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5. A device signaled I/O error alert.
The interface-control-check condition may also
include those channel-detected errors associated
with bus-in during data transfer that are not
indicated as channel-data check.
Detection of interface-control check causes the
current operation, if any, to be immediately
concluded.
Chaining Check
Chaining check is caused by channel overrun
during data chaining on input operations. Chaining
check occurs when the I/O data rate is too high to
be handled by the channel and by storage under
current conditions. Chaining check cannot occur
on output operations.
Chaining check causes the I/O device to be
signaled to conclude the operation. It causes
command chaining to be suppressed.
Contents of Channel-Status Word
The contents of the CSW depend on the reason the
CSW was stored and on the programming method
by which the information is obtained. The status
portion always identifies the reason the CSW was
stored. The subchannel-key, CCW-address, and
count fields may contain information pertaining to
the last operation or may be set to zero, or the
original contents of these fields at location 64 may
be left unchanged.
Information Provided
by
Channel-Status Word
Interruption conditions resulting from the execution
or conclusion of an operation at the sub channel
cause the whole CSW to be replaced. Such a CSW
can be stored only by an I/O interruption or by
TEST I/O or CLEAR I/O. Except for situations
associated with command chaining and equipment
malfunctioning, the storing can be caused by PCI
or channel end and by the execution of HALT I/O
or HALT DEVICE on the selector channel. The
contents of the CSW are related to the current
values of the corresponding quantities, although the
count is unpredictable after program check,
protection check, and chaining check, and after an
interruption due to the PCI flag.
A CSW stored upon the execution of a chain of
operations pertains to the last operation which the
channel executed or attempted to initiate.
Informatiop concerning the preceding operations is
not preserved and is not made available to the
program.
When an unusual situation causes command
chaining to be suppressed, the premature conclusion
of the chain is not explicitly indicated in the CSW.
A CSW associated with a conclusion due to a
situation occurring at channel-end time contains
channel end and identifies the unusual situation.
When the device signals the unusual situation with
control-unit end or device end, the channel-end
indication is not made available to the program,
and the channel provides the current subchannel
key, CCW address, and count, as well as the
unusual indication, with control-unit end or device
end in the CSW. The CCW-address and count
fields pertain to the operation that was executed.
When the execution of a chain of commands is
concluded by an unusual situation detected during
initiation of a new operation, the CCW -address
and count fields pertain to the rejected command.
Except for situations resulting from equipment
malfunctioning, conclusion at initiation time can
occur because of attention, unit check, unit
exception, or program check, and causes both the
channel-end and device-end bits in the CSW to be
set to zeros.
A CSW associated with status signaled after the
operation at the sub channel has been concluded
contains zeros in the subchannel-key,
CCW -address, and count fields, provided the status
is not cleared during START I/O or ST ART I/O
FAST RELEASE and provided logout pending is
not indicated. This status includes attention,
control-unit end, and device end (and channel end
when it occurs after the conclusion of an operation
on the selector channel by HALT I/O or HALT
DEVICE).
When the above status indications, other than
logout pending, are cleared during START I/O or
START I/O FAST RELEASE, only the status
portion of the CSW is stored, and the original
contents of the subchannel-key, CCW-address,
deferred-condition-code, logout-pending, and count
fields in location 64 are preserved. Similarly, only
the status bits of the CSW are changed when the
command is rejected or the operation at the
subchannel is concluded during the execution of
START I/O or START I/O FAST RELEASE or
whenever HALT I/O or HALT DEVICE causes
CSW status to be stored.
Errors detected during execution of the I/O
operation do not affect the validity of the CSW
unless channel-control check or interface-control
check are indicated. Channel-control check
indicates that equipment errors have been detected
which can cause any part of the CSW, as well as
the
110
address, to be invalid. Interface-control
check indicates that the address identifying the
Chapter 12. Input/Output Operations
12-55
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