3 Not operational
The condition code set when CLEAR CHAN­ NEL causes the CLRCH function to be
performed is shown for all possible
states of the I/O system in the figure "Condition Codes Set by CLEAR CHANNEL." The condition code set when CLEAR CHAN­ NEL causes the TCH function to be
performed is shown for all possible
states of the I/O system in the figure "Condition Codes Set by TEST CHANNEL" in
the definition of the instruction TEST CHANNEL. See the section "States of the Input/Output System" in this chapter for a detailed definition of the A, I, W, and N states.
N Channel A I W H +
3++
Available
Interruption Pending
Working Not Operational On certain channels, when the work­
ing state precludes performing the I/O-system reset on the I/O inter­
face, condition code 2 is set.
++ On certain channels, when the not­
operational state is due to a
channel-check-stop condition, the
instruction is executed, and condi­
tion code 0 is set. Condition Codes Set by CLEAR CHANNEL Programming Note CLEAR CHANNEL should be used to reset an I/O-device association with an I/O interface when I/O devices are shared
with other systems or have multiple
paths to the same system. In those
cases when I/O devices are shared,
before using CLEAR CHANNEL, steps should
be taken to protect against compromising
data integrity until the desired 1/0- device association can be reestablished. CLEAR CHANNEL may cause a channel that
is not operational because of a
channel-check-stop condition to be
restored. Before a not-operational
channel can be restored or system reset
signaled on an I/O interface, on some
models CLEAR CHANNEL must be issued to
all channels. On other models, CLEAR CHANNEL, when issued to a subset of the
channels, can cause a not-operational
channel to be restored and system reset
to be signaled on an I/O interface.
Refer to the SL publication for the
model to determine the appropriate
recovery action. CLEAR I/O [S] '9D01' o 16 20 31
The CLRIO function causes the current
operation with the addressed device to
be discontinued and the state of the
operation at the time of the discontin­
uation to be indicated in the stored CSW. Bits 8-14 of the instruction are
ignored. Bit positions 16-31 of the
second-operand address identify the
channel, subchannel, and I/O device to
which the instruction applies.
Either a TIO or CLRIO function is
performed, depending on the channel and
the block-multiplexing-control bit, bit
o of control register O. The TID func­
tion is performed when the CLRIO func­
tion is not implemented by the channel
or when the block-multiplexing-control
bi tis zero.
The TIO function is described in the
definition of the TEST I/O instruction.
When the subchannel is available,
interruption-pending with another
device, or working with another device,
no channel action is taken, and condi­
tion code 0 is set. Channels not capa­
ble of determining subchannel states
while in the working state may set
condition code 2.
When the subchannel is either working
with the addressed device or
interruption-pending with the addressed
device, the CLRIO function causes condi­
tion code 1 to be set and causes the
channel to discontinue the operation
with the addressed device by storing the
status of the operation in the CSW and
making the subchannel available. When
the channel is working with the
addressed device, the device is signaled
to terminate the current operation.
Some channels may, instead, indicate
busy and cause no channel action.
When any of the following conditions
occurs, the CLRIO function causes the CSW to be stored at real storage
locations 64-71. The contents of the
entire CSW pertain to the I/O device
addressed by the instruction.
1. The channel is available or
interruption-pending, and the
subchannel (1) contains an inter­
ruption condition for the addressed
device because of the ending of an I/O operation at the subchannel or
(2) is working with the addressed
device. The subchannel-key, Chapter 13. Input/Output Operations 13-17

command-address, and count fields
describe the state of the operation
at the time of the execution of the
instruction. If the subchannel is interruption-pending for reasons
other than the completion of an I/O operation at the subchannel, the
fields in the CSW other than the
unit-status field are all set to
zeros. If the operation has not
yet been initiated at the device,
the deferred condition code is 1.
2. The channel is working with the
addressed device. The subchannel­
key, command-address, and count
fields describe the state of the
operation at the time the instruc­
tion is executed. (Some channels
alternatively indicate busy under
this condition.)
3. The channel is working with a
device other than the one
addressed, and the subchannel
(1) contains a pending interruption
condition for the addressed device
because of the ending of an I/O operation at the subchannel or
(2) is working with the addressed
device.
In the former case, the
subchannel-key, command-address,
and count fields describe the state
of the operation at the time CLEAR I/O is executed. If the operation
has not yet been initiated at the
device, the deferred condition code
i s 1.
In the latter case, if the subchan­
nel is interruption-pending for
reasons other than the completion
of an I/O operation at the subchan­
nel, the fields in the CSW other
than the unit-status field are all
set to zeros. Some channels alternatively indi­
cate busy under the above condi­
tions (channel working).
13-18 System/370 Principles of Operation 4. The channel recognizes an equipment
error during the execution of the
instruction. The CSW identifies
the error condition. The states of
the channel and the I/O operations
in progress are unpredictable. The
limited channel logout, if stored,
indicates a sequence code of 000. When the CLRIO function cannot be
executed because of a pending logout
that affects the operational capability
of the channel, a full CSW is stored.
The fields in the CSW are all set to
zeros, with the exception of the
logout-pending and channel-control-check
bits, which are set to ones. No channel
logout is associated with this status. Program Exceptions:
Privileged operation
Resulting Condition Code:
o No operation in
subchannel for
device
1 CSW stored
2 Channel busy
3 Not operational
progress at the
the addressed
The condition code set when CLEAR I/O causes the CLRIO function to be
performed is shown for all possible
states of the I/O system in the figure
"Condition Codes Set by CLEAR I/O." The
condition code set when CLEAR I/O causes
the TIO function to be performed is
shown for all possible state of the I/O system in the figure "Condition Codes
Set by TEST I/O" in the definition of
the TEST I/O instruction. See the
section "States of the Input/Output System" in this chapter for a detailed
definition of the A, I, W, and N states.