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disconnected and command chaining to be
immediately suppressed.
When HALT DEVICE is issued to a channel
operating in burst mode, the halt signal is issued to
the device involved in the burst-mode operation
only if that device is the one to which the HALT
DEVICE is addressed.
If
the operation thus
terminated is in the data-transfer portion of the
operation, data transfer is immediately terminated,
and the device is disconnected from the channel.
If
the terminated burst involves a selector channel
executing a chain of operations and the device has
already provided channel end for the current
operation, HALT DEVICE causes the device to be
disconnected and command chaining to be
immediately suppressed.
If,
on a selector channel,
the device involved in the burst is not the one to
which the HALT DEVICE is addressed, no action
is taken.
If,
on a multiplexer channel, the device
involved in the burst is not the one to which the
HAL T DEVICE is addressed, HALT DEVICE
causes any operation for the addressed device to be
terminated at the addressed subchannel by
suppressing any further data transfer or command
chaining for that device.
When HALT I/O or HALT DEVICE is issued to
a channel not operating in burst mode, the
addressed device is selected, and the halt signal is
issued as the device responds. On a multiplexer
channel, command chaining, if indicated in the
subchannel, is immediately suppressed.
The termination of an operation by HALT I/O
or HALT DEVICE on the selector channel results
in up to four distinct interruption conditions. The
first one is generated by the channel upon
execution of the instruction and is not contingent
on the receipt of status from the device. The
channel-status bits reflect the unusual situations, if
any, detected during the operation.
If
HALT I/O
or HALT DEVICE is issued before all data
specified for the operation has been transferred,
incorrect length is indicated, subject to the control
of the SLI flag in the current CCW. The execution
of HALT I/O or HALT DEVICE itself is not
reflected in CSW status, and all status bits in a
CSW due to this interruption condition can be
zero. The channel is available for the initiation of
a new I/O operation as soon as the interruption
condition is cleared.
The second interruption condition on the
selector channel occurs when the control unit
signals channel end. The selector channel handles
this condition as any other interruption condition
from the device after the device has been
disconnected from the channel, and provides zeros
in the subchannel-key, CCW-address, count, and
channel-status fields of the associated CSW.
Channel end is not made available to the program
when HALT I/O or HALT DEVICE is issued to a
channel executing a chain of operations and the
device has already provided channel end for the
current operation.
Finally, the third and fourth interruption
conditions occur when control-unit end, if any, and
device end are signaled. These signals are handled
as for any other I/O operation.
The termination of an operation by HALT I/O
or HALT DEVICE on a multiplexer channel causes
the normal interruption conditions to be generated.
If
the instruction is issued when the subchannel is
in the data-transfer portion of an operation, the
subchannel remains in the working state until
channel end is signaled by the device, at which
time the subchannel is placed in the interruption-
pending state.
If
HALT I/O or HALT DEVICE is
issued after the device has signaled channel end
and the subchannel is executing a chain of
operations, channel-end is not made available to
the program, and the subchannel remains in the
working state until the next status byte from the
device is received. Receipt of a status byte
subsequently places the subchannel in the
interruption-pending state.
The CSW associated with the interruption
condition in the subchannel contains the status byte
provided by the device and the channel.
If
HALT
I/O or HALT DEVICE is issued before all data
areas associated with the current operation have
been exhausted or filled, incorrect length is
indicated, subject to the control of the SLI flag in
the current CCW. The interruption condition is
processed as for any other type of termination.
The termination of a burst operation by HALT
I/O or HALT DEVICE on a block-multiplexer
channel may, depending on the model and the type
of subchannel, take place as for a selector channel
or may allow the subchannel to remain in the
working state until the device provides ending
status.
Programming Note
The count field in the CSW associated with an
operation terminated by HALT I/O or HALT
DEVICE is unpredictable.
Chapter 12. Input/Output Operations
12-43
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