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Command Specifications
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 Section 3.2, "Look-Ahead Cache Feature" and Section 3.3, "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.
MS (multiple selection)
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"
0: The "minimum prefetch count" and "maximum prefetch 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.
4-46
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.
parameters indicate the data block count to be prefetched with the logical
data block count.
C141-C009
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