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

Programmiing Note
The instruction HALT I/O provides the program 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 im­
mediately 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
operations and permits the program to terminate
data transmission on a communication line.
Start I/O SIO [S] __ 9_C_0_0 ______ __ B_2 __ _____ D_2 o 16 20 31
Start I/O Fast Release SIOF [S] 9COl I B2 I D2 o 31
A write, read, read backward, control, or sense oper­
ation is inlitiated with the addressed I/O device and
subchannel. The instruction is executed only when
the CPU is in the supervisor state. Bits 8-14 of the
instruction are ignored.
Either an SIO or SIOF function is performed,
depending on the instruction, the channel, and the
block-multiplexing control: control register 0, bit 0. The SIO function causes the operation to be initiated
only after the device is selected. The SIOF function
causes the: operation to be initiated independently of
the device. The instruction START I/O always caus­
es the SIO function to be performed, as does
START I/O FAST RELEASE when block multi­
plexing is not specified. When block multiplexing is
specified, START I/O FAST RELEASE, depending
on the channel, may cause either the SIO or the SIOF function to be performed.
Bits 16-31 of the second-operand address identify
the channel, subchannel, and I/O device to which
the instruction applies. The CAW, at location 72,
contains the protection key for the subchannel and
the address of the first CCW. This CCW specifies
the operation to be performed, the main-storage area
to be used, and the action to be taken when the op­
eration is completed. 204 Syst€:!m/370 Principles of Operation
For the SIO function, the I/O operation is initiat­
ed if the addressed I/O device and sub channel are
available, the channel is available or is in the inter­
ruption pending state, and errors or exceptional con­
ditions have not been detected. The I/O operation is
not initiated when the addressed part of the I/O system is in any other state or when the channel or
device detects any error or exceptional condition
during execution of the instruction.
For the SIOF function, the I/O operation is initi­
ated if the subchannel is available, the channel is
available or is in the interruption-pending state, and
errors or exceptional conditions have not been de­
tected. The I/O operation is not initiated when the
sub channel and channel are in any other state or
when the channel or device detects any error or ex­
ceptional condition during execution of the instruc­
tion. The device state or device-detected errors are
not relevant during instruction execution but are
indicated in a CSW stored during a subsequent inter­
ruption.
When the channel is either available or in the
interruption-pending state and the subchannel is
available before the execution of the instruction, the
following conditions cause a CSW to be stored, in a
manner determined by whether an SIO or SIOF func­
tion is performed. The SIO function causes the sta­
tus portion of the CSW to be replaced by a new set
of status bits. The status bits pertain to the device
addressed by the instruction. The contents of the
other fields of the CSW are not changed. When the SIOF function is performed, the first condition caus­
es the same action as for the SIO function. The re­
maining conditions will be indicated in a subsequent
interruption, during which the entire CSW will be
stored.
1. The channel detects a programming error in the
contents of the CAW or detects an equipment
error during execution of the instruction. The
CSW identifies the error condition. The
channel-end and busy bits are off, unless, for
the SIO function, the error was detected after
the device was selected, and the device was
found to be busy, in which case the busy bit, as
well as any bits indicating pending interruption
conditions, are on. The interruption conditions
indicated in the CSW have been cleared at the
device. The I/O operation has not been initiat­
ed. No interruption conditions are generated at
the I/O device or subchannel. The state of the PCI bit in the CSW is unpredictable.
2. The channel detects a programming error asso­
ciated with the first CCW or, if CIDA is speci­
fied, with the first IDAW; or, for the SIOF function, the channel detects an equipment