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266 Fibre Channel Interface Manual, Rev. D
12.23 Read (12) command
The Read (12) command requests that the target transfer data to the initiator.
Table 173: Read (12) command (A8h)
Field definitions (listed alphabetically)
Control
See Control Bytes in Section 11.2.1.6.
DPO (Disable Page Out)
The DPO bit is used to control replacement of logical blocks in the cache memory when the host has information on the
future usage of the logical blocks.
1 The disc drive assigns the logical blocks accessed by this command the lowest priority for being fetched into or
retained by the cache. The host knows the logical blocks accessed by the command are not likely to be accessed
again in the near future and should not be put in the cache memory nor retained by the cache memory.
0 The host expects that logical blocks accessed by this command are likely to be accessed again in the near future.
FUA (Force Unit Access)
1 The target accesses the media in performing the command prior to returning Good status. Read commands access
the specified logical blocks from the media (i.e., the data is not directly retrieved from the cache). In the case where
the cache contains a more recent version of a logical block than the media, the logical block is first written to the
media.
0 The target satisfies the command by accessing the cache memory. For read operations, any logical blocks that are
contained in the cache memory are transferred to the initiator directly from the cache memory.
Logical Block Address
The logical block at which the read operation begins, if RelAdr bit is zero (RelAdr function is not supported by drives
described in this manual).
Bit
Byte
76543210
010101000
1000
DPO FUA
00
RelAdr
Reserved
2
Logical Block Address (MSB)
3
Logical Block Address
4
Logical Block Address
5
Logical Block Address (LSB)
6
Transfer Length (MSB)
7
Transfer Length
8
Transfer Length
9
Transfer Length (LSB)
10
Reserved
11
Control
A8h
Table
number
173
173
173
173