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

Device lEnd
Device end is caused by the completion of an I/O operation at the device or, on some devices, by man­
ually changing the device from the not-ready to the
ready st.ate. The condition normally indicates that
the I/O device has become available for use for an­
other operation.
Each I/O operation causes a device-end condi­
tion, and there is only one device end to an opera­
tion. The device-end condition is not generated
when any programming or equipment malfunction is
detected during initiation of the operation. When
command chaining takes place, only the device end
of the last operation of the cJ1ain is made available to
the program unless an unusual condition is detected
during the initiation of a chained command, in which
case the chain is concluded without the device-end
indication.
The device-end condition associated with an I/O operation is generated either simultaneously with the
channel·-end condition or later. On data-transfer
operations on devices such as magnetic tape units,
the device concludes the operation at the time chan­
nel end is generated, and both device end and chan­
nel end occur together. On buffered devices, such as
an IBM 3211 Printer Model 1, the device-end condi­
tion occurs upon completion of the mechanical oper­
ation. For control operations, device end generat­
ed at the completion of the operation at the device.
The operation may be completed at the time channel
end is generated or later.
When command chaining is specified in the sub­
channel, receipt of the device-end signal, in the ab­
sence of any unusual conditions, causes the Ghannel to initiate a new I/O operation. Unit Check Unit check indicates that the I/O device or control
unit has detected an unusual condition that is de­
tailed by the information avai!able to a sense com­
mand. Unit check may indicate that a programming
or an equipment error has been detected, that the
not-ready state of the device has affected the execu­
tion of the command or instruction, or that an ex­
ceptional condition other than the one identified by
unit exception has occurred. The unit-check bit
provides a summary indication of the conditions
identified by sense data.
An error condition causes the unit-check indica­
tion only when it occurs during the execution of a
command or TEST I/O, or during some activity
associated with an I/O operation. Unless the error
condition pertains to the activity initiated by a com­
mand and is of immediate significance to the pro­
gram, the condition does not cause the program to
232 System/370 Principles of Operation
be alerted after device end has been cleared; a mal­ funCtion may, however, cause the device to become
not ready. U nit check is indicated when the existence of the
not-ready state precludes a satisfactory execution of
the command, or when the command, by its nature,
tests the state of the device. When no interruption
conditiop is pending for the addressed device at the
control unit, the control unit signals unit check when
TEST I/O or the no-operation control command is
issued to a not-ready device. In the case of no­
operation, the command is rejected, and channel end
and device end do not accompany unit check. Unless the command is designed to cause unit
check, such as rewind and unload on magnetic tape,
unit check is not indicated if the command is proper­
ly executed even though the device has become not
ready during or as a result of the operation. Similar­
ly, unit check is not indicated if the command can be
executed with the device not ready. The IBM 2150 Console, for example, accepts and executes the
alarm control order when the printer is not ready.
Selection of a device in the not-ready state does not
cause a unit-check indication when the sense com­
mand is issued, and whenever an interruption condi­
tion is pending for the addressed device at the con­
trol unit.
If the device detects during the initiation sequence
that the command cannot be executed, unit check is
presented to the channel and appears without chan­
nel end, control unit end, or device end. Suchunit
status indicates that no action has peen taken at the
device in response to the command. If the condition
precluding proper execution of the operation occurs
after execution has been started, unit check is accom­
panied by channel end, control unit end, or device
end, depending on when the condition was detected.
Any errors associated with an operation, but detect­
ed after device end has been cleared, are indicated
by signaling unit check with attention.
Errors, such as invalid command code or invalid
command code parity, do not cause unit check when
the device is working or contains a pending interrup­
tion condition at the time of selection. Under these the device responds by providing the
busy bit and indicating the pending interruption con­
dition, if any. The command code invalidity is not
indicated.
Concluding of an operation with the unit-check
indication callses command chaining to be pressed. Unit check is presented in combination with chan­
nel end and status modifier to initiate the command
retry procedure.