Explanation (Continued):
& Either a CSW is not stored or busy and status modifier are stored.
$ Unit status of either zeros or busy and status modifier is stored.
m Unit status of busy may be stored, or an I/O interruption may not
occur. Asynchronous status is any unit status that is not related to the
termination of an I/O operation at the subchannel. # Except when the I/O interruption is caused by a deferred condition
code 1 for START I/O FAST RELEASE.
+ The entries in this column apply only when the CLRIO function is
executed. When CLEAR I/O causes the TIO function to be executed, the
entries in the TIO column apply. y. When the control unit is the type that never supplies status to the TIO function, unit status consisting solely of status modifier is
stored, and no interruption conditions are cleared.
Indications of Busy in CSW (Part 2 of 2)
Channel End
Channel end is caused by the completion
of the portion 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 control unit no longer requires
channel facilities to perform the opera­
tion.
Each I/O operation initiated at the
device causes channel end to be
signaled, and there is only one channel end for an operation. Channel end is
not signaled when programming errors or
equipment malfunctions are detected
during initiation of the operation.
When command chaining takes place, only
the channel end of the last operation of
the chain is made available to the
program. Channel end is not made avail­
able to the program when a chain of
commands is prematurely concluded
because of an unusual situation indi­
cated with control-unit end or device
end, or during the initiation of a
chained or retried command.
The instant within an I/O operation when
channel end is signaled depends on the operation 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 veri­
fy the writing, channel end mayor may
not be delayed until verification is
performed, depending on the device. When magnetic tape is being read, chan­
nel end occurs when the next interblock
gap on tape reaches the read-write head. On devices equipped with buffers, chan­ nel 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 oper­
ations channel end may be delayed until
completion of the operation. Operations that do not cause any data to be trans­
ferred 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 indicated (1) when the
completion of an I/O operation occurs at
the device, (2) when the I/O device signals that a change from the not-ready
to the ready state has occurred,
(3) when the termination of an activity
has occurred which previously caused a
response of busy to the channel, and
(4) when the I/O device signals that an
asynchronous condition has been recog­
nized. Device end normally indicates
that the I/O device has become available
for use in another operation.
Each I/O operation initiated at the device causes device end, and there is
only one device end for an operation. Device end is not generated when any
programming or equipment malfunction is detected during initiation of the opera­
tion. 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 condi-
Chapter 13. Input/Output Operations 13-67

tion is detected during the initiation
of a chained or retried command, in which case the chain is concluded with­
out device end.
Device end associated with an I/O opera­
tion is generated either simultaneously
with channel end or later. For data­
transfer operations on some I/O devices,
the operation is complete at the time
channel end is generated, and both
device end and channel end occur togeth­
er. The time at which device end is
presented depends upon the I/O-device
type and the kind of command executed.
For most I/O devices, device end is
presented when the I/O operation is
completed at the I/O device. In some
cases, for reasons of performance,
device end is presented before the I/O operation has actually been completed at
the I/O device. However, in all cases,
when device end is presented, the I/O device is available for execution of an
immediately following CCW if command
chaining was specified in the previous CCW. During execution of control
commands, device end may be presented
with channel end or later. When command chaining is specified,
receipt of the device-end signal, in the
absence of any unusual situations, caus­ es the channel to initiate a new I/O operation.
When the state of a device is changed
from not ready to ready, either device
end or device end, attention, and unit
exception are indicated. Refer to the SL publication for the I/O device to
determine which indication is given.
A device is considered to be not-ready
when operator intervention is required
in order to make the device ready. A
not-ready condition can occur, for exam­
ple, 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
Device end is also accompanied by other
status where conditions are recognized
that are unrelated to the execution of
an I/O operation.
13-68 System/370 Principles of Operation
Unit check indicates that the I/O device
or control unit has detected an unusual
situation that is detailed by the infor­
mation available to a sense command.
Unit check may indicate that a program­
ming 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 indi­
cation when it occurs during the
execution of a command or the TIO func­
tion or during some activity associated
with an I/O operation. Unless the error
pertains to the activity initiated by a
command or 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 exist­ ence 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 the TIO function 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.
Unless the command is designed to cause
unit check, such as the rewind-and­
unload command for magnetic tape, unit
check is not indicated if the command is
properly executed even though the device
has become not ready during or as a
result of the operation. Similarly,
unit check is not indicated if the
command can be executed with the device
not ready. Selection of a device that
is not ready does not cause a unit check
when the sense command is issued or when
an interruption condition is pending for
the addressed device at the control
unit.
If the device detects during the initi­
ation sequence that the command cannot
be executed, unit check is signaled to
the channel without 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 situation preclud­
ing proper execution of the operation
occurs after execution has been started,
unit check is accompanied by channel
end, control-unit end, or device end,
depending on when the situation was