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.

Condition SIO or SIOF=I Subchannel available DE or attention in device B,eI
Device working, CU available B
CU end or channel end in CU:
for the addressed device B,eI
for another device B,SM
CU working B,SM Interruption pending in subchannel for the
addressed device because of:
chaining terminated by busy condition
other type of termination Subchannel working
CU available CU working Explanation: B Busy bit appears in CSW. cl Interruption condition cleared; status appears in CSW.
CU Control unit.
DE Device end.
SM Status-modifier bit appears in CSW.
CSW not stored, or I/O interruption cannot occur.
Busy bit is off.
CSW Status Stored By TIO CLRIO+ HIO or HDV I/O Interrupt #
-,eI -,cl B
-,eI -,eI
B,SM -,eI
B,SM B,SM
B,eI -,eI B,eI
-,eI -,eI -,cl B,SM =I 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 independently of the device, the same status is stored by an I/O interruption
with the CSW also indicating deferred condition code 1.
# Except when the I/O interruption is caused by a deferred condition code 1 for
START I/O FAST RELEASE.
+ The entries in this column apply only when the CLRIO function is executed. When
CLEAR I/O is executed as{ TEST I/O, the entries in the TIO column apply. Indications of Busy in CSW 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 condi­
tion to be generated, and there is only one Channel
End for an operation. The channel-end condition is
not generated when programming errors or equip­
ment malfunctions are detected during initiation of
the 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 condition is not made available to the program
when a chain of commands is prematurely concluded
because of an unusual condition indicated with con­
trol unit end or device end or during the initiation of
a chained command.
The instant within an I/O operation when chan­
nel end is generated depends on the operation and
the type of device. For operations such as writing on
magnetic tape, the channel-end condition occurs
when the block has been written. On devices that
verify the writing, channel end mayor may not be
delayed until verification is performed, depending on
the device. When magnetic tape is being read, the
channel-end condition occurs when the gap on tape
reaches the read-write head. On devices equipped
with buffers, such as the IBM 3211 Printer Model 1,
the channel-end condition occurs upon completion
of data transfer between the channel and the buffer.
During control operations, channel end is generated
when the control information has been transferred to
the devices, although for short operations the condi­
tion may be delayed until completion of the opera­
tion. Operations that do not cause any data to be
transferred can provide the channel-end condition
during the initiation sequence.
A channel-end condition pending in the control
unit causes the control unit to appear busy for initia­
tion of new operations.
Channel end is presented in combination with
status modifier and unit check to initiate the com­
mand retry procedure. Input/Output Operations 231