Advertisement
causes a condition code 0, rather than a condition
code 1, to be set. The subsequent termination of
the I/O operation causes an interruption condition
to be generated. During command retry, the CCW
may be refetched.
Programming Note
The following possible results of a command retry
must be anticipated by the program:
1. A CCW with the PCI flag set to one may, if
retried because of command retry, cause
multiple PCI interruptions to occur.
2. A channel program consisting of a single,
unchained CCW specifying an immediate
command may cause a condition code 0 rather
than a condition code 1 to be set. This setting
of the condition code occurs if the control unit
signals command retry at the time initial status
is signaled to the channel. An interruption
condition is generated upon completion of the
operation.
3. If a CCW used in an operation is changed
before that operation has been successfully
completed, the results are unpredictable.
4. A CSW stored after the initiation of a retry but
before the presentation of device end, as when
an interruption condition due to the PCI flag is
taken, contains the address of the command to
be retried plus 8.
5. If a HALT I/O, HALT DEVICE, or CLEAR
1/
a
instruction is issued after the initiation of a
retry but before the presentation of device end,
the CSW contains the address of the command
to be retried plus 8.
6. On a multiplexer channel, chained CCWs which
might ordinarily have been executed in a burst
may, upon the occurrence of command retry,
cause multiplexing to occur, with the result that
the channel becomes unexpectedly available.
7. Command chaining may occur even though the
CCW does not indicate command chaining.
This can occur if immediate retry is not
requested and the control unit or device
presents a status of device end and status
modifier.
Conclusion of Input/Output Operations
When the operation or sequence of operations
initiated by START 1/
a
or START I/O FAST
RELEASE is ended, the channel and the device
generate status. Status can be brought to the
attention of the program by means of an I/O
interruption, by TEST I/O or CLEAR I/O, or, in
12-40
IBM 4300 Processors Principles of Operation
certain cases, by START I/O or START I/O FAST
RELEASE. This status, as well as an address and
a count indicating the extent of the operation
sequence, are presented to the program in the form
of a channel-status word (CSW).
Types
0/
Conclusion
Normally an I/O operation at the subchannellasts
until the device signals channel end. Channel end
can be signaled during the sequence initiating the
operation, or later. When the channel detects
equipment malfunctioning or an I/O system reset is
performed, the channel disconnects the device
without receiving channel end. The program can
force a device to be disconnected prematurely by
issuing CLEAR I/O, HALT I/O, or HALT
DEVICE.
Conclusion at Operation Initiation
After the addressed channel and subchannel have
been verified to be in a state where START I/O or
START I/O FAST RELEASE can be executed,
certain tests are performed on the validity of the
information specified by the program and on the
availability of the addressed control unit and I/O
device. This testing occurs during the execution of
START I/O, either during or subsequent to the
execution of START I/O FAST RELEASE, and
during command chaining.
A data-transfer operation is initiated at the
subchannel and device only when no programming
or equipment errors are detected by the channel
and when the device responds with zero status
during the initiation sequence. When the channel
detects or the device signals any unusual situations
during the initiation of an operation, the command
is said to be rejected.
Rejection of the command during the execution
of START I/O or START I/O FAST RELEASE is
indicated by the setting of the condition code in the
PSW. Unless the device is not operational, the
reasons for the rejection are detailed by the portion
of the CSW stored by START I/O or START I/O
FAST RELEASE. The device is not started, no
interruption conditions are generated, and the
subchannel is available subsequent to the initiation
sequence. The device is immediately available for
the initiation of another operation, provided the
command was not rejected because the device was
busy or not operational.
When an unusual situation causes a command to
be rejected during initiation of an I/O operation by
command chaining, an interruption condition is
Advertisement
Chapters
