device or the indication of the reasons
for which an I/O operation has been
concluded. The CSW is formed, or parts
of it are replaced, in the process of I/O interruptions and possibly during
the execution of START I/O, START I/O FAST RELEASE, TEST I/O, CLEAR I/O, HALT I/O, HALT DEVICE, and STORE CHANNEL ID.
The CSW is stored at real storage
location 64 and is available to the
program at this location until the time
the next I/O interruption occurs or
until another I/O instruction causes its
contents to be replaced, whichever
occurs first.
The information placed in the CSW by an I/O interruption pertains to the channel
and device which are identified by the I/O address stored during the inter­
ruption. The information placed in the CSW by START I/O, START I/O FAST RELEASE, TEST I/O, CLEAR I/O, HALT I/O, HALT DEVICE, or STORE CHANNEL ID
pertains to the channel and (except for STORE CHANNEL ID) the device addressed
by the instruction.
The CSW has the following format: IKeylslllccl CCW Address 0 4 6 8 31 Count 32 40 48 63
The fields in the CSW are allocated as
follows:
Subchannel Key: Bits 0-3 form the
access key used in the chain of oper­
ations at the subchannel. Suspended Bit 4, when stored as
one, indicates that the subchannel asso­
ciated with the information in the CSW has the execution of a channel program
currently suspended. The S condition
can only be indicated in the CSW stored
as a result of an I/O interruption
because of the program-controlled­
interruption (PCI) condition.
logout Pending (1): Bit 5, when one,
indicates that an I/O instruction cannot
be executed until a logout has been
cleared. Bit 45, channel-control check,
will always be one when bit 5 is one.
Deferred Condition Code eCC): Bits 6
and 7 indicate whether situations have
been encountered subsequent to the
setting of a condition code 0 for START I/O FAST RELEASE or RESUME I/O that
would have caused a different
condition-code setting for START I/O. The possible setting of these bits, and
their meanings, are as follows: Setting of
Bit 6 Bit 7 Meaning 0 0 Normal I/O interruption 0 1 Deferred condition code
i s 1
1 0 (Reserved)
1 1 Deferred condition code
is 3 CCW Address: Bits 8-31 form an absolute
address that is 8 higher than the
address of the last CCW used. Status: Bits 32-47 identify the status
of the device and the channel that
caused the storing of the CSW. Bits
32-39, the unit status, indicate situ­
ations detected by the device or control
unit. Bits 40-47, the channel status,
are provided by the channel and indicate
situations associated with the subchan­
nel. The 16 bits are designated as
follows:
Bit Designation
32 Attention
33 Status modifier
34 Control-unit end
35 Busy
36 Channel end
37 Device end
38 Unit check
39 Unit exception 40 Program-controlled interruption
41 Incorrect length
42 Program check
43 Protection check
44 Channel-data check
45 Channel-control check
46 Interface-control check
47 Chaining check Count: Bits 48-63 form the
count for the last CCW used. UNIT STATUS residual
The following status indications are
generated by the I/O device or control
unit. The timing and causes of these
status indications for each type of
device are specified in the Sl publica­
tion for the device.
When the I/O device is accessible from
more than one channel, status due to
channel-initiated operations is signaled
to the channel that initiated the asso­
ciated I/O operation. The handling of
status not associated with I/O oper- Chapter 13. Input/Output Operations 13-63

ations, such as attention, unit
exception, and device end because of
transition from the not-ready to the
ready state, depends on the type of
device and situation and is specified in
the SL publication for the device. (See
"Device End" in this chapter.)
Attention
Attention is signaled when the device
detects an asynchronous condition that
is significant to the program. The
condition may also be described by other
status indications that accompany atten­
tion. Attention is interpreted by the
program and is not associated with the
initiation, execution, or conclusion of
an I/O operation.
The device can signal attention to the
channel when no operation is in progress
at the I/O device, control unit, or
subchannel. Attention can be signaled
with device end upon completion of an
operation, and it can be signaled to the
channel during the initiation of a new I/O operation. An I/O device may pres­
ent attention accompanied by device end
and unit exception when a not-ready-to­
ready-state transition is signaled.
(See "Device End" in this chapter.) The
handling and presentation of attention
to the channel depends on the type of
device.
When the device signals attention during
the initiation of an operation, the
operation is not initiated. Attention
causes command chaining and command
retry to be suppressed.
Status Modifier
Status modifier is generated by the
device when the device cannot provide
its current status in response to the TIO function, when the control unit is
busy, when the normal sequence of
commands has to be modified, or when
command retry is to be initiated.
When status modifier is signaled in
response to the TIO function and status
modifier is the only status bit that is
set to one, this indicates that the
device is unable to execute the TIO function and has not provided its
current status. The interruption condi­
tion, which may be pending at the device
or subchannel, has not been cleared, and
the CSW stored contains zeros in the
subchannel-key, CCW-address, and count
fields.
When the status-modifier bit in the CSW is set to one together with the bit, it indicates that the busy status
13-64 System/370 Principles of Operation pertains to the control unit associated
with the addressed I/O device. The
control unit appears busy when it is
executing a type of operation that
precludes the acceptance and execution
of any command or the instructions TEST I/O, HALT I/O, and HALT DEVICE or, for
some control units, when it contains an
interruption condition for a device
other than the one addressed. The
interruption condition may be due to
control-unit end, due to channel end
following execution of the CLRIO func­
tion, or, on a selector channel or
block-multiplexer channel operating with
multiplexing inhibited, due to channel
end following the execution of HALT I/O or HALT DEVICE. The busy state occurs
for operations such as backspace file,
in which case the control unit remains
busy after providing channel end, for
operations concluded by CLEAR I/O, and
for operations concluded on the selector
channel by HALT I/O or HALT DEVICE, and
temporarily occurs on control units such
as the IBM 3705 Communication Controller
after initiation of an operation on a
device accommodated by the control unit.
A control unit accessible from two or
more channels may appear busy when it is
communicating with another channel.
Presence of status modifier and device
end means that the normal sequence of
commands must be modified. The handling
of this status combination by the chan­
nel depends on the operation. If
command chaining is specified in the
current CCW and no unusual situations
have been detected, presence of status
modifier and device end causes the chan­
nel to fetch and chain to the CCW whose
storage address is 16 higher than that
of the current CCW. If the I/O device
signals status modifier at a time when
no command chaining is specified, or
when any unusual situations have been
detected, no action is taken in the
channel, and the status-modifier bit and
any other status bits presented by the
device are set to ones in the CSW. Status modifier is set to one in combi­
nation with unit check and channel end
to initiate the command-retry procedure. Control units that recognize special
conditions that must be brought to the
attention of the program present status
modifier along with other status indi­
cations in order to modify the meaning
of the status. The status presented is
unrelated to the execution of an I/O operation.
Control-Unit End
Control-unit end indicates that the
control unit has become available for
use for another operation.