turely by issuing CLEAR CHANNEL, CLEAR I/O, HALT I/O, or HALT DEVICE. After the addressed channel and subchan­
nel 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 test­ ing 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 and command
retry.
A data-transfer operation is initiated
at the subchannel and device only when
the CCW contains the S flag set to zero,
when no programming or equipment errors
are detected by the channel, and when
the device responds with zero status or
signals command retry during the initi­
ation sequence. When the channel
detects or the device signals any unusu­
al 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 I/O 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 I/O device is not started, no inter­
ruption conditions are generated, and
the subchannel is available subsequent
to the initiation sequence. The I/O device is immediately available for the
initiation of another operation,
provided the command was not rejected
because the device or control unit 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
or command retry, an interruption condi­
tion is generated, and the subchannel is
not available until the condition is
cleared. The reasons for the rejection
are indicated to the program by means of
the corresponding status bits in the CSW. The not-operational state of the I/O device, which during the execution
of START I/O and in some cases during
the execution of START I/O FAST RELEASE
causes condition code 3 to be set,
instead causes the interface-control­
check bit to be set to one when detected
during command chaining or command
retry. The new operation at the I/O device is not initiated.
When START I/O FAST RELEASE;s executed
by a channel independent of the
addressed device, tests for most
program-specified information, for
control-unit and device availability,
for control-unit and device status, and
for most errors may be performed subse­
quent to the execution of START I/O FAST
RELEASE. Some situations which would
have caused a condition code 1 or 3 to
be set had the instruction been START I/O instead cause an interruption condi­
tion to be generated. The CSW, when
stored, indicates that the interruption
condition is a deferred condition code 1
or 3.
When START I/O FAST RELEASE is executed
and start-I/O-fast queuing is provided
for the addressed subchannel, control­
unit or device busy indications, when
presented in the absence of other indi­
cations, may not result in the
generation of an interruption condition
indicating deferred condition code 1.
Instead the I/O operation may remain
pending at the subchannel with the
subchannel in the working state until
the corresponding no-longer-busy indi­
cation is presented to the channel by
the control unit or device. Subsequent­
ly, when the no-longer-busy indication
is presented to the channel, the channel
again attempts to initiate the pending I/O operation at the device. (See also
"START I/O FAST RELEASE" in this
chapter.)
When the resume function is performed by
the channel, tests for program-specified
information, for control-unit and device
availability, for control-unit and
device status, and for errors are
performed as for START I/O FAST RELEASE
executed independent of the addressed
device. Any unusual or error conditions
(except control unit or device busy)
detected while attempting to resume
channel-program execution at the device
causes an interruption condition to be
generated. The CSW, when stored, indi­
cates that the interruption condition is a deferred condition code 1 or 3. Control-unit or device-busy conditions
encountered when the resume function is
performed by the channel are handled as
for START I/O FAST RELEASE when start­ I/O-fast queuing is provided. That is,
control-unit or device-busy indications
may not result in the generation of an
interruption condition. Instead, the
channel program may remain pending at
the subchannel until the no-longer-busy
indication is presented by the control
unit or device.
Immediate Operations Any command except that for the TIO function may cause the I/O device to Chapter 13. Input/Output Operations 13-55

signal channel end immediately upon
receipt of the command code. An I/O operation causing channel end to be signaled during the initiation sequence is called an immediate operation. When the first CCW designated by the CAW during a START I/O or START I/O FAST
RELEASE executed as a START I/O initi­
ates an immediate operation with command
chaining not indicated and command retry
not occurring, no interruption condition
is generated. In this case, channel end
is brought to the attention of the
program by causing START I/O or START I/O FAST RELEASE to store the CSW status
portion. The subchannel is immediately
made available to the program. The I/O operation, however, ;s initiated, and,
if channel end is not accompanied by
device end, the device remains busy.
Device end, when subsequently provided
by the device, causes an interruption
condition to be generated.
An immediate operation initiated by the
first CCW designated by the CAW during a
START I/O FAST RELEASE executed inde­
pendent of the addressed device appears
to the program as a nonimmediate
command. That is, any status generated
by the device for the immediate command,
or for a subsequent command if command
chaining occurs, causes an interruption
condition to be generated.
When command chaining is specified after an immediate operation and no unusual
situations have been detected during the
execution, or when command retry occurs
for an immediate operation, neither
START I/O nor START I/O FAST RELEASE
causes the immediate storing of CSW status. The subsequent commands in the
chain are handled normally, and channel
end for the last operation of the chain
of CCWs generates an interruption condi­
tion even if the I/O device provides the
signal immediately upon receipt of the
command code.
Whenever immediate completion of an I/O operation is signaled, no data has been
transferred to or from the device as a
result of that operation.
Since a count of zero is not valid, any CCW specifying an immediate operation
must contain a nonzero count. When an
immediate operation is executed,
however, incorrect length is not indi­
cated to the program, and command
chaining is performed when so specified.
Programming Note Control operations for which the entire
operation is specified in the command
code may be executed as immediate oper­
ations. Whether the control function is
executed as an immediate operation
13-56 System/370 Principles of Operation
depends on the and type of
device and is specified in the SL publi­
cation for the device. Conclusion of Data Transfer
When the device accepts a command, the
subchannel is set up for data transfer.
The subchannel is in the working state
during this period. Unless the channel
detects equipment malfunctioning or the
operation is concluded by CLEAR CHANNEL, CLEAR I/O, or, on the selector channel,
the operation is concluded by CLEAR CHANNEL, CLEAR I/O, HALT I/O, or HALT DEVICE, the subchannel-working state
lasts until the channel receives the
channel-end signal from the I/O device.
When no command chaining or command
retry is specified or wBen chaining is
suppressed because of unusual
situations, channel end causes the oper­
ation at the subchannel to be terminated
and an interruption condition to be
generated. The status bits in the asso­
ciated CSW indicate channel end and any
unusual situations. The I/O device can
signal channel end at any time after
initiation of the operation, and the
signal may occur before any data has
been transferred.
For operations not involving data trans­
fer, the I/O device normally controls
the timing of channel end. The duration
of data-transfer operations may be vari­
able and may be controlled by the I/O device or the channel.
Excluding I/O-system reset, equipment
errors, CLEAR CHANNEL, CLEAR I/O, HALT DEVICE, and HALT I/O, the channel
signals the device to conclude data
transfer whenever any of the following
events occurs:
1. The storage areas specified for the
operation are exhausted or filled.
2. A program check is detected.
3. A protection check is detected.
4. A chaining check The first event occurs when the channel
has stepped the count to zero in the
last CCW associated with the operation.
A count of zero indicates that the chan­
nel has transferred all information
specified by the program. The other
three events are due to errors and cause
premature conclusion of data transfer.
In every case, the conclusion is
signaled in response to a service
request from the device and causes data
transfer to cease. If the device has no
blocks defined for the operation (such
as writing to magnetic tape), it
concludes the operation and generates
channel end. •