Read - HP C2488A Technical Reference Manual

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