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4.3.2
DISCONNECT/RECONNECT EXAMPLE
In the above READ example, the drive may require a time-consuming
physical seek to reach the requested data.
In order to improve
system throughput,
the ACB-5500 disconnects from the Initiator,
freeing the SCSI bus to allow other Initiator requests to be sent
to other controllers or other devices on the same controller
during the time that would otherwise be unusable.
A typical disconnection is performed after the READ Command has
been transferred and before data is transferred.
The ACB-5500
indicates that the Initiator must store its active state informa-
tion by sending a Store Pointers message, then sends a Disconnect
message to indicate that the SCSI bus will be freed up, but that
the operation will continue later.
When the physical motions of the device are complete, the ACB-
5500 reselects the Initiator and passes an Identify message to
it.
The Identify message provides the necessary information for
the In i t i at
0
r
t o r e - act i vat e
th e s t
0
red s tat e
i n for mat ion.
Th e
read operation then continues as previously described.
4.3.3
LINKED COMMAND EXAMPLE
The Link function defines a relationship between commands which,
when combined with the RELATIVE ADDRESS BIT, allows previous
operations to modify subsequent commands.
Linked operation makes
high performance I/O .functions possible by providing a relative
addressing capability and allowing multiple command execution
without invoking the host software and without requiring a new
SELECTION phase.
As one example of a linked operation, the Initiator may want to
restrict any Read operations to a certain set of tracks.
This
may be done by -linking a SET LIMITS command to a READ command.
After normal execution of the SET LIMITS command, the ACB-5500
presents a LINKED COMMAND COMPLETE message instead of a COMMAND
COMPLETE message.
The LINKED COMMAND COMPLETE indicates to the
Initiator that it must now set up for the next command, a READ.
The ACB-5500 requests the Command Descriptor Block, interprets
the READ, and continues a normal READ command, but limited to the
set of logical blocks specified by the SET LIMITS command.
The linked command structure can similarly be used by the SEARCH
EQUAL command, followed by a disk READ of the data found.
4.3.4
COMMAND QUEUING EXAMPLE
In a multi-host environment,
(say,
file server)
the data are
shared among hosts.
When one host is using drive A,access from
the other hosts to the same drive will get busy status since the
drive can only do one task at one time.
The other hosts must
continue to poll the status of drive A until the busy goes away.
500504-00
4-17
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