Read Capacity - HP C2488A Technical Reference Manual

Read Capacity
Read Capacity
The READ CAPACITY command provides a means for the Initiator to request information
regarding the capacity of the logical unit.
Read Capacity Command Descriptor Block (COB)
Bit
Byte
7
I
6
I
5
I
"
I
3
I
2
I
1
I
0
00
Opcode = 25H
01 Logical Unit Number
I
Reserved
I
RelAdr=O
02-05
(MSB=02)
Logical Block Address
(LSB=05)
06-07 Reserved
08 Reserved
I
PMI
09 Control Byte
PMI (Partial Media Indicator). A PMI bit of zero (0) indicates that the information returned
in the READ CAPACITY Data phase will be the logical block address and block length (in
bytes) of the last logical block of the logical unit. The Logical Block Address field in the CDB
must be set to zero for this option. If the PMI bit is zero (0) and the logical block address is
not zero, the Target will return a Status of CHECK CONDITION, a Sense Key of ILLEGAL
REQUEST, and an Additional Sense Code of ILLEGAL FIELD IN CDB.
A PMI bit of one (1) indicates that the information returned in the Data phase will be the
logical block address and block length (in bytes) of the last logical block address after which
a substantial delay in data transfer will be encountered. This logical block address shall be
greater than or equal to the logical block address specified in the CDB. (Implementor's Note:
This function is intended to assist storage management software in determining whether there
is sufficient space on the current track, cylinder, etc. to contain a frequently accessed data
structure such as a file directory or file index without incurring an access delay. The address
returned will normally be the last block on the addressed track.) .
Read Capacity Data Format
The format of the information returned by the Target during the Data In phase of the
command is as follows:
Bit
Byte 7
I
6
I
5
I
"
I
3
I
2
I'
1
I
00-03 (MSB=OO) Logical Block Address (LSB=03)
04-07 (MSB=04) Block Length (LSB=07)
0
DRAFT SCSI Command Descriptions A·33
1/18/93 09:58