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error condition. The state of the channel and
the progress of the I/O operation are
unpredictable.
When HALT 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 occurs in this case.
When HALT I/O causes data transfer to be
terminated, the control unit associated with the
operation remains unavailable until the data-
handling portion of the operation in the control
unit is terminated. Termination of the data-
transfer portion of the operation is signaled by the
generation of channel end, which may occur at the
normal time for the operation, earlier, or later,
depending on the operation and type of device.
If
the control unit is shared, all devices attached to
the control unit appear in the working state until
the channel-end signal is accepted by the CPU.
The I/O device executing the terminated operation
remains in the working state until the end of the
inherent cycle of the operation, at which time
device end is generated.
If
blocks of data at the
device are defined, such as reading on magnetic
tape, the recording medium is advanced to the
beginning of the next block.
When HALT I/O is issued at a time when the
subchannel is available and no burst operation is in
progress, the effect of the HALT I/O signal
depends on the type of device and its state and is
specified in the SL publication for the device. The
HAL T I/O signal has no effect on devices that are
not in the working state or are executing a
Channel
Subchannel
A
Control Unit
A
- Device
1
,~
Available
Interruption pending
Working
Not operational
CSW stored
A
N
A
0
3
N
A
3
1*
0
N
3
mechanical operation in which data is not
transferred, such as rewinding tape or positioning a
disk-access mechanism.
If
the device is executing a
type of operation that is variable in duration, the
device interprets the signal as one to terminate the
operation. Attention or device-end signals at the
device are not reset.
Program Exceptions:
Privileged Operation
Resulting Condition Code:
o
Interruption pending in subchannel
1
CSW stored
2
Burst operation terminated
3
Not operational
The condition code set by HALT I/O for all
possible states of the I/O system is shown in the
figure "Condition Codes Set by HALT 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.
"
,
Programming Note
The instruction HALT I/O provides the program
with a means of terminating an I/O operation
before all data specified in the operation has been
transferred or before the operation at the device
has reached its normal ending point.
It
permits the
program to immediately free the selector channel
for an operation of higher priority. On the
byte-multiplexer channel, HALT I/O provides a
means of controlling real-time operations and
permits the program to terminate data transmission
on a communication line.
N
3
#
When a.device-not-operational response is received in
selecting the addressed device, a condition code 3 is set.
Note: Underscored condition codes pertain to situations that can
occur only on the multiplexer channel.
Condition Codes Set by HALT I/O
12-20
IBM 4300 Processors Principles of Operation
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