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 byI/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 whenSTART 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, forTEST I/O, the subchannel on the multiplexor channel con
tains an interruption condition for the addressed de
vice. The instruction HALTI/O can cause the control
unit to be interrogated when issued to a device shar
ing a control unit and operating in the multiplex
mode.
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
terrogation.
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 anI/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 device address associated with control unit end
depends on the type ofI/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
Busy indicates that theI/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
modifier.
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
anI/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 deviceCU working
Interruption pend. in subchannel
for the addressed device
because of:
chaining terminated by
attention
other type of termination
Subchannel workingCSW STATUS STORED BY:
START TEST HALTI/O I/O I/O I/O INT.
B,cl
B
B,cl
B,SM
B,SM
--,cl
B
--,cl
B,SM
B,SM
,cl
--,cl
--,cl --,cl
--,cl B,cl
--,clCU 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 anI/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.
EachI/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
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
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
the control unit had responded with busy and status
modifier in the unit status byte.
control unit is still executing a previously initiated op
eration, but the sub channel is available or, for
tains an interruption condition for the addressed de
vice. The instruction HALT
unit to be interrogated when issued to a device shar
ing a control unit and operating in the multiplex
mode.
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
terrogation.
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
which the control unit may be busy temporarily and
which includes control unit end with busy and status
modifier.
The device address associated with control unit end
depends on the type of
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-
busy for initiation of new operations.
Busy
Busy indicates that the
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
modifier.
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
hyphen (--) indicates that the busy bit is off; an
asterisk
an
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.
DE or attention in device
Device working,
for another device
Interruption pend. in subchannel
for the addressed device
because of:
chaining terminated by
attention
other type of termination
Subchannel working
START TEST HALT
B,cl
B
B,cl
B,SM
B,SM
--,cl
B
--,cl
B,SM
B,SM
,cl
--,cl
--,cl
--,cl
--,cl
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
channel. The condition indicates that the sub channel
has become available for use for another operation.
Each
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