Read
The READ command requests
that
the
Target transfer
data
to
the
Initiator. The Target
accepts
both
the
non-extended (6-byte) and extended (lO-byte) CDB formats.
Read (S-Byte) Command Descriptor Block (CDB)
Bit
Byte
7
I
6
I
5
I
4
I
3
I
2
I
.1
I
00
Opcode
= 08H
01
Logical
Unit
Number
J
Logical Block
Address
(MSB)
02-03
(MSB=02)
Logical Block
Address
(LSB=03)
04
Transfer
Length
05
Control
Byte
Read (10-Byte) Command Descriptor Block (CDB)
Bit
Byte
7
I
6
I
5
1
4
J
3
1
2
I
1
I
00
Opcode
= 28H
Read
0
0
01
Logical
Unit
Number
I
DPO=O
I
FUA
I
Reserved
I
RelAdr
02-05
(MSB=02)
Logical Block
Address
(LSB=05)
06
Reserved
07-08
(MSB=07)
Transfer
Length
(LSB=08)
09
Control
Byte
Logical Block Address. This field specifies
the
logical block
at
which the read operation will
begin.
DPO (Disable Page Out). Not supported. Must be set to 0 (zero).
FUA (Forced Unit Access). A zero (0) instructs the target
to
satisfy
data
demand from cache;
a one (1) instructs the target
to
satisfy
data
demand from
the
media.
RelAdr (Relative Address). A one (1) 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 has accessed a block of
data
on the logical unit.
A RelAdr
bit
of zero (0) 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.
Transfer Length. This field specifies
the
number of contiguous logical blocks of
data
to
be
transferred. When using
the
non extended (6-byte) CDB format, a Transfer Length
of
zero
indicates
that
256
logical blocks will be transferred. When using the extended (lO-byte) CDB
format, a Transfer Length of zero indicates
that
no logical blocks will be transferred. This
condition is not considered an error (no SEEK occurs, and no
data
is
transferred).
The most recent
data
value written in the addressed logical block( s) will be returned.
DRAFT
SCSI
Command Descriptions A-31
1/18/93
09:58
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