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SCSI Commands
MODE SELECT
Command Descriptor Block
6-Byte Command Descriptor Block
Field Descriptions
The Mode Select command allows the host to assign device parameters to the drive.
After a power-on or SCSI reset condition, the drive sets its device parameters to the
default values. By issuing a Mode Select command, the host can change the device
parameters. The parameters are transferred to the drive as data formatted in a
parameter list.
Parameters assigned by the Mode Select command remain in effect until the drive
receives a subsequent Mode Select command or a reset. The Mode Select
parameters are not unique to the initiator that assigned the parameters.
In multiple-initiator systems, all initiators that access the drive use the assigned
parameters. However, when a Mode Select command changes parameters that
apply to other initiators, the drive generates a Unit Attention condition for all initiators
except the one that issued the Mode Select command. The Additional Sense Code
and Additional Sense Code Qualifier are set to Mode Parameters Changed.
The Mode Select command immediately checks for invalid parameters or invalid
combinations of parameters before executing. If an exception is found, the drive
returns a Check Condition, and the request sense data is set to Illegal Request.
All Mode Select parameters may be rounded up or down, as appropriate. A Mode
Sense command may be issued after a Mode Select command to determine which
parameters have been rounded.
Bits
Byte
7
6
0
1
Ignored LUN
2
3
4
5
Field
Bytes
Bits
Operation
1
Code
Ignored
1
5-7
Logical
Unit
PF
1
4
Parameter
4
List Length
Reserved
5
4
3
Operation Code (15h)
PF
Reserved
Reserved
Reserved
Parameter List Length
Control
Description
The Operation Code for Mode Select is 15h.
The LUN field is residual from the SCSI-1 days and is ignored.
Always 1, indicating SCSI-2 format.
Specifies the number of bytes in the Mode Select parameter list to
be transferred from the host to the drive. A value of 0 is a valid en-
try and indicates no data.
All reserved bits must be 0.
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