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5.5.3
Saved pointers
Saved pointers point to initiator storage locations where command, status and data information are stored for a
task that was saved at some point in the past. There is one set of saved pointers for the current task for each
target on the interface bus that is currently active (whether or not it is currently connected to the initiator). The
saved command pointer always points to the first byte of the Command Descriptor Block (see Section 7.2) for
the "current" task for each target, the saved status pointer always points to the first byte of the area used for the
status associated with the command, and the saved data pointer points to some location (not necessarily the
beginning) in the area used for data associated with the command. When a SCSI target port disconnects the
initiator saves the current pointers. Before a SCSI target port disconnects it may send a SAVE DATA POINT-
ERS message to the initiator, which copies the data pointer that is for the current command for that target into
the location set aside for the target's saved pointers. When a SCSI target port reconnects, the initiator performs
a restore pointers operation that copies the saved pointers for the reconnected target into the initiator current
pointer registers so that the current command for that target may continue its operation from where it left off
before disconnecting. If the target had sent a Save Data Pointer message previously, the current data pointer
points to the place in the data store area where operations left off. The data pointer otherwise points to the
beginning of the data area, unless the data pointers were modified by a MODIFY DATA POINTERS message
from the target prior to disconnecting. The MODIFY DATA POINTERS message adds a value to the data
pointer that allows data to be taken, upon reconnection, from a location before or after the last byte transferred
location.
Since the data pointer value may be modified by the target before the task ends, it should not be used to test
for actual transfer length, because the value may no longer be valid for that purpose.
5.6
Command processing considerations and exception conditions
5.6.1
Command processing considerations and exception conditions overview
The following subclauses describe some aspects of command processing, including exception conditions and
error handling that are specific to this standard.
5.6.2
Asynchronous event notification
Notification of an asynchronous event is performed using the SEND command with the AER bit set to one. The
information identifying the condition being reported shall be returned during the data out delivery phase of the
SEND command (see SCSI Primary Commands-2 standard).
An error condition or unit attention condition shall be reported once per occurrence of the event causing it. The
target may choose to use an asynchronous event notification or to return CHECK CONDITION status on a sub-
sequent command, but not both. Notification of command-related error conditions shall be sent only to the initi-
ator that requested the task.
The asynchronous event notification protocol may be used to notify processor devices that a system resource
has become available. If a SCSI target port chooses to use this method, the sense key in the sense data sent
to the processor device shall be set to Unit Attention.
The asynchronous event notification protocol shall be used only with SCSI devices that return processor device
type with an AERC bit of one in response to an INQUIRY command. The INQUIRY command should be sent to
logical unit zero of each SCSI device responding to selection. This procedure shall be conducted prior to the
first asynchronous event notification and shall be repeated whenever the SCSI device deems it appropriate or
when an event occurs that may invalidate the current information. (See SYNCHRONOUS DATA TRANSFER
REQUEST message, Section 4.3.16, for examples of these events.)
Each SCSI device that returns processor device type with an AERC bit of one shall be issued a TEST UNIT
READY command to determine that the SCSI device is ready to receive an asynchronous event notification. A
SCSI device returning CHECK CONDITION status is issued a REQUEST SENSE command. This clears any
pending unit attention condition. A SCSI device that returns processor device type with an AERC bit of one and
returns Good status when issued a TEST UNIT READY command shall accept a SEND command with an
AER bit of one.
Note.
A SCSI device that uses asynchronous event notification at initialization time should provide means to
defeat these notifications. This may be done with a switch or jumper wire. SCSI devices that implement
saved parameters may alternatively save the asynchronous event notification permissions either on a
SCSI Interface Product Manual, Rev. B
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