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When TEST I/O cannot be executed because of
a pending logout which affects the operational
capability of the channel or subchannel, a full CSW
is stored. The fields in the CSW are all set to
zeros, with the exception of the logout-pending bit
and the channel-control-check bit, which are set to
ones. No channel logout is associated with this
status.
When the TIO function is used to clear an
interruption condition from the sub channel and the
channel has not yet accepted the condition from
the device, the function causes the device to be
selected and the interruption condition in the
device to be cleared. During certain I/O
operations, some types of devices cannot provide
their current status in response to TEST I/O.
Some magnetic-tape control units, for example, are
in such a state when they have provided channel
end and are executing the backspace-file operation.
When TEST I/O is issued to a control unit in such
a state, the unit-status field of the CSW has the
busy and status-modifier bits set to ones, with zeros
in the other CSW fields. The interruption
condition in the device and in the sub channel is not
cleared.
On some types of devices, the device never
provides its current status in response to TEST
I/O, and an interruption condition can be cleared
only by permitting an I/O interruption. When
TEST I/O is issued to such a device, the unit-status
field has the status-modifier bit set to one, with
zeros in the other CSW fields. The interruption
condition in the device and in the subchannel, if
any, is not cleared.
12-26
IBM 4300 Processors Principles of Operation
However, at the time the channel assigns the
highest priority for interruptions to a condition
associated with an operation at the subchannel, the
channel accepts the status from the device and
clears the corresponding condition at the device.
When the TIO function is addressed to a device for
which the channel has already accepted the
interruption condition, the device is not selected,
and the condition in the subchannel is cleared
regardless of the type of device and its present
state. The CSW contains unit status and other
information associated with the interruption
condition.
On the byte-multiplexer channel, the TIO
function causes the addressed device to be selected
only after the channel has serviced all outstanding
requests for data transfer for previously initiated
operations.
Program Exceptions:
Privileged Operation
Resulting Condition Code:
o
Available
1
CSW stored
2
Channel or sub channel busy
3
Not operational
The condition code set by the TIO function for
all possible states of the I/O system is shown in the
figure" Condition Codes Set by TEST I/O." See
the section "States of the Input/Output System" in
this chapter for a detailed definition of the A, I, W,
and N states.
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