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The parameters defined in this page descriptor control the range of look-ahead data in the
Look-Ahead cache feature and enable or disable the caching operation. See 2.2, "Look-Ahead
Cache Feature" and 2.3, "Write Cache" concerning details of the Look-Ahead cache feature
and parameter setting methods.
a. RCD (read cache disable)
This bit can be specified, and its operation is as specified.
This bit specifies whether or not to activate the cache operation for a read command.
"1" : Prohibits operation of the Look-Ahead cache function.
The IDD reads ahead all of the data requested by the READ command or READ
EXTENDED command from the disk and transfers it to the INIT. Moreover, it does not
read ahead data blocks after the requested data.
"0" : Specifies operation of the Look-Ahead cache function.
If part or all of the data, including logical data blocks of headers, requested by a READ
command or READ EXTENDED command exists in the data buffer, the IDD transfers that
data without accessing the disk. Also, depending on the instructions for the bytes beyond
byte 4 of the parameter page, it reads ahead the data blocks after the requested data in the
data buffer.
b. Multiple selection (MS)
This bit specifies how to specify the "minimum prefetch count" (bytes 6 and 7) and
"maximum prefetch count" (bytes 8 and 9) parameters in this page descriptor.
1:
The "minimum prefetch count" and "maximum prefetch count" parameters indicate a
multiplier. The number of data blocks to be prefetch is calculated that the value in the
"transfer byte count" in the CDB specified by the READ or READ EXTENDED
command X multiplier.
0:
The "minimum prefetch count" and "maximum prefetch count" parameters indicate
the data block count to be prefetched with the logical data block count.
This bit cannot be changed, and the IDD ignores this bit specification and operates
according to the default value ('0').
c. WCE (write cache enable)
This bit can be specified, and its operation is as specified.
This bit specifies whether or not to activate the cache operation for a write command.
"1" : This enables the WRITE cache.
WRITE DATA remains in the buffer memory, the cache is made the object even for a read
command, and when all of the write data has been received, "GOOD" status is reported
without disconnecting.
C141-E039-01EN
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