Notes
1. A can complete
compatibility between HALT DEVICE and HALT I/O on channels that
execute HALT DEVICE as HALT I/O by the following
conventions:
a. On a channel,
do not issue HALT DEVICE to a
multiplexing device when a
burst operation could be in
progress on the channel.
b. On a byte-multiplexer channel,
do not issue HALT DEVICE to a
device on a shared subchannel
while that subchannel is work­
ing with a device other than
the one addressed.
c. On a selector channel in the
working state, do not issue
HALT DEVICE to any device other
than the one with which the
channel is working.
2. A block-multiplexer channel may
execute HALT DEVICE as a block­
multiplexer or selector channel.
However, when a block-multiplexer
channel is operating with multi­
plexing inhibited, HALT DEVICE causes the HDV function to be
performed rather than the HIO func­
tion.
3. The execution of HALT DEVICE always
causes data transfer between the
addressed device and the channel to
be terminated. The condition code
and the C5W (when stored) indicate
whether the control unit was
signaled to terminate its operation
during the execution of the
instruction. If the control unit
was not signaled to terminate its
operation, the condition code and
the C5W (when stored) imply the
situations under which the
execution of a HALT DEVICE for the
same address will cause the control
unit to be signaled to terminate. Condition code 0 indicates that
HALT DEVICE cannot signal the
control unit until an interruption
condition on the same subchannel is
cleared. Condition code 1 with control­
unit-busy status in the C5W indi­
cates that HALT DEVICE cannot
signal the control unit until the
control-unit-end status is received
from that control unit. Condition code 1 with
status field of the
that the addressed
selected and signaled
the operation,
zeros in the C5W indicates
device was
to terminate
if any. Condition code 2 indicates that the control unit cannot be signaled
until the channel is not working.
The end of the working state can be
detected by noting an interruption
from the channel by noting the
results of repeatedly executing
HALT DEVICE. Condition code 3 indicates that
manual intervention is required in
order to allow HALT DEVICE to
signal the unit to termi­
nate.
HALT I/O [5] '9EOO' o 16 20 31
Execution of the current I/O operation
at the addressed I/O device, subchannel,
or channel is terminated. The subse­
quent state of the subchannel depends on
the type of channel.
Bits 8-14 of the instruction are ignored. Bits 16-31 of the
second-operand address identify the
channel and, when the channel is not
working, identify the subchannel and the I/O device to which the instruction
applies.
The HIO function is performed by the
HALT I/O instruction and, on some chan­
nels and under certain circumstances, by
HALT DEVICE. When the channel is either available or
interruption-pending, with the subchan­
nel either available or working, HALT I/O causes the addressed I/O device to
be selected and to be signaled to termi­
nate the current operation, if any. If
the subchannel is available, its state
is not affected. If, on the byte­
multiplexer channel, the subchannel is
working, data transfer is immediately
terminated, but the subchannel remains
in the working state until the device
provides the next status byte, whereupon
the subchannel is placed in the
interruption-pending state.
When the channel is either available or
interruption-pending with the subchannel
working but the I/O operation is either
not yet initiated at the device or is
suspended, HALT I/O causes the suspended
or pending I/O operation to be termi­
nated and an interruption condition to
be recognized. The C5W stored when the
interruption occurs contains zeros in
the unit-status and channel-status
fields. The command-address field Chapter 13. Input/Output Operations 13-23

contains the address of the first or the
suspended CCW, plus 8, and the deferred
condition code is 1.
When HALT I/O is issued to a channel
operating in the burst mode, data trans­
fer for the burst operation is termi­ nated, and the I/O device performing the
burst operation is immediately discon­
nected from the channel. The subchannel
and I/O-device address in the instruc­
tion is ignored in this case.
The termination of a burst operation by
HALT I/O on the 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 a
status byte from the I/O device. When
HALT I/O causes a burst operation on the
byte-multiplexer channel to be termi­ nated, the subchannel associated with
the burst operation remains in the work­
ing state until the I/O device next
provides status, whereupon the subchan­
nel enters the interruption-pending
state. The termination of a burst oper­
ation by HALT I/O on a block-multiplexer
channel may, depending on the model and
the design of the subchannel, take place
as for a selector channel or may allow
the subchannel to remain in the working
state until the device next provides
status. On the byte-multiplexer channel operat­
ing in the byte-multiplex mode, the I/O device is selected and the instruction
executed only after the channel has
serviced all outstanding requests for
data transfer for previously initiated
operations, including the operation to
be halted. If the control unit does not
accept the signal to halt the operation
because it is in the not-operational or
busy state, the subchannel, if working
with a device, is set up to signal
termination of device operation the next
time the device requests or offers a
byte of data. If command chaining or
command retry is indicated in the
subchannel, it is suppressed if the
device presents status.
When the addressed subchannel is
interruption-pending, with the channel
available or interruption-pending, HALT I/O does not cause any action.
When any of the following conditions
occurs, HALT I/O causes the status
portion, bits 32-47, 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 I/O pertains only to the execution
of HALT I/O and does not describe the I/O operation that is terminated at the
addressed subchannel. The extent of
data transfer, and the status at the
termination of the operation at the
subchannel, are provided in the CSW 13-24 System/370 Principles of Operation
associated with the interruption condi­
tion due to the termination.
1. The addressed device was selected
and signaled to halt the current
operation. The CSW contains zeros ln the status field unless an
equipment error is detected.
2. The channel attempted to select the
addressed device, but the control
unit could not accept the halt
signal because it was executing a
previously initiated operation or
had an interruption condition asso­
ciated with a device other than the
one addressed. The signal to
terminate the operation has not
been transmitted to the device, and
the subchannel, if in the working
state with an I/O operation in
progress at the device, will signal
termination the next time the
device identifies itself. The CSW unit-status field contains ones in
the busy and status-modifier bit
positions. The channel-status
field contains zeros unless an
equipment error is detected.
When a channel detects an equipment
malfunction during the execution of HALT I/O, a CSW mayor may not be immediately
stored, depending on the state of the
subchannel or the channel model. When
the subchannel is interruption-pending and a malfunction occurs during the
execution of HALT I/O, condition code 0 is set, and the channel-status field of
the subsequently stored CSW mayor may not indicate channel-control check,
along with the other ending-status
information, depending on whether the
malfunction affected the I/O operation.
When the channel recognizes a malfunc­
tion during the execution of HALT I/O and the subchannel is working, condition
code 0 or 1 may be set, depending on the
channel model. If the channel sets
condition code 1, the contents of the
immediately stored CSW identify the
malfunction. If the channel sets condi­
tion code 0, the contents of the
subsequently stored CSW identify the
malfunction and may also indicate other
status information describing the termi­
nated operation. Consult the SL
publication for each system model to
determine implementation.
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 may not become avail­
able until the data-handling portion of