Read Command (08H) - Seagate SCSI Interface Product Manual

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5.19

Read command (08h)

The Read command (Table 119) requests that the drive transfer data to the initiator.
Table 119: Read command (08h)
Bit 7
Byte
0 0
1 0
Logical Unit Number [1]
2 Logical Block Address
3 Logical Block Address (LSB)
4 Transfer Length [3]
5 0
Notes.
[1] The Logical Unit Number must be zero.
[2] The Logical Block Address specifies that logical block at which the read operation shall begin.
[3] The Transfer Length specifies the number of contiguous logical blocks of data to be transferred. A Transfer
Length of zero indicates that 256 logical blocks shall be transferred. Any other value indicates the number
of logical blocks that shall be transferred.
[4] See "Control byte," paragraph 4.2.6.
The data value most recently written in the addressed logical block shall be returned.
Read data transfers with the initiator do not begin until at least one full sector of data is available in the drive
data buffer. For multiple sector reads, the transfer of data continues until the number of blocks specified in byte
4 of the CDB has been read and transferred or until an unrecoverable error is detected.
Data transfer could stop if the option to stop on recovered error is selected.
For systems that support disconnection, the drive disconnects when a valid Read command is received. The
drive reconnects depending on the value of the Buffer Full Ratio Set in Page 2 of the Mode Select Data (see
Section 5.10). After data transfer has been initiated with an initiator, the drive does not disconnect unless an
internal error recovery procedure is required or the data transfer to an initiator is interrupted for more than 1
millisecond.
The initiator must accept all data presented to the initiator after sending this command until the drive sends
Completion Status during a Status phase.
Note. The drive may disconnect and reconnect while executing this command and the initiator may prema-
turely terminate this command by creating the Reset condition or by sending an Abort, Abort Tag,
Clear Queue, or Bus Device Reset message.
Sense Data is valid after this command is executed and Completion Status is sent. If the Address Valid bit in
the Sense Data is true (1), the Sense Data Logical Block Address (Information bytes) points to the last Logical
Block accessed by the drive. If the Address Valid bit in the Sense Data is false (0), the Sense Data Logical
Block Address bytes are not valid.
The drive contains a large buffer and implements an optional "prefetch" and segmented cache function
whereby the requested read data is read into the buffer, plus an additional amount, depending on the cache
control parameters. See paragraph "Prefetch and multisegmented cache control" in the individual drive's Prod-
uct Manual, Volume 1, for more information on this.
This command is terminated with a Reservation Conflict status and no data is read if any reservation access
conflict (see Section 5.35) exists.
6 5
0 0
0 0
0 0
SCSI Interface Product Manual, Rev. J
4 3
0 1
Logical Block Address (MSB) [2]
0 0
2 1
0 0
0 Flag [4] Link [4]
0
0