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4.4 Maintenance, Diagnostic Commands
C141-C013 273
(4) Mode = 0, 0, 1, 1: Buffer descriptor
When this mode is specified, the HDD transfers only the 4-byte buffer descriptor
to the INIT. the HDD's data buffer attributes are indicated in the 4-byte buffer
descriptor. Zero must be specified in the "Buffer offset" field in the CDB when
this mode is specified. The HDD 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.
Table 4.64 READ BUFFER command: buffer descriptor
Bit
Byte
7 6 5 4 3 2 1 0
0 X'02' Addressing Boundary
1 (MSB)
2 Buffer Capacity
3 (LSB)
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 HDDs report 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.
Exercise caution regarding the following points when using this
command.
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.
(5) Mode = 1, 0, 1, 0 : Echo buffer
In this mode the HDD transfers data to the INIT from the echo buffer. The echo
buffer shall transfer the same data as when the WRITE BUFFER command with
the mode field set to echo buffer was issued. The BUFFER ID and BUFFER
OFFSET fields are ignored in this mode.