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(4)
Mode = 0, 1, 1: Buffer descriptor
When this mode is specified, the IDD transfers only the 4-byte buffer descriptor to the INIT.
the IDD’s data buffer attributes are indicated in the 4-byte buffer descriptor. Zero must be
specified in the “Buffer address” field in the CDB when this mode is specified. The IDD
transfers the data length specified in the “Transfer byte length” field in the CDB or 4 bytes,
whichever portion of data is smaller, to the INIT. When zero is specified in the “Transfer byte
length” field, this command is terminated without executing a data transfer.
Bit
Byte
7
6
5
4
3
2
1
0
0
X’02’
Addressing Boundary
1
X’3C’
Buffer Capacity (MSB)
2
X’C0’
Buffer Capacity
3
X’00’
Buffer Capacity (LSB)
Figure 4.32 READ BUFFER command: buffer descriptor
The “Addressing boundary” field in the buffer descriptor indicates the addressing boundary in
the data buffer which can be specified in the WRITE BUFFER Command and the READ
BUFFER Command as a “Power” when expressed as a “Power of 2.” The IDD reports X ‘02’
(=2
2
), indicating that it is possible to specify the address in 4-byte units. Also, the “Buffer
capacity” field indicates the byte length of the size of the data buffer which can be operated by
the WRITE BUFFER and READ BUFFER commands.
IMPORTANT
If the WRITE BUFFER command or READ BUFFER command is
used under a multi-initiator or multitask environment, it is
necessary to be careful of the contents of the data buffer being
changed by another initiator or a command that issues another
task during the interval between completion of WRITE BUFFER
command execution and execution of the READ BUFFER
command. In order to avoid this problem, it is necessary in the
case of a multi-initiator environment, to reserve the IDD by the
RESERVE UNIT command.
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