executed before
the
Ordered-Queue command. All Simple-Queue commands received after
an Ordered-Queue command are executed after
the
Ordered-Queue command.
• Clear Queue Message. Clears from
the
queue all
I/O
processes from all initiators
in
the
queue for
the
specified logical unit (LUN). All active
I/O
processes for
that
LUN are
terminated. All pending
status
and
data
for
that
LUN are cleared.
No
status
or message
is sent for any of
the
I/O
processes. A Unit Attention condition is generated for all other
initiators with
I/O
processes
that
either were active or were queued for
that
target.
The
device goes
to
the
Bus Free phase following successful receipt
.of
this message.
The
additional sense code for
the
Unit Attention is: Commands Cleared by Another Initiator.
• Abort Tag Message.
Instructs
the
target
to
abort
a tagged command.
The
target
clears
the
current
I/O
process.
If
the
target
has already
started
execution
of
the
I/O
process,
the
execution is halted. Any pending
status
or
data
for
the
I/O
process is cleared
and
no
status
or ending message is sent
to
the
initiator. Pending
status,
data,
and commands for
other
active or queued
I/O
processes are not affected.
Command queuing is activated by
the
Target when
the
Initiator
precedes a command with
a one-byte queue
tag
message
that
specifies
the
type
of command queuing, followed by a
one-byte queue
tag
value
that
uniquely defines
the
I/O
process.
The
queue
tag
message values
are:
20H = Head of Queue Tag
21H
= Simple Queue Tag
22H
= Ordered Queue Tag
A queue
tag
value can be any hex value from
OOh
to
FFh.
An
Initiator
can assign 256 queue
tags
to
a LUN. A queue
tag
value becomes available for reassignment after
the
I/O
process
it
is assigned
to
is completed.
If
more
than
one command with
the
same queue
tag
value is sent
to
a LUN, all commands are aborted for
that
initiator.
The
Clear Queue and Abort Tag operations are activated by one-byte messages with no
follow-on values. These message formats are:
OEH
= Clear Queue
ODH
=
Abort
Tag
If
an Initiator sends a tagged command
to
a Target when
the
command queue is full,
the
Target will send a Queue Full
status
to
the
Initiator. The
Initiator
should wait a period of
time,
and
then
send
the
command again.
Untagged commands are also accepted by
the
drives with
the
following restrictions:
• An untagged command sent by an
Initiator
will not be accepted
if
there are any
other
commands (tagged or untagged) in
the
queue
that
were sent by
the
same
Initiator.
• A tagged command sent by an
Initiator
will not
be
accepted
if
there is
an
untagged
command already in
the
queue
that
was sent by
the
same Initiator.
The drives will clear
the
queue in either case.
Note
"
DRAFT
1/18/93 09:58
Error
recovery is an exception
to
the
above restrictions because
the
Initiator
is
required
to
send
an
untagged REQUEST SENSE command if
an
error occurs
while a Target is executing a command.
Product Features 3·5
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