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considered 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 the
section "Status Modifier" earlier in this
chapter.
2. The control unit detected an unusual situation
during the portion of the operation after
channel end had been signaled to the channel.
The indication of the unusual situation
accompanies control-unit end.
If
the control unit remains busy with the
execution of an operation after signaling channel
end but has not detected any unusual situations and
has not been interrogated by the program, control-
unit end is not generated. Similarly, control-unit
end is not provided 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
temporary, 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 commensurate with the
program time required to handle an 1/
°
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.
Control-unit end can be signaled with channel
end, with device end, or between the two. When
control-unit end is signaled by means of an I/O
interruption in the absence of any other status, the
interruption may be identified by any address
assigned to the control unit. A control-unit end
may cause the control unit to appear busy for the
initiation of new operations with any attached
device. Alternatively, a control-unit end may be
assigned by the control unit to a specific device
address, and only that device would appear busy
for the initiation of new operations.
Busy
Busy indicates that the I/O device or control unit
cannot execute the command or instruction because
( 1)
it is executing a previously initiated operation,
(2)
it contains an interruption condition,
(3)
it is
shared by channels or 1/
°
devices and the shared
facility is not available, or
(4)
a self-initiated
function is being performed. The status associated
with the interruption condition for the addressed
device, if any, accompanies the busy status. If bus)
applies to the control unit, busy is accompanied by
status modifier.
The figure "Indications of Busy in CSW" lists
the situations for devices connected to only one
channel when the busy bit is set to one in the CSW
and when busy is accompanied by status modifier.
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.
Chapter 12. Input/Output Operations
12-49
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