ceipt of the signal from the device. The channel-end
indication in this case is not made available to the
program.
Termination by HALTI/O The instruction HALT r/ a causes the current operation
at the addressed channel or subchannel to be termi
nated immediately. The method of termination differs
from that used upon exhaustion of count or upon de
tection of programming errors to the extent that ter
mination by HALT r/o is not contingent on the receipt
of a service request from the device.
When HALT r/o is issued to a channel operating in
the burst mode, the channel issues the haIt-r/o signal
to the device regardless of the current activity in the
channel and on the interface. If the channel is involved
in the data-transfer portion of an operation, data trans
fer is immediately terminated, and the device is dis
connected from the channel. If HALTI/O is addressed
to a selector channel executing a chain of operations
and the device has already provided channel end for
the current operation, the instruction causes the de
vice to be disconnected and the chain-command Hag
to be removed.
When HALTI/O is issued to the multiplexor channel
and the channel is not operating in the burst mode,
the haIt-I/o signal is sent to the device whenever the
addressed subchannel is in the working state. The sub
channel may be transferring data, or it may have al
ready received channel end for the current operation
and may be waiting for device end to initiate a new
operation by command chaining. In either case, HALT
r/o causes the device to be selected, and the halt-I/o
signal is issued as the device responds. When com
mand chaining is indicated in the subchannel, HALTI/O causes the chain-command Hag to be turned off.
Termination of an operation by HALTI/O on the se
lector channel results in up to four distinct interrupt
ion conditions. The first one is generated by the chan
nel upon execution of the instruction and is not con
tingent on the receipt of status from the device. The
command address and count in the associated csw
indicate how much data have been transferred, and
the channel-status bits reHect the unusual conditions
if any, detected during the operation. If HALT r/o issued before all data specified for the operation have
been transferred, incorrect length is indicated, subject
to the control of theSLI Hag in the current ccw. The
execution of HALTI/O itself is not reHected in csw
status, and all status bits in a csw due to this inter
ruption condition can be zero. The channel is available
for the initiation of a newI/O operation as soon as the
interruption condition is cleared.
The second interruption condition on the selector
106
channel occurs when the control unit generates the
channel-end condition. The selector channel handles
this condition as any other interruption condition from
the device with the sub channel available and provides
zeros in the protection key, command address, count,
and channel status fields of the associated csw. The
channel-end condition is not made· available to the
program when HALTI/O is issued to a channel execu
ting 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 generated. These conditions are handled as for any
otherI/O operation.
Termination of an operation by HALTI/O on the
multiplexor 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 interrupt
ion-pending state. If HALT r/o is issued after the de
vice has signaled channel end and the sub channel is
executing a chain of operations, the channel-end con
dition is not made available to the program, andthe sub channel remains in the working state until the next
status byte from the device is received. Receipt of a
status byte subsequently places the sub channel in the
interruption-pending state.
The csw associated with the interruption condition
in the sub channel contains the status bytes provided
by the device and the channel, and indicates at what
point data transfer was terminated. If HALT r/o is is
sued before all data areas associated with the current
opcration have been exhausted or filled, incorrect
length is indicated, subject to the control of theSLI Hag in the current ccw. The interruption condition is
processed as for any other type of termination.
Termination Due to Equipment Malfunction
When channel equipment malfunctioning is detected
or invalid signals are received over theI/O interface,
the recovery procedure and the subsequent states of
the subchannels and devices on the channel depend
on the type of error and on the model. Normally, the
program is alerted of the termination by anI/O inter
ruption, and the associated csw indicates the channel
control-check or interface-control-check condition. In
channels sharing common equipment with theCPU, malfunctioning detected by the channel may be indi
cated by a machine-check interruption, in which case
no cswis stored. Equipment malfunctioning may cause
the channel to perform the malfunction-reset function.
indication in this case is not made available to the
program.
Termination by HALT
at the addressed channel or subchannel to be termi
nated immediately. The method of termination differs
from that used upon exhaustion of count or upon de
tection of programming errors to the extent that ter
mination by HALT r/o is not contingent on the receipt
of a service request from the device.
When HALT r/o is issued to a channel operating in
the burst mode, the channel issues the haIt-r/o signal
to the device regardless of the current activity in the
channel and on the interface. If the channel is involved
in the data-transfer portion of an operation, data trans
fer is immediately terminated, and the device is dis
connected from the channel. If HALT
to a selector channel executing a chain of operations
and the device has already provided channel end for
the current operation, the instruction causes the de
vice to be disconnected and the chain-command Hag
to be removed.
When HALT
and the channel is not operating in the burst mode,
the haIt-I/o signal is sent to the device whenever the
addressed subchannel is in the working state. The sub
channel may be transferring data, or it may have al
ready received channel end for the current operation
and may be waiting for device end to initiate a new
operation by command chaining. In either case, HALT
r/o causes the device to be selected, and the halt-I/o
signal is issued as the device responds. When com
mand chaining is indicated in the subchannel, HALT
Termination of an operation by HALT
lector channel results in up to four distinct interrupt
ion conditions. The first one is generated by the chan
nel upon execution of the instruction and is not con
tingent on the receipt of status from the device. The
command address and count in the associated csw
indicate how much data have been transferred, and
the channel-status bits reHect the unusual conditions
if any, detected during the operation. If HALT r/o
been transferred, incorrect length is indicated, subject
to the control of the
execution of HALT
status, and all status bits in a csw due to this inter
ruption condition can be zero. The channel is available
for the initiation of a new
interruption condition is cleared.
The second interruption condition on the selector
106
channel occurs when the control unit generates the
channel-end condition. The selector channel handles
this condition as any other interruption condition from
the device with the sub channel available and provides
zeros in the protection key, command address, count,
and channel status fields of the associated csw. The
channel-end condition is not made· available to the
program when HALT
ting 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 generated. These conditions are handled as for any
other
Termination of an operation by HALT
multiplexor 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 interrupt
ion-pending state. If HALT r/o is issued after the de
vice has signaled channel end and the sub channel is
executing a chain of operations, the channel-end con
dition is not made available to the program, and
status byte from the device is received. Receipt of a
status byte subsequently places the sub channel in the
interruption-pending state.
The csw associated with the interruption condition
in the sub channel contains the status bytes provided
by the device and the channel, and indicates at what
point data transfer was terminated. If HALT r/o is is
sued before all data areas associated with the current
opcration have been exhausted or filled, incorrect
length is indicated, subject to the control of the
processed as for any other type of termination.
Termination Due to Equipment Malfunction
When channel equipment malfunctioning is detected
or invalid signals are received over the
the recovery procedure and the subsequent states of
the subchannels and devices on the channel depend
on the type of error and on the model. Normally, the
program is alerted of the termination by an
ruption, and the associated csw indicates the channel
control-check or interface-control-check condition. In
channels sharing common equipment with the
cated by a machine-check interruption, in which case
no csw
the channel to perform the malfunction-reset function.