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and the type of device. For operations such as
writing on magnetic tape, channel end occurs when
the block has been written. On devices that verify
the writing, channel end mayor may not be
delayed until verification is performed, depending
on the device. When magnetic tape is being read,
channel end occurs when the gap on tape reaches
the read-write head. On devices equipped with
buffers, channel end occurs upon completion of
data transfer between the channel and the buffer.
During control operations, channel end is generated
when the control information has been transferred
to the devices, although for short operations
channel end may be delayed until completion of the
operation. Operations that do not cause any data
to be transferred can provide channel end during
the initiation sequence.
Channel end in the control unit may cause the
control unit to appear busy for the initiation of new
operations.
Channel end is presented in combination with
status modifier and unit check to initiate the
command-retry procedure.
Device End
Device end is caused by the completion of an I/O
operation at the device, by manually changing the
device from the not-ready to the ready state, or by
the termination of an activity which previously
caused a response of busy to the channel. Device
end normally indicates that the I/O device has
become available for use in another operation.
Each I/O operation causes device end, and there
is only one device end to an operation. Device end
is not generated when any programming or
equipment malfunction is detected during initiation
of the operation. When command chaining takes
place, only the device end of the last operation of
the chain is made available to the program unless
an unusual situation is detected during the
initiation of a chained command, in which case the
chain is concluded without device end.
Device end associated with an I/O operation is
generated either simultaneously with channel end
or later. For data-transfer operations on devices
such as magnetic-tape units, the device concludes
the operation at the time channel end is generated,
and both device end and channel end occur
together. On buffered devices, device end occurs
upon completion of the mechanical operation. For
control operations, device end is generated at the
completion of the operation at the device. The
operation may be completed at the time channel
end is generated or later.
When command chaining is specified, receipt of
the device-end signal, in the absence of any
unusual situations, causes the channel to initiate a
new I/O operation.
When the state of a device is changed from not
ready to ready, a device end is generated. Some
devices generate attention and unit exception along
with device end when they change from the
not-ready to ready state. A device is considered to
be not-ready when operator intervention is required
in order to make the device available. A not-ready
condition can occur, for example, because of any of
the following:
1. An unloaded condition for magnetic tape
2. Card equipment out of cards or with the
stacker full
3. A printer out of paper
4. Error conditions that need operator
intervention
5. The unit having changed from the enabled to
the disabled state
Unit Check
Unit check indicates that the I/O device or control
unit has detected an unusual situation that is
detailed by the information available to a sense
command. Unit check may indicate that a
programming or equipment error has been detected,
that the not-ready state of the device has affected
the execution of the command or instruction, or
that an exceptional situation other than the one
identified by unit exception has occurred. The
unit-check bit provides a summary indication of the
sense data.
An error causes the unit-check indication only
when it occurs during the execution of a command
or TEST I/O, or during some activity associated
with an I/O operation. Unless the error pertains to
the activity initiated by a command and is of
immediate significance to the program, the error
does not cause the program to be alerted after
device end has been cleared; a malfunction may,
however, cause the device to become not ready.
Unit check is indicated when the existence of the
not-ready state precludes a satisfactory execution
of the command, or when the command, by its
nature, tests the state of the device. When no
interruption condition is pending for the addressed
device at the control unit, the control unit signals
unit check when TEST I/O or the no-operation
control command is issued to a not-ready device.
In the case of no-operation, the command is
rejected, and channel end and device end do not
accompany unit check.
Chapter 12. Input/Output Operations
12-51
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