The fields in the csw are allocated for the following
purposes: Protection Key: Bits 0-3 form the storage protection
key used in the chain of operations initiated by the
last START I/O. Command Address: Bits 8-31 form an 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 are obtained over the I/O interface and
indicate conditions detected by the device or the con­
trol unit. Bits 40-47 are provided by the channel and
indicate conditions associated with the subchannel.
Each of the 16 bits represents one type of condition,
32 Attention 40 Pro gram -con trolled
33 Status modifier 41 Incorrcct length
34 Control unit end 42 Program check
35 Busy 43 Protection check
36 Channel end 44 Channel data check
37 Device end 45 Channel control check
38 Unit cheek 46 Interface control check
39 Unit exception 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 de­
vice or control unit and are indicated to the channel
over the I/O interface. The timing and causes of these
conditions for each type of device are specified in the SRL publication for the device.
When the I/O device is accessible from more than
one subchannel, status is signaled to the sub channel
that initiated the associated I/O operation. The han­
dling of conditions not associated with I/O operations
depends on the type of device and condition and is
specified in the SRL publication for the device.
The channel does not modify the status bits received
from the I/O device. These bits appear in the csw as
received over the interface.
Attention is caused upon the generation of the atten­
tion signal at the I/O device. The attention signal can
be generated at any time and is interpreted by the
program. Attention is not associated with the initiation,
execution, or termination of any I/O operation.
The attention condition cannot be indicated to the
program while an operation is in progress at the I/O device, control unit, or subchannel. Otherwise, the
handling and presentation of the condition to the chan­
nel depend on the type of device.
Status Modifier Status modifier is generated by the device when the
normal sequence of commands has to be modified or
when the control unit detects during the selection
sequence that it cannot execute the command or in­
struction as specified.
When the status-modifier condition is provided in
response to TEST I/O, presence of the bit indicates that
the device cannot execute the instruction and that no
bits pertaining to the current status of the device have
been provided. The status of the device and subchan­
nel is not changed, and the csw stored by TEST I/O contains zeros in the key, command address, channel
status, and count fields. The 2702 Transmission Con­ trol is an example of a type of device that cannot
execute TEST I/O. When the status-modifier bit appears in the csw to­
gether 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 or is in
a state that precludes the acceptance of any command
or the instruction TEST I/O and HALT I/O. This occurs
for operations such as backspace tape file, in which
case the control unit remains busy after providing
channel end, and for operations terminated on the
selector channel by HALT I/O. The combination of
busy and status modifier can be provided in response
to any command as well as TEST I/O and HALT I/O. Presence of both busy and status modifier in response
to TEST I/O is handled the same way as when status
modifier alone is on. Once the execution of a command has been initi­
ated, the status-modifier indication can be provided
only together with device end. 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, pres­
ence of the bit 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 I/O device signals
the status-modifier condition at a time when the chain­
command flag is off or when any unusual conditions
have been detected, no action is taken in the channel,
and the status-modifier bit is placed in the csw.
Programming Note
When the multiplexor channel detects a programming
error during command chaining, the interruption con­
dition is queued at the I/O device. On devices such as
the 2702 Transmission Control, queuing of the con­
dition may generate the status-modifier indication,
which subsequently appears in the csw associated
with the termination of the operation. Input/Output Operations 109
Control Unit End
Control unit end indicates that the control unit has
become available for use for another operation.
The control-unit-end condition is provided only by
control units shared by I/O devices and only when one
or both of the following conditions has occurred:
1. The program has caused the control unit to be
interrogated while the control unit was executing an
operation. The control unit is considered to have been
interrogated when START I/O, TEST I/O, or HALT I/O has 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. START I/O and TEST I/O cause interrogation of the control unit when the
control unit is still executing a previously initiated op­
eration, but the sub channel is available or, for TEST I/O, the subchannel on the multiplexor channel con­
tains an interruption condition for the addressed de­
vice. The instruction HALT I/O can cause the control
unit to be interrogated when issued to a device shar­
ing a control unit and operating in the multiplex
2. The control unit detected an unusual condition
during the portion of the operation after channel end
had been signaled to the channel.
If the control unit remains busy with the execution
of an operation after signaling channel end but 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 modi­
fier in the response to the instruction causing the in­
When the busy state of the control unit is tem­
porary, control unit end is included with busy and
status 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 short with respect to 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
The device address associated with control unit end
depends on the type of I/O device. The address can be
fixed for the control unit, may identify the device on
which the terminated operation was executed, or may
be the device address specified in the instruction caus­
ing the control unit to be interrogated.
The control-unit-end condition can be signaled with
channel end, device end, or between the two. A pend- 110 ing control unit end causes the control unit to appear
busy for initiation of new operations.
Busy indicates that the I/O device or control unit can­
not execute the command or instruction because it is
executing a previously initiated operation or because
it contains an interruption condition. The interruption
condition for the addressed device, if any, accompa­
nies the busy indication. If the busy condition applies
to the control unit, busy is accompanied by status
The following table lists the conditions when the
busy bit (B) appears in the csw and when it :is ac­
companied by the status-modifier bit (SM). A double
hyphen (--) indicates that the busy bit is off; an
asterisk indicates that csw status is not stored or
an I/O interruption cannot occur; and the (cl) indi­
cates that the interruption condition is cleared and
the status appears in the csw. The abbreviation DE
stands for device end, while cu stands for control unit. CONDITION Subchannel available
DE or attention in device
Device working, CU available CU end or channel end in CU: for the addressed device
for another device CU working
Interruption pend. in subchannel
for the addressed device
because of:
chaining terminated by
other type of termination
Subchannel working CSW STATUS STORED BY:
--,cl --,cl
--,cl B,cl
--,cl CU available CU working B,SM The busy bit is included in the status associated
with a pending interruption condition from the sub­
channel only when a chain of commands has been
prematurely terminated because of attention and no
interruption was pending in the channel at the time
of chaining.
Channel End
Channel end is caused by the completion of the por­
tion of an I/O operation involving transfer of data or control information between the I/O device and the
channel. The condition indicates that the sub channel
has become available for use for another operation.
Each I/O operation causes a channel-end condition
to be gencrated, and there is only one channel end for
an operation. When command chaining takes place,
only the channel end of the last operation of the chain
is made available to the program. The channel-end
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