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Fibre Channel Interface Manual, Rev. D 211
12.13 Mode Sense (6) command
The Mode Sense command provides a means for the disc drive to report its medium, logical unit, or peripheral
device parameters to the initiator. It is a command complementary to the Mode Select command.
Field definitions (listed alphabetically)
Allocation Length
The number of bytes that the initiator has allocated for returned Mode Sense data. Any value other than zero indicates the
maximum number of bytes transferred.
0 No Mode Sense data is transferred. This condition is not considered an error.
The disc drive terminates the data when allocation length bytes have been transferred or when all available Mode Sense
data has been transferred to the initiator, whichever is less.
Control
See Control Bytes in Section 11.2.1.6.
DBD (Disable Block Descriptors)
0 The drive may return zero or more block descriptors in the returned Mode Sense data, at the drive’s discretion.
Seagate FC-AL products return one block descriptor if the DBD bit is zero.
1 The drive does not return any block descriptors in the returned Mode Sense data. The Block Descriptor Length field
of the Mode Sense header contains 00h to indicate a block descriptor length of zero.
Page Code
This field allows the initiator to select one or all of the pages of Mode parameters supported by the target. Page codes that
may be supported by the disc drive are summarized in Table 145 on page 215 (also see the individual drive’s Product Man-
ual, Volume 1).
PCF (Page Control Field)
The content of mode parameter bytes is determined by the value of this field. The disc drive returns the same Page Length
for each supported page regardless of the value of PCF. The block descriptor contains its normal values regardless of the
value of the PCF. Unsupported fields or bits within a page are returned as zeros for all PCF values. PCF is defined below.
Bit 7 Bit 6 Page control description
0 0 Return Current values. The Current values are the values currently being used by the disc drive to control its
operation. After a Power On Reset, a hard Reset, or a Bus Device Reset message the Current values are
equal to the Saved values (if Saved values can be retrieved) or the Default values (if Saved values cannot be
retrieved). The Current value of a parameter is updated by a Mode Select command if the Mode Select com-
mand ends with Good status returned.
0 1 Return Changeable values. The changeable values of any page is a mask that indicates the parameters that
are changed via a Mode Select command and the parameters that are not. Each returned parameter byte
contains ones where a field or bit may be changed and zeros where a field or bit may not be changed.
Table 142: Mode Sense command (1Ah)
Bit
Byte
76543210
000011010
10000
DBD
000
Reserved
2 PCF Page Code
300000000
4
Allocation Length
5
Control
1Ah
Table
number
142
142
142
142
142