Section 5
SCSI Programming Guidelines
5.11.16
READ CAPACITY COMMAND: OPCODE = 2S{hex)
The
READ
CAPACITY command (Tahle 5-31) provides a means for the initiator to request information
regarding the capacity
of
the disk drive.
TABLE
5-31
READ
CAPACITY Command
Bit
7
I
(»
5
I
4
I
3
I
2
I
1
0
Byte
0
OPCODE=25H
1
LUN=<>
I
RESERVED
=0
bR=O
2-5
(MSB)
LOGICAL BLOCK ADDRESS
(LSB)
6-7
RESERVED
=0
8
VU:::()
..
RESERVED=O
PMI
9
VU:::()
RESERVED
=0
I
F
L
PMI
• PARTIAL MEDIUM INDICATOR (Byte 9, Bit
0)
- Set to zero indicates that the
information returned in the READ CAPACITY
data will be the logical block address and block length
(in bytes)
of
the last logical block
of
the logical unit. The logical block address in the command
descriptor block will be set to zero for this option.
A
PMI
bit set to one indicates that the information returned 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 will be greater than
or
equal to the logical block
address specified in the command descriptor block.
The
eight bytes
of
READ
CAPACITY data shown in Table 5-32 will be sent during the DATA
IN
phase
of
the
command.
TABLE
5-32
READ
CAPACITY Data
Byte
DESCRIPTION
~3
(MSB)
LOGICAL BLOCK ADDRESS
(LSB)
4-7
(MSB)
BLOCK LENGTH
(LSB)
NOTE: .
Due
to
the physical organization
of
sectors and logical block assignments
on
the ProDrive, the
system designer can predict the initial
LBA
for all cylinders with a high level
of
confidence.
Formatting assigns consecutive logical block addresses to sectors
on
a cylinder, with only rare
exceptions. The designer can be relatively certain that records stored beginning at a cylinder
boundary will occupy contiguous physical sectors (skipping occasional defective sectors according
to
the defect map) and thus.will be optimally accessible. However, exceptions
do
occur due to
grown defects and sector ID errors. Therefore it is recommended that the designer rely
on
the
READ
CAPACITY command to check for optimal file assignment.
5·60
SCSI
Programming
Guidelines
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