Control Unit End
Control unit end indicates that the control unit has
become available for use for another operation.
The control-unit-end condition is provided only by
control units shared by I/O devices and only when one
or both of the following conditions has occurred:
1. The program has caused the control unit to be
interrogated while the control unit was executing an
operation. The control unit is considered to have been
interrogated when START I/O, TEST I/O, or HALT I/O has been issued to a device on the control unit, and
the control unit had responded with busy and status
modifier in the unit status byte. START I/O and TEST I/O cause interrogation of the control unit when the
control unit is still executing a previously initiated op­
eration, but the sub channel is available or, for TEST I/O, the subchannel on the multiplexor channel con­
tains an interruption condition for the addressed de­
vice. The instruction HALT I/O can cause the control
unit to be interrogated when issued to a device shar­
ing a control unit and operating in the multiplex
mode.
2. The control unit detected an unusual condition
during the portion of the operation after channel end
had been signaled to the channel.
If the control unit remains busy with the execution
of an operation after signaling channel end but has not
been interrogated by the program, control unit end
is not generated. Similarly, control unit end is not pro­
vided when the control unit has been interrogated and
could perform the indicated function. The latter case
is indicated by the absence of busy and status modi­
fier in the response to the instruction causing the in­
terrogation.
When the busy state of the control unit is tem­
porary, control unit end is included with busy and
status modifier in response to the interrogation even
though the control unit has not yet been freed. The
busy condition is considered to be temporary if its
duration is short with respect to the program time
required to handle an I/O interruption. The 2702 Transmission Control is an example of a device in
which the control unit may be busy temporarily and
which includes control unit end with busy and status
modifier.
The device address associated with control unit end
depends on the type of I/O device. The address can be
fixed for the control unit, may identify the device on
which the terminated operation was executed, or may
be the device address specified in the instruction caus­
ing the control unit to be interrogated.
The control-unit-end condition can be signaled with
channel end, device end, or between the two. A pend- 110 ing control unit end causes the control unit to appear
busy for initiation of new operations.
Busy
Busy indicates that the I/O device or control unit can­
not execute the command or instruction because it is
executing a previously initiated operation or because
it contains an interruption condition. The interruption
condition for the addressed device, if any, accompa­
nies the busy indication. If the busy condition applies
to the control unit, busy is accompanied by status
modifier.
The following table lists the conditions when the
busy bit (B) appears in the csw and when it :is ac­
companied by the status-modifier bit (SM). A double
hyphen (--) indicates that the busy bit is off; an
asterisk indicates that csw status is not stored or
an I/O interruption cannot occur; and the (cl) indi­
cates that the interruption condition is cleared and
the status appears in the csw. The abbreviation DE
stands for device end, while cu stands for control unit. CONDITION Subchannel available
DE or attention in device
Device working, CU available CU end or channel end in CU: for the addressed device
for another device CU working
Interruption pend. in subchannel
for the addressed device
because of:
chaining terminated by
attention
other type of termination
Subchannel working CSW STATUS STORED BY:
START TEST HALT I/O I/O I/O I/O INT.
B,cl
B
B,cl
B,SM
B,SM
--,cl
B
--,cl
B,SM
B,SM
,cl
--,cl
--,cl --,cl
--,cl B,cl
--,cl CU available CU working B,SM The busy bit is included in the status associated
with a pending interruption condition from the sub­
channel only when a chain of commands has been
prematurely terminated because of attention and no
interruption was pending in the channel at the time
of chaining.
Channel End
Channel end is caused by the completion of the por­
tion of an I/O operation involving transfer of data or control information between the I/O device and the
channel. The condition indicates that the sub channel
has become available for use for another operation.
Each I/O operation causes a channel-end condition
to be gencrated, and there is only one channel end for
an operation. When command chaining takes place,
only the channel end of the last operation of the chain
is made available to the program. The channel-end

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 an I/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 HALT I/O, channel end causes the subchannel
to be in the interruption-pending state.
Device End
Device end is caused by the completion of an I/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 the I/O device has
become available for use for another operation.
Each I/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 an I/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 terminates the 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 new I/O operation.
Unit Check Unit check is caused by any programming or equip­
ment errors detected by the I/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 TEST I/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 Exception Unit 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 an I/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 Operations III