[10] Read Cache Disable (RCD).
0: SCSI Read commands may access the cache or the media.
1: SCSI Read commands must access the media. Data cannot come from the cache.
[11] DEMAND READ RETENTIN PRIORITY. The cache replacement algorithm does not distinguish between
retention in the cache of host-requested data and prefetch data. Therefore, this half byte is always 0.
[12] WRITE RETENTION PRIORITY. The cache replacement algorithm does distinguish between retention in
the cache of host-requested data and prefetch data. Therefore, this half byte is always 0.
[13] DISABLE PREFETCH TRANSFER LENGTH. PREFETCH is disabled for any SCSI Read command
whose requested transfer length exceeds this value.
[14] MINIMUM PREFETCH specifies the minimum number sectors to prefetch, regardless of the delay it may
cause to other commands.
[15] MAXIMUM PREFETCH specifies the maximum number of logical blocks that may be prefetched. The
prefetch operation may be aborted before the MAXIMUM PREFETCH value is reached, but only if the
MINIMUM PREFETCH value has been satisfied.
[16] MAXIMUM PREFETCH Ceiling specifies an upper limit on the number of logical blocks computed as the
maximum prefetch. If the MAXIMUM PREFETCH value is greater than the MAXIMUM PREFETCH CEIL-
ING, the value is truncated to the MAXIMUM PREFETCH CEILING value.
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