Defe"ed Condition Code (CC): Bits 6 and 7 indi­
cate whether conditions have been encountered sub­
sequent to the setting of a condition code 0 for
START I/O FAST RELEASE 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
o
o
Bit 7
o
1
o
Meaning Normal I/O interruption
Deferred condition code is 1
(Reserved)
Deferred condition code is 3
CCW Address: Bits 8-31 form an absolute address
that is eight higher than the address of the last CCW
used.
Status: Bits 32-47 identify the conditions in the de­
vice and the channel that caused the storing of the
CSW. Bits 32-39, the unit status, are obtained over
the I/O interface and indicate conditions detected
by the device or the control unit. Bits 40-47, the
channel status, are provided by the channel and indi­
cate conditions associated with the subchannel. Each
of the 16 bits represents one type of condition, 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 I ncorrect length 42 Program check
43 Protection check
44 Channel data check
45 Channel control check
46 I nterface control check
47 Chaining check
Count: Bits 48-63 form the residual count for the
last CCW used.
Unit Status Conditions
The following conditions are detected by the I/O device or control unit and are indicated to the chan­
nel over the I/O interface. The timing and causes of
these conditions for each type of device are specified
in the SL or SRL publication 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 associated I/O operation. The handling of condi­
tions not associated with I/O operations, such as
attention or device end due to transition from the
not-ready to the ready state, depends on the type of
device and condition and is specified in the SL or
SRL publication for the device.
The channel does not modify the status bits re­
ceived from the I/O device. These bits appear in the
CSW as received over the interface.
Attention
Attention is generated when the device detects an
asynchronous condition that is significant to the
program. The condition is interpreted by the pro­
gram and is not associated with the initiation, execu­
tion, or concluding of an I/O operation.
The device can signal the attention condition to
the channel when no operation is in progress at the I/O device, control unit, or subchannel. Attention
can be indicated with device end upon completion of
an operation, and it can be presented to the channel
during the initiation of a new I/O operation. Other­ wise, the handling and presentation of the condition
to the channel depends on the type of device.
When the device signals attention during the initi­
ation of an operation, the operation is not initiated.
Attention accompanying device end causes com­
mand chaining to be suppressed.
Status Modifier
Status modifier is generated by the device when the
device cannot provide its current status in response
to TEST I/O, when the control unit is busy, when
the normal sequence of commands has to be modi­
fied, or when command retry is to be initiated.
When the status-modifier condition is signaled in
response to TEST I/O and the bit appears in the
CSW in the absence of any other status bit, presence
of the bit indicates that the device cannot execute
the instruction and has not provided its current sta­
tus. The interruption condition, which may be pend­
ing at the device or subchannel, has not been
cleared, and the CSW stored by TEST I/O contains
zeros in the key, command address, and count fields.
The 2702 Transmission Control is an example of a
type of device that cannot execute TEST I/O. When the status-modifier bit appears in the CSW
together with the busy bit, it indicates that the busy
condition 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 com-
Input/Output Operations 229

mand or the instructions TEST I/O, HALT I/O, and HALT DEVICE or when it contains an inter­
ruption condition for a device other than the one
addressed. The interruption condition may be due to
control unit end, due to channel end following the
execution of CLEAR I/O, or, on the selector chan­
nel, due to channel end following the execution of
HALT I/O or HALT DEVICE. The busy state oc­
curs for operations such as backspace tape file, in
which ease the control unit remains busy after pro­
viding 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 the 2702 Transmission
Control after initiation of an operation on a device
accommodated by the control unit. A control unit
accessible from two or more channels appears busy
when it is communicating with another channel. Presence of the status modifier and device end
means that the normal sequence of commands must
be modified. The handling of this set of bits by the channel depends on the operation. If command
chaining is specified in the current CCW and no
unusual conditions have been detected, presence of
status modifier and device end causes the channel to
fetch and chain to the CCW whose main-storage
address is 16 higher than that of the current CCW.
If the 1/ ° device signals the status modifier condi­
tion at a time when no command chaining is speci­
fied, or when any unusual conditions have been de­
tected, 1110 action is taken in the channel, and the
status modifier bit is placed in the CSW. Status modifier is presented in combination with
unit check and channel end to initiate the command
retry procedure.
Control Unit End
Control unit end indicates that the control unit has
become available for use for another operation.
The eontrol-unit-end condition is provided only
by control units shared by 1/ ° devices or control
units aceessible by two or more channels, and only
when one or both of the following conditions have
occurred:
1. The program had previously caused the control
unit to be interrogated while the control unit
was in the busy state. The control unit is con­
sidered to have been interrogated in the busy
state when a command or the instructions TEST I/O, HALT I/O, or HALT DEVICE
had been issued to a device on the control unit,
and the control unit had responded with busy
and status modifier in the unit status byte. See "Status Modifier." 230 System/370 Principles of Operation 2. The control unit detected an unusual condition
during the portion of the operation after chan­
nel end had been signaled to the channel. The
indication of the unusual condition accompa­
nies control unit end.
If the control unit remains busy with the execu­
tion of an operation after signaling channel end but
has not detected any unusual conditions and has not
been interrogated by the program, control unit end is
not generated. Similarly, control unit end is not pro­
vided when the control unit has been interrogated
and could perform the indicated function. The latter
case is indicated by the absence of busy and status
modifier in the response to the instruction causing
the interrogation.
When the busy state of the control unit is tempo­
rary, control unit end is included with busy and sta­
tus modifier in response to the interrogation even
though the control unit has not yet been freed. The
busy condition is considered to be temporary if its
duration is commensurate with the program time
required to handle an I/O interruption. The 2702 Transmission Control is an example of a device in
which the control unit may be busy temporarily and
which includes control unit end with busy and status
modifier.
The control unit end condition can be signaled
with channel end, device end, or between the two.
When control unit end is signaled by means of an 1/ ° interruption in the absence of any other status
conditions, the interruption may be identified by any
address assigned to the control unit. A pending con­
trol unit end causes the control unit to appear busy
for initiation of new operations.
Busy
Busy indicates that the I/O device or control unit
cannot execute the command or instruction because
it is executing a previously initiated operation or
because it contains a pending interruption condition.
The interruption condition for the addressed device,
if any, accompanies the busy indication. If the busy
condition applies to the control unit, busy is accom­
panied by status modifier.
The following table lists the conditions for devices
connected to only one channel when the busy bit
appears in the CSW and when it is accompanied by
the status-modifier bit. For devices shared by more
than one channel, operations related to one channel
may cause the control unit or device to appear busy
to the other channels.