information from the device. When HALT DEVICE causes a burst operation on a
byte-multiplexer channel to be termi­
nated, the subchannel associated with
the burst operation remains in the work­
ing state until the device next provides
status, whereupon the subchannel 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
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 terminate the oper­
ation because it is busy or in the not­
operational 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 or command
retry is indicated for the I/O operation
using the subchannel, it is suppressed.
When either of the two situations
numbered below occurs, HALT DEVICE caus­
es 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
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 termi­
nation of the operation at the
subchannel are provided in the CSW asso­
ciated with the interruption condition
caused by the termination. The two
situations are:
1. The addressed device is selected
and signaled to halt 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 I/O 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.
When a channel recognizes an equipment
malfunction during the execution of HALT DEVICE, a CSW mayor may not be imme­
diately 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 DEVICE, condition
code 0 may be set, and the subsequently
stored CSW mayor may not indicate the
malfunction, depending on whether or not
the malfunction affected the I/O opera­
tion. When the channel recognizes a
malfunction and the subchannel is work­
ing with the addressed device, 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 type
of malfunction. If the channel sets
condition code 0, the contents of the
subsequently stored CSW identify the
type of malfunction. In either case,
the state of the channel and the
progress of the I/O operation are unpre­
dictable. Refer to the SL publication
for the system model to determine its
particular implementation.
When 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 DEVICE causes data transfer to
be terminated, the subchannel associated
with the operation either (1) remains in
the working state until the channel-end
condition is received and the subchannel
enters the interruption-pending state or
(2) immediately enters the interrup­
tion-pending state, depending on the
type of channel. If the subchannel is
shared by other devices attached to the
control unit, I/O instructions addressed
to those devices set the condition code
appropriate to the subchannel states
described.
When HALT DEVICE causes data transfer to
be terminated, the control unit associ­
ated with the operation may not become
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 opera­
tion, or earlier, or later, depending on
the operation and type of device.
When HALT DEVICE causes data transfer to
be terminated, 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. Chapter 13. Input/Output Operations 13-21

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 signal depends
partially on the type of device and its
state. In all cases, the halt 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 dura­
tion, or in which data is transferred, the device interprets the signal as one
to terminate the operation. Pending
interruption conditions at the device are not reset. Program Exceptions:
Privileged operation
A Channel A I W* Subchannel Q Q A N CU/ Device 1* 3
A Available
I Interruption pending
W Working WH N d A li 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 when HALT DEVICE causes the HDV function to be performed
is shown for all possible states of the
I/O system in the figure "Condition Codes Set by HALT DEVICE." The condi­
tion code set when HALT DEVICE causes
the HIO function to be performed is
shown for all possible states of the I/O
system in the figure "Condition Codes Set by HALT I/O" in the description of
the HALT I/O instruction. See the
section "States of the Input/Output
System" in this chapter for a detailed
definition of the A, I, W, and N states.
I WH A I W* WH N o Q • N 3 W* = Working with a device other than the one addressed WH = Working with the addressed device N Not operational * CSW Stored
$ CSW stored. Condition code 0 (with no CSW stored) instead of condition code 1
may be set when a malfunction is detected. In the WHXX state, either condition code 1 (with CSW stored) or condition
code 2 may be set, depending on the channel. However, condition code 1 (with CSW stored) can be set only if the control unit has received the signal to
terminate.
+ In the W*IX and W*W*X states, either condition code 0 or 2 may be set. • In the W*NX state, either condition code 2 or 3 may be set, depending on the
model and the channel type. Note: Underscored condition codes pertain to situations that can occur only on the
multiplexer channel. Condition Codes Set by HALT DEVICE 13-22 System/370 Principles of Operation