Read 12 Command (A8H) - Seagate Ultra160 Product Manual

SCSI Interface Product Manual, Rev. B
8.21

Read 12 command (A8h)

The Read 12 command requests that the target transfer data to the initiator.
Table 184: Read 12 command (A8h)
Bit 7
Byte
0 1
0
1
LOGICAL UNIT NUMBER [1]
2 LOGICAL BLOCK ADDRESS (MSB) [5]
3 LOGICAL BLOCK ADDRESS [5]
4 LOGICAL BLOCK ADDRESS [5]
5 LOGICAL BLOCK ADDRESS (LSB) [5]
6 TRANSFER LENGTH (MSB) [6]
7 TRANSFER LENGTH [6]
8 TRANSFER LENGTH [6]
9 TRANSFER LENGTH (LSB) [6]
10 Reserved
11 CONTROL [7]
[1] The LOGICAL UNIT NUMBER must be zero.
[2] A Disable Page Out (DPO) bit of one indicates that the drive shall assign the logical blocks accessed by
this command the lowest priority for being fetched into or retained by the cache.
The DPO bit is used to control replacement of logical blocks in the cache memory when the host has infor-
mation on the future usage of the logical blocks. If the DPO bit is set to one, 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. If the DPO bit is zero, the host expects that
logical blocks accessed by this command are likely to be accessed again in the near future.
[3] A Force Unit Access (FUA) bit of one indicates that the target shall access the media in performing the
command prior to returning GOOD status. Read commands shall 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 shall first be written to the media.
An FUA bit of zero indicates that the target may satisfy the command by accessing the cache memory. For
read operations, any logical blocks that are contained in the cache memory may be transferred to the initi-
ator directly from the cache memory.
[4] A Relative Address (RELADR) bit of one indicates that the LOGICAL BLOCK ADDRESS field is a two's
complement displacement. This negative or positive displacement is to be added to the LOGICAL BLOCK
ADDRESS last accessed on the logical unit to form the LOGICAL BLOCK ADDRESS for this command.
This feature is only available when linking commands. The feature requires that a previous command in
the linked group have accessed a block of data on the logical unit.
A RELADR bit of zero indicates that the LOGICAL BLOCK ADDRESS field specifies the first logical block
of the range of logical blocks to be operated on by this command.
[5] The LOGICAL BLOCK ADDRESS field specifies the logical block at which the read operation shall begin,
if RELADR bit is zero (see note [4]).
[6] The TRANSFER LENGTH specifies the number of contiguous logical blocks of data that shall be trans-
ferred. A TRANSFER LENGTH of zero indicates that no logical blocks shall be transferred. This condition
shall not be considered an error. Any other value indicates the number of logical blocks that shall be trans-
ferred.
[7] See Table 61, "CONTROL field," in Section 7.2.1.
6 5
0 1
0 0
DPO [2]
4 3
0 1
FUA [3]
2 1
0 0
0 0
RELADR [4]
253
0
0