Additional Information; Scsi Protocol; Priority Of Scsi Status Byte Reporting - IBM Ultrastar 36LZX Specifications

11.0 Additional information

This chapter provides additional information or descriptions of various functions, features, or operating
models supported by the Target that are not fully described in previous chapters.

11.1 SCSI Protocol

There are various operating conditions that prevent the Target from executing a SCSI command. This
section describes each of these operating conditions and their relative priority.

11.1.1 Priority of SCSI Status Byte Reporting

After establishing the I_T_L nexus or I_T_L_Q nexus the Target must first determine whether command
execution is allowed. Execution is deferred until a later time if the command must be added to the com-
mand queue. Execution may also be prevented by an internal Target condition that requires the reporting
of a Check Condition, Queue Full, Busy or Reservation Conflict Status. There are several different
internal conditions to be active at the same time. The order in which the Target checks for each of these
conditions determines their priority (highest priority first) as follows:
1. Check Condition status for invalid Logical Unit Number. (see 11.1.2, "Invalid LUN in Identify
Message")
2. Check Condition status for Incorrect Initiator Connection (see 11.1.3, "Incorrect Initiator
Connection")
3. Busy Status or Queue Full Status (see 11.1.4, "Command Processing During Execution of Active
I/O process")
4. Check Condition status for Unit Attention condition (see 11.1.5, "Unit Attention Condition")
5. Check Condition status during Startup and Format operations (see 11.1.6, "Command Processing
During Startup and Format Operations")
6. Check Condition status for Deferred Error Condition (see 11.1.8, "Deferred Error")
7. Reservation Conflict status (see 11.1.10, "Command Processing While Reserved")
8. Check Condition status for invalid command opcode
9. Check Condition status for invalid command descriptor block
The highest priority internal condition that prevents command execution is reported by the Target
provided there is no bus error.
For all Check Conditions Sense data is built by the target provided a valid LUN address is known. Sense
data is cleared by the Target upon receipt of any subsequent command to the LUN from the initiator
receiving the Check Condition.
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