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chaining is indicated for the I/O operation using
the subchannel, it is suppressed.
When the channel is working in burst mode with
a device other than the one addressed, and the
subchannel is available, interruption-pending, or
working with a device other than the one
addressed, no action is taken.
If
the sub channel is
working with the addressed device, the sub channel
signals termination of the device operation the next
time the device requests or offers a byte of data, if
any.
If
chaining is indicated for the I/O operation
using the subchannel, it is suppressed.
When the channeCis working in burst mode with
a device other than the one addressed and the
subchannel is not operational, is interruption-
pending, or is working with a device other than the
one addressed, the resulting condition code may, in
some channels, be determined by the subchannel
state.
Termination of a burst operation by HALT
DEVICE on a selector channel causes the channel
and subchannel to be placed in the interruption-
pending state. Generation of the interruption
condition is not contingent on the receipt of status
information from the device. When HALT
DEVICE causes a burst operation on a byte-
multiplexer channel to be terminated, the
sub channel associated with the burst operation
remains in the working state until the device
provides ending status, whereupon the sub channel
enters the interruption-pending state. The
termination of a burst operation by 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.
. When any of the three situations numbered
below occurs, HALT DEVICE causes the 16-bit
unit-status and channel-status portion of the CSW
to be replaced by a new set of status bits. The
contents of the other fields of the CSW are not
changed. The CSW stored by HALT DEVICE
pertains only to the execution of HALT DEVICE
and does not describe the I/O operation, at the
addressed subchannel, that is terminated. The
extent of data transfer and the status at the
termination of the operation at the subchannel are
provided in the CSW associated with the
interruption condition caused by the termination.
The three situations are:
1. The addressed device is selected and signaled to
terminate the current operation, if any. The
CSW then contains zeros in the status field
unless a machine malfunction is detected.
2. The control unit is busy and the device cannot
be given the signal to terminate the operation.
The CSW unit-status field contains ones in the
busy and status-modifier bit positions. The
channel-status field contains zeros unless a
machine malfunction is detected.
3. The channel detects a machine malfunction
during the execution of HALT DEVICE. The
status bits in the CSW then identify the type of
malfunction. The state of the channel and the
progress of the I/O operation are
unpredictable.
If
HALT DEVICE 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 D·EVICE causes data transfer to be
terminated, the control unit associated with the
operation remains not available until the
data-handling portion of the operation in the
control unit is concluded. Conclusion of this
portion of the operation is signaled by the
generation of channel end. This may occur at the
normal time for the operation, or 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 on that
channel until the channel-end condition 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, as in read-type
operations on magnetic tape, the recording medium
is advanced to the beginning of the next block.
When HALT DEVICE is issued at a time when
the subchannel is available and no burst operation
is in progress, the effect of the HALT DEVICE
signal depends partially on the type of device and
its state. In all cases, the HALT DEVICE signal
has no effect on devices that are not in the working
state or are executing a 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
unpredictable in duration, or in which data is
transferred, the device interprets the signal as one
to terminate the operation. Pending attention or
device-end conditions at the device are not reset.
Chapter 12. Input/Output Operations
12-17
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