Condition Code 0 indicates that HALT DE­
VICE cannot signal the control unit until an inter­
ruption condition on the same subchannel is cleared.
Condition Code 1 with Control-Unit-Busy Sta­
tus in the CSW indicates that HALT DEVICE cannot signal the control unit until the control-unit­
end status is received from that control unit.
Condition Code 1 with Zeros in the Status
Field of the CSW indicates that the addressed
device WillS selected and signaled to terminate the
current operation, if any.
Condition Code 2 indicates that the control unit
cannot be signaled until the end of a busy condition
in the channel. The end of the busy condition can
be detected by noting an interruption from the chan­
nel or by noting the results of repeatedly executing
HALT DEVICE.
Condition Code 3 indicates that manual interven­
tion is required to allow HALT DEVICE to signal
the control unit to terminate.
Halt I/O HIO [S] ____ 9_E_0_0 ________ ___ ________ o 16 20 31
Execution of the current 110 operation at the ad­
dressed 110 device, subchannel, or channel is termi­
nated. The subsequent state of the sub channel de­
pends on the type of channel. The instruction HALT 110 is executed only when the CPU is in the supervi­
sor state. 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 sub channel and the 110 device to which
the instruction applies.
When the channel is either available or in the
interruption pending state, with the subchannel ei­
ther available or working, HALT 110 causes the
addressed device to be selected and to be signaled to the current operation, if any. If the sub­
channel is available, its state is not affected. If, on
the byte-multiplexer channel, the sub channel 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 HALT 110 is issued to a channel operating
in the burst mode, data transfer for the burst opera­
tion is terminated, and the device performing the
burst operation is immediately disconnected from 202 System/370 Principles of Operation
the channel. The subchannel and 110 device address
in the instruction, in this case, is ignored.
The termination of a burst operation by HALT 110 on the selector channel causes the channel and
sub channel 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
device. When HALT 110 causes a burst operation
on the byte-multiplexer channel to be terminated,
the subchannel associated with the burst operation
remains in the working state until the device pro­
vides channel end, whereupon the subchannel enters
the interruption pending state. The termination of a
burst operation by HALT 1/0 on a block­
multiplexer channel may, depending on the model
and the type of subchannel, take place as for a selec­
tor channel or may allow the sub channel to remain
in the working state until the device provides ending
status. On the byte-multiplexer channel operating in the
byte-interleave mode, the device is selected and the
instruction executed only after the channel has ser­
viced all outstanding requests for data transfer for
previously initiated operations, including the opera­
tion to be halted. If the control unit does not accept
the HALT II Q signal because it is in the not opera­
tional or control-unit-busy state, the subchannel, if
working, is set up to signal termination of device
operation the next time the device requests or offers
a byte of data. If command chaining is indicated in
the sub channel and the device presents status next,
chaining is suppressed.
When the addressed sub channel has a pending
interruption condition, with the channel in the avail­
able or interruption pending state, HALT 110 does
not cause any action.
When any of the following conditions occurs,
HALT 110 causes the status portion, bit positions
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 110 pertains only to the execution of HALT 1/0 and does not describe under what conditions the 1/0 operation at the addressed sub channel is concluded.
The extent of data transfer, and the conditions at the
termination of the operation at the subchannel, are
provided in the CSW associated with the interrup­
tion condition due to the termination.
1. The addressed device has been selected and
signaled to terminate the current operation.
The CSW contains zeros in the status field un­
less an equipment error is detected.
2. The channel attempted to select the addressed
device, but the control unit could not accept
the HALT 110 signal because it is executing a

previously initiated operation or has pending
an interruption condition associated with a de­
vice other than the one addressed. The signal
to terminate the operation has not been trans­
mitted to the device, and the subchannel, if in
the working state, has been set up to signal
termination the next time the device identifies
itself. The CSW unit-status field contains the
busy and status modifier bits. The channel­
status field contains zeros unless an equipment
error is detected.
3. The channel detected an equipment malfunc­
tion during the execution of HALT I/O. The
status bits in the CSW identify the error condi­
tion. 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 condition which affects the opera­
tional 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 is associated with this sta­
tus.
When HALT I/O causes data transfer to be termi­
nated, 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 oper­
ation is signaled by the generation of channel end,
which may occur at the normal time for the opera­
tion, earlier, or later, depending on the operation and
type of device. If the control unit is shared, all de­
vices attached to the control unit appear in the Channel A Subchannel A
o !*# 3
A
working state until the channel end condition is ac­
cepted 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
sub channel 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 or SRL publication for the device. The
HALT I/O signal has no effect on devices that are
not in the working state or are executing an opera­
tion of a fixed duration, such as rewinding tape or
positioning a disk-access mechanism. If the device is
executing a type of operation that is variable in du­
ration, the device interprets the signal as one to ter­
minate the operation. Pending attention or device
end conditions 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 pos­
sible states of the I/O system is shown graphically as
follows. See "States of the Input/Output System" for a detailed definition of the A, I, W, and N states.
o "-*# Control Unit
- Device
A I W N 1
1 *1
1 *1
1 *1 3 A I W N 1 1 *1 1* 1 1* 13
1 A Available I Interruption pending
W Working
N
Not operational CSW stored
# 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 conditions that can occur only on the multiplexer channel. Condition Codes Set by HALT I/O Input/ Output Operations 203