Read Extended Command (28H) - Seagate SCSI Interface Product Manual

SCSI Interface Product Manual, Rev. J
5.25

Read Extended command (28h)

The Read Extended command requests that the target transfer data to the initiator. This command is imple-
mented with the drive specific parameters shown in Table 131.
This command operates the same as the Read command (see Section 5.19) except that in the CDB for this
command a four byte Logical Block Address and a two byte Transfer Length may be specified.
The data value most recently written in the addressed logical block is returned to the Host.
This command shall be terminated with a Reservation Conflict status if any reservation access conflict (see
Section 5.35) exists, and no data shall be read.
Table 131: Read Extended command (28h)
Bit 7
Byte
0 0
1 0
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 0
7 Transfer Length (MSB) [6]
8 Transfer Length (LSB) [6]
9 0
Notes.
[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 avail-
able 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.
6 5
0 1
0 0
DPO [2]
0 0
0 0
4 3
0 1
FUA [3]
0 0
0 0
2 1
0 0
0 0
RelAdr [4]
0 0
0 Flag [7] Link [7]
173
0
0
0