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Product Manual - Disc Drive SCSI-2 Interface (Vol. 2; Ver. 2), Rev. E
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4.8
Parameter rounding
Certain parameters sent to a target with various commands contain a range of values. Targets may choose
to implement only selected values from this range. When the target receives a value that it does not support,
it either rejects the command (CHECK CONDITION status with ILLEGAL REQUEST sense key) or it rounds
the value received to a supported value. The target shall reject unsupported values unless rounding is per-
mitted in the description of the parameter.
Rounding of parameter values, when permitted (Rounding is enabled by Mode Select command, page code
00h, byte 2, bit 2) shall be performed as follows:
A target that receives a parameter value that is not an exact supported value shall adjust the value to one that
it supports and shall return CHECK CONDITION status with a sense key of RECOVERED ERROR. The
additional sense code shall be set to ROUNDED PARAMETER. The initiator is responsible to issue an
appropriate command to learn what value the target has selected.
IMPLEMENTORS NOTE: Generally, the target should adjust maximum-value fields down to the next lower
supported value than the one specified by the initiator. Minimum-value fields should be rounded up to the
next higher supported value than the one specified by the initiator. In some cases, the type of rounding (up or
down) is explicitly specified in the description of the parameter.
4.9
Command Processing Considerations and Exception conditions
The following sections describe some exception conditions and errors associated with command processing
and the sequencing of commands.
4.9.1
Programmable Operating definition
Some applications require that the operating definition of a logical unit be modified to meet the special re-
quirements of a particular initiator. The program-controlled modification of the operating definition is provided
to allow operating systems to change the operating definition of a more recently developed targets to one
which is more compatible with the operating system. This ability requires that the system comply with the low-
level hardware definitions of SCSI-2.
The parameters that can be changed by modifying the operating definition of a logical unit include the vendor
identification, the device type, the device model, the SCSI compliance level, the SCSI specification level, the
command set, and other parameters. The low-level hardware parameters including signal timing and parity
definitions cannot be changed by modifying the operating definition. The present operating definition of a
logical unit with respect to an initiator can be determined at any time by execution of an Inquiry command. In
some vendor-specific cases, it may also be necessary to perform other commands including Mode Sense
and Read Capacity.
Each logical unit begins at a particular operating definition. If the logical unit supports the Change Definition
command, the present operating definition can be changed to any other operating definition supported by the
logical unit. The actual details of the operating definition of a logical unit are vendor-specific. If the operating
definition is changed to one that does not include the Change Definition command, the target continues to
accept the Change Definition command.
If an error occurs during execution of a Change Definition command, the original operating definition remains
in effect after the command is executed. The new operating definition becomes active only after successful
execution of the Change Definition command.
Since new operating definitions may preclude the execution of I/O processes that are already in progress,
the target may disconnect to allow completion of any I/O processes that are in progress. Operating definition
changes that may cause conflicts with the normal operation from other initiators shall be indicated to those
initiators by generating a unit attention condition for each other initiator. The additional sense code shall be
set to CHANGED OPERATING DEFINITION.
An initiator may request a list of the operating definitions that the target supports and descriptive text for each
operating definition using the INQUIRY command.
