Control-unit
control units
control units
channels, and
the following
end is provided only by
shared by I/O devices or
accessible by two or more
only when one or both of
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
considered to have been interro­
gated in the busy state when a
command or the instructions START I/O, START I/O FAST RELEASE (when
not executed independent of the
device), 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 unusu­
al condition 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 indi­
cated 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 I/O interruption.
The IBM 3705 Communications Controller 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. Control-unit end may be
signaled at other times and may be
accompanied by other status bits. 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 device address
assigned to the control unit which is
associated with a device in the avail­
able state, even if the device is not
ready or absent. 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 initi­
ation of new operations.
When control-unit end is signaled to the
channel in the absence of any other
status to indicate that the control-unit
busy period previously indicated to the
program is ended, and the control unit
is available, the control-unit-end
status normally causes the channel to
recognize an interruption condition to
present the control-unit end to the
program. However, when start-I/O-fast
queuing or the suspend-and-resume facil­
ity is provided and the device address
with which the control unit signals the
control-unit end is associated with a working subchannel that has a pending I/O operation or has a suspended
channel-program execution, the channel
recognizes the channel-available­
interruption (CAl) condition instead.
The control-unit-end status is discarded
by the channel and the state of the
associated subchannel remains unchanged
in this case. (See the section
"Channel-Available Interruption," earli­
er in this chapter.)
Busy indicates that the I/O device or
control unit cannot execute the command
or instruction because (1) it is execut­
ing a previously initiated operation,
(2) it contains an interruption condi­
tion, (3) it is shared by channels or I/O devices and the shared facility is
not available, or (4) a self-initiated
function is being performed. The status
associated with the interruption condi­
tion for the addressed device, if any,
accompanies the busy status. If busy
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
indicates 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 13. Input/Output Operations 13-65

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 condition in
subchannel for the addressed
device because of: Chaining terminated by busy
device Chaining or retry terminated
by busy CU Other type of termination
Asynchronous status~ Subchannel working CU available CU working
Explanation:
B Busy bit in CSW is one. SID, SIOF*, or RIO* B,cl
B
B,cl
&
B,SM * B,el * * CSW Status Stored by HIO or I/O TIO CLRIO+ HDV IRPTn NB,cl~ * B~ * NB,cl~ * $~ * B,SM~ * B,cl B,cl
B,SM,cl B,SM,cl
NB,cl
NB,cl * * NB,cl
NB,cl
NB
NB * * * * * * * NB
B,SM
NB,cl
m
NB,cl
NB,el
B,SM
B,cl
B,SM,cl
NB,el
NB,el= * * cl Interruption condition cleared; status is placed in CSW. CU Control unit.
DE Device end. HB Busy bit in CSW is zero.
SM Status-modifier bit in CSW is one. * CSW not stored, or I/O interruption cannot occur. * When a channel executes START I/O FAST RELEASE as START I/O, the CSW status stored for the two instructions is identical. When START I/O FAST RELEASE is executed independent of the device and when RESUME I/O is executed, the CSW status is stored by an I/O interruption with
the CSW also indicating deferred condition code 1, except when start­ I/O-fast queuing is provided for the subehannel. When start-I/O-fast
queuing is provided, a control-unit-busy or device-busy condition, in
the absence of other status, may not cause an interruption and, in­
stead, the I/O operation remains pending at the subchannel until the
no-longer-busy indication is received by the channel.
= When the device presents asynchronous status other than control-unit
end while a channel program is suspended at the subchannel, the
channel program is terminated, and an interruption condition is
generated; the status, with the busy bit included, is stored in the CSW when the interruption occurs, along with the deferred condition
code equal to zero and the command address equal to the address of
the suspended CCW + 8.
Indications of Busy in CSW (Part 1 of 2)
13-66 System/370 Principles of Operation