code (9C02) are
significant.
defined to be
2. Programs that issue RESUME I/O are
not executed correctly in models
that do not provide the suspend­
and-resume facility. In these
models, RESUME I/O is executed as
START I/O. This means that
programs that use RESUME I/O must
be designed to issue RESUME I/O only in models that provide the
suspend-and-resume facility. The
program can determine whether the
suspend-and-resume facility is
provided by issuing a mock START I/O with the suspend-control bit
set to one in the CAW. If the mock I/O operation is not terminated
with a channel-program-check indi­
cation because CAW bit 4 is not
equal to zero, the suspend-and­
resume facility is provided, and
the program may safely issue RESUME I/O. 3. Unlike a channel program initiated
by START I/O or START I/O FAST
RELEASE, a suspended channel
program being resumed may specify a CCW containing the transfer-in­
channel command as the first CCW executed when channel-program
execution is resumed.
START I/O SIO [S] '9COO' o 16 20 31
START I/O FAST RELEASE SIOF [S] '9C01' o 16 20 31
A write, read, read backward, control,
or sense operation is initiated with the
addressed I/O device and subchannel.
Bits 8-14 of the instruction are ignored
unless the suspend-and-resume facility is provided by the system model. When
the facility is provided, bits 0-15 of
the instruction are interpreted as
follows: Operation Code Interpretation 9COO 9C01 9C02 9C03-9CFF START I/O START I/O FAST RELEASE
RESUME I/O Invalid operation
Bits 16-31 of the second-operand address
identify the channel, subchannel, and I/O device to which the instruction
applies. The CAW, at real location 72,
contains the subchannel key, the
suspend-control bit, and the address of
the first CCW. This CCW specifies the
operation to be performed and the stor­
age area to be used, if any.
Either an SIO or SIOF function is
performed, depending on the instruction,
the channel, and the block­
multiplexing-control bit, bit 0 of
control register o. The instruction
START I/O always causes the SIO function
to be performed, as does START I/rr FAST
RELEASE when the block-multiplexing­
control bit is zero. When the bit is
one, START I/O FAST RELEASE may, depend­
ing on the channel, cause either the SIO or the SIOF function to be performed.
For the SIO function, the I/O operation
is initiated at the device if the
suspend flag is not 1 in the first CCW, the addressed I/O device and subchannel
are available, the channel is available
or interruption-pending, and errors or
exceptional situations 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 situations during
execution of the instruction.
For the SIOF function, the I/O operation is made pending at the subchannel if the
subchannel is available, the channel is
available or interruption-pending, and
no errors or exceptional conditions are
recognized during the execution of START I/O FAST RELEASE. Selection of the I/O device may be performed during the
execution of the instruction or may be
performed later. When an SIOF function
is performed, initiation of the I/O operation at the I/O device occurs
logically subsequent and asynchronous to
the execution of the instruction. When
the I/O operation is not initiated at
the I/O device during the execution of
the instruction, the I/O operation is
said to be pending at the subchannel
until channel and subchannel facilities
are available for initiation. When an I/O operation is made pending at the
subchannel, the subchannel enters the
working state and condition code 0 is
set for the instruction.
Status, other than control-unit end or
device end signaling the end of a previ­
ously signaled control-unit-busy or
device-busy condition, that is presented
Chapter 13. Input/Output Operations 13-27

by the device while the I/O operation is
pending at the subchannel causes the
pending I/O operation to be canceled.
An interruption condition is recognized,
and the status, with the busy bit
appended, is stored in the unit-status
field when the CSW is stored that clears
the interruption condition. The
deferred condition code is stored as 1
in the CSW in this case, and the CCW­ address field contains the address of
the first CCW plus 8.
When the channel attempts to initiate
the pending I/O operation at the I/O device, the detection of any error
condition by the channel or the I/O device causes the channel to terminate
the operation. The detection of any
exceptional condition by the channel or
the I/O device during the attempt to
initiate the I/O operation at the I/O device also causes the channel to termi­
nate the operation, except for certain
busy conditions when start-I/O-fast
queuing is provided for the subchannel.
When start-I/O-fast queuing is provided,
busy conditions detected during the
selection of the I/O device cause the
currently pending I/O operation at the
subchannel not to be initiated. Whether
the I/O operation remains pending or is
terminated when these conditions are
detected depends on the degree of
start-I/O-fast queuing provided and
conditions existing at the channel and
subchannel when detected. Control-unit or device-busy conditions
detected during the attempt to initiate
a pending I/O operation at the device
may cause the operation to remain pend­
ing.
If conditions are such that the I/O operation is not terminated but remains
pending at the subchannel, the operation
will remain pending until terminated for
some other reason or until the no­
longer-busy indication is received from
the control unit or device. When the
latter occurs, the channel again
attempts to initiate the pending I/O operation at the device.
When the channel is available or
interruption-pending, when the subchan­
nel is available before the execution of 510 or SIOF, and when the suspend-and­
resume facility is provided and the
first CCW contains a valid suspend (S)
flag, condition code 0 is set, but the
command in the first CCW is not trans­
ferred to the device. Instead, the
subchannel enters the subchannel-working
state with channel-program execution
suspended. When the SIOF function is
performed in this case, detection of a
valid S flag in the first CCW may occur
either before or after condition code 0 is set, depending on the system model.
13-28 System/370 Principles of Operation
When the channel is available or
interruption-pending, and the subchannel
is available before the execution of the
instruction, the following situations
cause a CSW to be stored. How the CSW is stored depends on whether an SIO or SIOF function is performed. The SIO function causes the status 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, situation 1 caus­
es the same action as for the 510 function; also, the control-unit and
device state may be tested, with the
result that situation 5 may cause the
same action as for the SID function. Or, situation 5 may be indicated in a
subsequent I/O interruption during which
the entire CSW is stored, or, when
start-I/O-fast queuing is provided,
situation 5 may not be indicated at all.
The remaining situations for the SIOF function are indicated in a subsequent I/O interruption, during which the
entire CSW is 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. If
selection of the device occurred
prior to detection of the error or
if the error condition was detected
during the selection of the device,
the device status is indicated in
the CSW. 2. The channel detects a programming
error associated with the first CCW or, if channel i ndi rect data
addressing is specified, with the
first IDAWi or, for the SIOF func­
tion, the channel detects an
equipment error after completion of
the instruction. The CSW identi­
fies the error. If selection of
the device occurred prior to
detection of the error, or if the
error condition was detected during
the selection of the device, the
device status is indicated in the CSW. 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 situations detected
during the operation. In the CSW, the channel-end bit is one, the
busy bit is zero, and other status
may be indicated. The I/O opera­
tion is initiated, but no
information has been transferred to
or from the storage area designated
by the CCW. No interruption condi­
tions are generated at the
subchannel, and the subchannel is
available for a new I/O operation.