C141-E039-01EN
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2.1.2
Setting the Operation Mode
(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 Fig. 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 item 3.1.5 for details
of the MODE SELECT command.
[Disconnect/Reconnect Parameter (Page code = 2)]
Bit
Byte
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
1
0
1
X‘0E’ (Page Length)
2
Buffer Full Ratio
Default Value: X‘20’ (=Half the character segment capacity)
Variable Range: X‘00’ to X‘FF’
3
Buffer Empty Ratio
Default Value: X‘20’ (=Half the character segment capacity)
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. The IDD's default cache
segment capacity is 127 Kbytes. Therefore, if the specified value (n) is 128, for example,
it means that a data volume of 63.5 Kbytes was specified (
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.
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