IBM DDRS-39130 - Ultrastar 9.1 GB Hard Drive, Specifications

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10.0 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.
10.1 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.
10.1.1 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
command 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 dif-
ferent 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 10.1.2, “Invalid L U N in Identify
Message”)
2. Check Condition status for Incorrect Initiator Connection(see 10.1.3, “Incorrect Initiator Connection”
on page 180)
3. Busy Status or Queue Full Status (see 10.1.4, “Command Processing During Execution of Active I/O
process” on page 180)
4. Check Condition status for Unit Attention condition(see 10.1.5, “Unit Attention Condition” on
page 183)
5. Check Condition status during Start-up and Format operations(see 10.1.6, “Command Processing
During Start-up and Format Operations” on page 183)
6. Check Condition status for Deferred Error Condition(see 10.1.8, “Deferred error” on page 184)
7. Reservation Conflict status(see 10.1.11, “Command Processing While Reserved” on page 186)
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 L U N address is known. Sense
data is cleared by the Target upon receipt of any subsequent command to the L U N from the initiator
receiving the Check Condition.
10.1.2 Invalid LUN in Identify Message
There are three different circumstances defined within the SCSI protocol when the response to an invalid
L U N will occur. Each of these result in a different response.
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Copyright IBM Corp. 1997
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