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2.1 Data Buffer
C141-E167 2 - 5
2.1.2 Operation mode setting
(1) MODE SELECT parameter
In order to make it possible to control from the INIT the reconnection processing start timing for
executing data transfers with the SCSI bus in read and write operations, the IDD is provided with the
MODE SELECT parameter (Disconnect/Reconnect Parameter: Page code = 2) shown in Figure 2.4.
The user can set the optimum operating state in the system environment as necessary. The IDD holds
the MODE SELECT parameter individually for each INIT's SCSI ID, so the specified parameter values
can differ for each INIT. See Section 3.1.5 for details of the MODE SELECT command.
[Disconnect/Reconnect Parameter (Page code = 2)]
Bit
Byte
76543210
0 00000010
1 X‘0E’ (Page Length)
2 Buffer Full Ratio
Default Value: X‘00’
Variable Range: X‘00’ to X‘FF’
3 Buffer Empty Ratio
Default Value: X‘00’
Variable Range: X‘00’ to X‘FF’
4
(Other parameters)
15
Figure 2.4 Parameters for controlling reconnection timing
a. Buffer full ratio
This parameter specifies the timing for the IDD to start reconnection to perform data transfer to
the INIT in a READ or READ EXTENDED command.
The value specified in this parameter (n) shows the amount of data read to the IDD's data buffer
from the disk media as a proportion [n/256] of the total capacity of the data buffer until
reconnection processing with the SCSI bus is started. (Note)
If it is possible to transfer the amount of data specified in this parameter to the INIT from the
data buffer, reconnection processing is executed and transfer of data to the INIT begins.
However, if the total amount of transfer data specified in the command does not fill the capacity
of the cache segment, the IDD executes reconnection processing and begins transferring data to
the INIT when it becomes possible to transfer the amount of data blocks from the data buffer
which is equivalent to the proportion of the total volume of transfer data blocks specified in the
command that is specified by this parameter.
For example, if a read operation of 16 blocks with a logical data block length of 512 bytes (8
Kbytes) is requested, when the value specified in this parameter is 32, reconnection processing is
performed when it becomes possible to transfer 2 blocks of data [16 x (32/256)], from the data
buffer.
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