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

error after completion of the instruction. The CSW identifies the error condition. The
channel-end and busy bits are off, unless the
error was detected after the device was select­
ed, and the device was found to be busy, in
which case the busy bit, as well as any bits in­
dicating 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 initiated. No inter­
ruption conditions are generated at the I/O device or subchannel. The state of the PCI bit
in the CSW is unpredictable.
3. An immediate operation was executed, and
either (1) no command chaining is specified
and no command retry occurs, or (2) chaining
is suppressed because of unusual conditions
detected during the operation. The CSW con­
tains the channel-end bit and any other indica­
tions provided by the channel or the device.
The busy bit is off. The I/O operation has
been initiated, but no information has been
transferred to or from the storage area desig­
nated by the CCW. No interruption conditions
are generated at the subchannel, and the sub­
channel is available for a new I/O operation. If
device end is not indicated, the device remains
busy, and a subsequent device-end condition is
generated. The CSW contains the PCI bit if
specified in the first CCW.
4. The I/O device contains a pending interruption
condition, or the control unit contains a pend­
ing interruption condition for the addressed
device. The CSW unit-status field contains the
busy bit, identifies the interruption condition,
and may contain other bits provided by the
device or control unit. The interruption condi­
tion is cleared. The channel-status field indi­
cates any error conditions detected by the chan­
nel and contains the PCI bit if specified in the
first CCW.
5. The I/O device or the control unit is executing
a previously initiated operation, or the control
unit has pending an interruption condition asso­
ciated with a device other than the one ad­
dressed. The CSW unit-status field contains
the busy bit or, if the control unit is busy, the
busy and status-modifier bits. The channel­
status field indicates any error conditions de­
tected by the channel and contains the PCI bit
if specified in the first CCW. When the SlOP function is performed, the control unit busy
condition may cause the same action as the SIO function. Page of GA22-7000-4 Revised September 1, 1975
By TNL: GN22-0498
6. The I/O device or control unit detected an
equipment or programming error during the
initiation, or the addressed device is in the not­
ready state. The CSW identifies the error con­
dition. The channel-end and busy bits are off,
unless the device was found to be busy, in
which case the busy bit, as well as any bits in­
dicating 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 initiated. No inter­
ruption conditions are generated at the I/O device or sub channel. The CSW contains the
PCI bit if specified in the first CCW.
When the SIO or SlOP function cannot be execut­
ed because of a pending logout condition which af­
fects 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 is associat­
ed with this status.
When the SlOP function causes condition code 0 to be set and subsequently a condition is encoun­
tered which would have caused a condition code 1 to
be set had the function been SIO, a deferred­
condition-code-l I/O interruption condition is gener­
ated. In the resulting I/O interruption, a full CSW is stored, and the deferred condition code appears
in the CSW. On the byte-multiplexer channel, both the SIO and SlOP functions cause the addressed device to be
selected and the operation to be initiated only after
the channel has serviced all outstanding requests for
data transfer for previously initiated operations.
Program Exceptions:
Privileged operation
Resulting Condition Code:
o I/O operation initiated and channel proceeding
with its execution
1 CSW stored
2 Channel or sub channel busy
3 Not operational
The condition code set by ST ART I/O and START I/O PAST RELEASE for all possible states
of the I/O system is shown graphically as follows. See "States of the Input/Output System" for a de­
tailed definition of the A, I, W, and N states.
Input/Output Operations 205