the operation in the control unit is
terminated. Termination of the data­
transfer portion of the operation is
signaled by the generation of channel
end, which may occur at the normal time
for the operation, earlier, or later,
depending on the operation and type of
device.
When HALT I/O 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
interruption-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 I/O 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 I/O 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 subchannel is available and no burst Channel A
Subchannel
A N CU/ Device 1* 3
A
I
W
N
Available
Interruption pending
Working
Not operational CSW stored
A
I N Q 1 A 1* A
operation is in progress, the effect of
the halt signal depends on the type?f I/O device and its state and is speCl­
fied in the SL publication for the I/O device. The halt signal has no effect
on I/O devices that are not in the work­
ing state or are executing a mechanical
operation in which data is not trans­
ferred, such as rewinding tape or
positioning a disk-access mechanism. If
the I/O device is executing a type of
operation that is variable in duration,
the I/O device interprets the signal as
one to terminate the operation. Atten­
tion or device-end signals at the device
are not reset. Program Exceptions: Privileged operation
Resulting Condition Code: o
1
2
3
Interruption pending
subchannel CSW stored
Burst operation terminated
Not operational
in
The condition code set by HALT I/O for
all possible states of the I/O system is
shown in the figure "Condition Codes Set
by HALT I/O." 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 W
2
I 0 N
3 * When a device-nat-operational
selecting the addressed device, Condition code 0 may be set if a
response is received in
a condition code 3 is set.
malfunction is detected.
Note: Underscored condition codes pertain to situations that can
occur only on the multiplexer channel. Condition Codes Set by HALT I/O Chapter 13. Input/Output Operations 13-25

Programming Note
The instruction HALT I/O provides the
program with a means of terminating an I/O operation before all data specified
in the operation has been transferred or
before the operation at the device has
reached its normal ending point. It
permits the program to immediately free
the selector channel for an operation of
higher priority. On the byte­
multiplexer channel, HALT I/O provides a
means of controlling real-time oper­
ations and permits the program to
terminate data transmission on a commu­
nication line.
RESUME I/O RIO [S]
'9C02'
o 16 20 31
Depending on whether the suspend-and­
resume facility is provided by the
system model, an RIO or SIO function is
performed. The RIO function is
performed when the suspend-and-resume
facility is provided by the system
model; otherwise, the SIO function is
performed.
The SIO function is described in the
definition of the instruction START I/O. Execution of the RIO function causes a
currently suspended channel-program
execution to be resumed with the device
if the suspend flag in the CCW causing
suspension has been set to zero. If the
suspend flag remains set to one or if
the I/O operation is not currently
suspended, the instruction has no effect
on the channel-program execution. The
instruction is executed only when the
CPU is in the supervisor state; other­
wise, a privileged-operation exception
is recognized, and the operation is
suppressed.
Bits 16-31 of the second-operand address
identify the channel, subchannel, and I/O device to which the instruction
applies.
The RIO function is performed independ­
ent of the state of the channel,
subchannel, and device so long as the
channel is operational.
If the channel is not operational,
condition code 3 is set, and no action
takes place. If the channel is opera­
tional, condition code 0 is set. If the
suspend-and-resume facility is not
provided for the addressed subchannel or
if the addressed subchannel is not oper-
13-26 System/370 Principles of Operation ational, no further action takes place.
If the suspend-and-resume facility is
provided for the addressed subchannel
and a channel-program execution is
currently suspended or in the process of
being suspended at the subchannel, the
channel is signaled to perform the
resume function. A channel-program
execution is in the process of being
suspended if a channel-command word
(CCW) has been fetched which contains a
valid S flag but the suspend function
has not yet been completed.
The RIO function is performed by the
channel logically subsequent to and
asynchronous to the execution of the
RESUME I/O that provided the stimulus.
The RIO function causes the channel to
perform a modified SIOF function by
using the CCW that previously caused the
channel to perform the suspend function
as the first CCW of the resumed
channel-program execution. Resumption
of channel-program execution appears to
the device to be the initiation of a new I/O operation not chained to the previ­
ous operation.
Program Exceptions:
Privileged operation
Resulting Condition Code:
o RIO function performed
1
2
3 Channel not operational
The condition code set by RESUME I/O for
all possible states of the I/O system is
shown in the figure "Condition Codes Set
by RESUME I/O." See the section "States
of the Input/Output System" in this
chapter for a detailed definition of the
A, I, W, and N states.
Channel
A Available
I Interruption pending
W Working N Not operational
Condition Codes Set by RESUME I/O Programming Notes 1. Programs designed to be executed in
models that do not provide the
suspend-and-resume facility may not
be executed properly in models that
provide the facility because bits
8-14 of the RESUME I/O operation