condition, however, is not made available to the pro
gram when a chain of commands is prematurely ter
minated because of an unusual condition indicated
with control unit end or device end. The channel-end
condition is not generated when programming or
equipment errors are detected during initiation of the
operation.
The instant within anI/O operation when channel
end is generated depends on the operation and the
type of device. For operations such as writing on mag
netic tape, the channel-end condition 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, the channel-end
condition occurs when the gap on tape reaches the
read-write head.On devices equipped with buffers,
such as a line printer, the channel-end condition oc
curs 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 the condition may be delayed until com
pletion of the operation.Operations that do not cause
any data to be transferred can provide the channel
end condition during the initiation sequence.
A channel-end condition pending in the control unit
causes the control unit to appear busy for initiation of
new operations. Unless the operation has been per
formed on the selector channel and has been termi
nated by HALTI/O, channel end causes the subchannel
to be in the interruption-pending state.
Device End
Device end is caused by the completion of anI/O op
eration at the device or, on some devices, by manually
changing the device from the not-ready to the ready
state. The condition indicates that theI/O device has
become available for use for another operation.
EachI/O operation causes a device-end condition,
and there is only one device end to an operation.
When command chaining takes place, only the device
end of the last operation of the chain is made avail
able to the program. The device-end condition is not
generated when any programming or equipment er
rors are detected during initiation of the operation.
The device-end condition associated with anI/O operation is generated either simultaneously with the
channel-end condition or later.On data transfer op
erations on devices such as magnetic tape units, the
device terminatesthe operation at the time channel
end is generated, and both device end and channel
end occur together.On buffered devices, such as a
line printer, the device-end condition 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 in the current ccw is spe
cified, receipt of the device-end signal, in the absence
of any unusual conditions, causes the channel to ini
tiate a newI/O operation.
Unit CheckUnit check is caused by any programming or equip
ment errors detected by theI/O device or control unit.
The errors responsible for the unit check are detailed
by the information available to a sense command. The
unit-check bit provides a summary indication of the
errors identified by sense data.
The unit-check condition is generated only when
the error is detected during the execution of TESTI/O or a command. The device does not alert the program
of any equipment malfunction occurring at a time
when the device is not executing an operation and
does not have a pending interruption condition. Mal
functioning detected at this time may cause the device
to become not ready; unit check in this case is sig
naled to the program the next time the device is
selected.
If the device detects during the initiation sequence
that the command cannot be executed, unit check is
presented to the channel and appears in the csw with
out channel end, control unit end, or device end.Such unit status indicates that no action has been taken at
the device in response to the command. If the con
dition precluding proper execution of the operation
occurs after execution has been started, unit check is
accompanied by channel end, control unit end, or de
vice end, depending on when the condition was de
tected.
Termination of an operation with the unit-check
indication causes command chaining to be suppressed.
Unit ExceptionUnit exception is caused when the I/O device detects
a condition that usually does not occur. The condition
includes conditions such as recognition of a tape mark
and does not necessarily indicate an error. It has only
one meaning for any particular command and type of
device.
The unit-exception condition can be generated only
when the device is executing anI/O operation. If the
device detects during the initiation sequence that the
operation cannot be executed, unit exception is pre
sented to the channel and appears in the csw without
Input/Output OperationsIII
gram when a chain of commands is prematurely ter
minated because of an unusual condition indicated
with control unit end or device end. The channel-end
condition is not generated when programming or
equipment errors are detected during initiation of the
operation.
The instant within an
end is generated depends on the operation and the
type of device. For operations such as writing on mag
netic tape, the channel-end condition occurs when the
block has been written.
writing, channel end mayor may not be delayed until
verification is performed, depending on the device.
When magnetic tape is being read, the channel-end
condition occurs when the gap on tape reaches the
read-write head.
such as a line printer, the channel-end condition oc
curs 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 the condition may be delayed until com
pletion of the operation.
any data to be transferred can provide the channel
end condition during the initiation sequence.
A channel-end condition pending in the control unit
causes the control unit to appear busy for initiation of
new operations. Unless the operation has been per
formed on the selector channel and has been termi
nated by HALT
to be in the interruption-pending state.
Device End
Device end is caused by the completion of an
eration at the device or, on some devices, by manually
changing the device from the not-ready to the ready
state. The condition indicates that the
become available for use for another operation.
Each
and there is only one device end to an operation.
When command chaining takes place, only the device
end of the last operation of the chain is made avail
able to the program. The device-end condition is not
generated when any programming or equipment er
rors are detected during initiation of the operation.
The device-end condition associated with an
channel-end condition or later.
erations on devices such as magnetic tape units, the
device terminates
end is generated, and both device end and channel
end occur together.
line printer, the device-end condition 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 in the current ccw is spe
cified, receipt of the device-end signal, in the absence
of any unusual conditions, causes the channel to ini
tiate a new
Unit Check
ment errors detected by the
The errors responsible for the unit check are detailed
by the information available to a sense command. The
unit-check bit provides a summary indication of the
errors identified by sense data.
The unit-check condition is generated only when
the error is detected during the execution of TEST
of any equipment malfunction occurring at a time
when the device is not executing an operation and
does not have a pending interruption condition. Mal
functioning detected at this time may cause the device
to become not ready; unit check in this case is sig
naled to the program the next time the device is
selected.
If the device detects during the initiation sequence
that the command cannot be executed, unit check is
presented to the channel and appears in the csw with
out channel end, control unit end, or device end.
the device in response to the command. If the con
dition precluding proper execution of the operation
occurs after execution has been started, unit check is
accompanied by channel end, control unit end, or de
vice end, depending on when the condition was de
tected.
Termination of an operation with the unit-check
indication causes command chaining to be suppressed.
Unit Exception
a condition that usually does not occur. The condition
includes conditions such as recognition of a tape mark
and does not necessarily indicate an error. It has only
one meaning for any particular command and type of
device.
The unit-exception condition can be generated only
when the device is executing an
device detects during the initiation sequence that the
operation cannot be executed, unit exception is pre
sented to the channel and appears in the csw without
Input/Output Operations