mains in the working state until the end of the inher­
ent cycle of the operation, at which time device end
is generated. If blocks of data at the device are de­
fined, 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 across the I/O interface, such as
rewinding tape or positioning a disk access mecha­
nism. If the device is executing a type of operation
that is unpredictable in duration, or in which data is
transferred across the I/O interface, the device in­
terprets the signal as one to terminate the operation.
Pending attention or device end conditions at the devke are not reset.
Program Exceptions:
Privileged operation
Resulting Condition Code:
o Subchannel busy with another device
or interruption pending
1 CSW stored
2 Channel working
3 Not operational
The condition code set by HALT DEVICE for all
possible states of the I/O system is shown graphical- Channel A Subchannel A II W# Q Q
A
ly. See "States of the Input/Output System" for a
detailed definition of the A, I, W, and N states.
Programming Note
Some selector and byte-multiplexer channels de­
signed prior to the defining of HALT DEVICE (for
example, the 2860), will execute HALT DEVICE as
HALT I/O. A program can ensure complete com­
patibility between HALT DEVICE and HALT I/O on such channels by observing the following conven­
tions:
1. On a byte-multiplexer channel, do not issue
HALT DEVICE to a multiplexing device when
a burst operation is in progress on the channel.
2. On a byte-multiplexer channel, do not issue
HALT DEVICE to a device on a shared sub­
channel while that subchannel is working with
a device other than the one addressed.
3. 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 work­
ing.
The execution of HALT DEVICE always causes
data transfer across the I/O interface for the ad­
dressed device to be terminated. The condition code
and the CSW (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 CSW (when stored) imply
the conditions under which the execution of a HALT
DEVICE for the same address will cause the control
unit to be signaled to terminate.
W#
P Q 1. .:!: + ! Control Unit
- Device I A ,I ,WIN I IA I I ,WIN I IA III
W IN, I AI11W IN I A
W
N
*
1* 1* 1* 3 Available Interruption pending
Working
1* 1* 1* 3
- W=I= = Working with a device other than the
one addressed W# = Working with the addressed device
Not operational CSW stored
Condition Codes Set by HALT DEVICE
1* 1* 1* 3
@ In the W#XX state, either condition code 1 (with CSW stored) or
condition c'ode 2 may be set, depending on the channel and the
conditions in the channel. Condition code 1 (with CSW stored) can be
set only if the control unit has received the signal to terminate.
+ In the W=I=IX and W=I=W=I=X states, either condition code 0 or 2 may be
set, depending on the channel and the conditions in the channel. $ In the state, either condition code 2 or 3 may be set, depending
on the channel type and system model. Note: Underscored condition codes pertain to conditions that can occur only on the multiplexer channel. Input/Output Operations 201

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