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3.2 Data Access Commands
C141-E167
3 - 65
3.2 Data Access Commands
3.2.1 READ (08)
Bit
Byte
76543210
0 X‘08’
1 LUN Logical Block Address (MSB)
2 Logical Block Address
3 Logical Block Address (LSB)
4 Transfer Block Count
50000000Link
This command reads the number of blocks of data in continuous logical data blocks specified in the
"Transfer block count" field with the logical data block on the disk media specified in the "Logical
block address" field in the CDB as the top.
The transfer block count can be specified up to a maximum of 256 logical data blocks. When zero is
specified in the "Transfer block count" field in byte 4 of the CDB, it specifies transfer of 256 logical
data blocks and when a value other than zero is specified, it specifies the number of logical data
blocks that should be transferred.
When transfer of multiple data blocks is instructed by this command, when the data blocks which are
to be processed come to a track boundary, cylinder boundary or zone boundary, a head switch or
cylinder switch is executed automatically and reading of the specified number of blocks is
performed.
When the specifications in the "Logical block address" field and "Transfer block count" field in the
CDB exceed the maximum logical block address in the IDD, that command is terminated with a
CHECK CONDITION status (ILLEGAL REQUEST [=5] / Logical block address out of range [=21-
00]) and the reading of data from the disk media is not executed.
Error recovery processing during execution of this command can be specified by the MODE
SELECT parameter. If retry processing and data correction processing are not prohibited, when this
command is completed normally, or when it is completed with a "RECOVERED ERROR [=1]"
sense key report, the data transferred to the INIT by this command are error free. When a
correctable error check is detected, the IDD first corrects the data errors in the data buffer, then
transfers those data to the INIT.
When disconnect processing is permitted, the starting timing for reconnection processing in order to
execute data transfer on the SCSI bus can be specified by the MODE SELECT parameter (buffer full
ratio). Also, if the Look-Ahead cache feature is permitted, a caching operation is performed using
the IDD's data buffer by this command. See Chapter 2, "Data Buffer Management," concerning
details of data buffer operation and the Look-Ahead cache feature.