Channel Data Check
Channel data check indicates that a machine error
has been detected in the information transferred to
or from main storage during an I/O operation, or
that a parity error has been detected on the data on
bus-in during an input operation. This information
includes the data read or written, as well as the in­
formation transferred as data during a sense or con­
trol operation. The error may have been detected
anywhere inboard of the I/O interface: in the chan­
nel, in main storage, or on the path between the two.
Channel data check may be indicated for data with
an invalid checking block code (CBC) in main stor­
age when that data is referred to by the channel but
does not participate in the operation.
Whenever a parity error on I/O input data is indi­
cated by means of channel data check, the channel
forces correct parity on all data received over the I/O interface, and all data placed in main storage
has valid CBC. When, on an input operation, the
channel attempts to store less than a complete
checking block, and invalid CBC is detected on the
checking block in storage, the contents of the loca­
tion remain unchanged, with invalid CBC. On an
output operation, whenever a channel data check is
indicated, all bytes that came from a checking block
with invalid CBC have been transmitted on the in­
terface with parity errors.
A condition indicated as channel data check caus­
es command chaining to be suppressed, but does not
affect the execution of the current operation. Data
transfer proceeds to normal completion, if possible,
and an I/O interruption condition is generated when
the device presents channel end. A logout may be
performed, depending on the channel. Accordingly,
the detection of the error may affect the state of the
channel and the device.
Channel Control Check
Channel control check is caused by any machine
malfunctioning affecting channel controls. The con­
dition includes invalid CBC on CCW and data ad­
dresses and invalid CBC on the contents of the
CCW. The condition also includes those channel­
detected errors associated with data transfer that are
not indicated as channel data check, as well as those I/O interface errors detected by the channel that are
not indicated as interface control check. Conditions
responsible for channel control check may cause the
contents of the CSW to to invalid and conflicting.
The CSW as generated by the channel has valid
CBC.
Detection of the channel-control-check condition
causes the current operation, if any, to be immedi­
ately concluded.
In some situations, the channel-control-check
condition may be reported as an external-damage or
system-damage machine-check condition. Channel
control check is set whenever CSW bit S, logout
pending, is set.
Interface Control Check
Interface control check indicates that an invalid sig­ nal has occurred on the I/O interface. The condi­
tion is detected by the channel and ususally indicates
malfunctioning of an I/O device. It can be due to
the following reasons:
1. The address or status byte received from a de­
vice has invalid parity.
2. A device responded with an address other than
the address specified by the channel during
initiation of an operation.
3. During command chaining the device appeared
not operational.
4. A signal from a device occurred at an invalid
time or had invalid duration.
S. A device signaled I/O error alert by raising the
disconnect-in-line on the I/O interface.
Detection of the interface control check condition
causes the current operation, if any, to be immedi­
ately concluded.
Chaining Check
Chaining check is caused by channel overrun during
data chaining on input operations. The condition
occurs when the I/O data rate is too high for the
particular resolution of data addresses. Chaining
check cannot occur on output operations.
Detection of the chaining-check condition causes
the I/O device to be signaled to conclude the opera­
tion. It causes command chaining to be suppressed.
Contents of Channel Status Word
The contents of the CSW depend on the condition
causing the storing of the CSW and on the program­
ming method by which the information is obtained.
The status portion always identifies the condition
that caused storing of the CSW. The protection key,
command 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
Conditions associated with the execution or conclud­
ing of an operation at the subchannel cause the
whole CSW to be replaced. Such a CSW can be
stored only by an I/O interruption or by TEST I/O Input/Output Operations 235

or CLEAR I/O. Except for conditions associated
with command chaining and equipment malfunction­
ing, the storing can be caused by the PCI or channel­
end condition and by the execution of HALT I/O or
HALT DEVICE on the selector channel. The con­
tents of the CSW are related to the current values of
the corresponding quantities, although the count is unpredietable after program check, protection check,
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 thc channel
executed or attempted to initiate. Information con­
cerning the preceding operations is not preserved
and is not made available to the program.
When an unusual condition causes command
chaining to be suppressed, the premature concluding
of the chain is not explicitly indicated in the CSW. A CSW associated with a concluding due to a condi­
tion occurring at channel-end time contains the
channel-end bit and identifies the unusual condition.
When the device signals the unusual condition with
control unit end or device end, the channel-end indi­
cation is not made available to the program, and the
channel provides the current protection key, com­
mand address, and count, as well as the unusual indi­
cation, with the control-unit-end or device-end bit in
the CSVv. The command address and count fields
pertain to the operation that was t1xecuted. When the execution of a chain of commands is
concluded by an unusual condition detected during
initiation of a new operation, the command address
and count fields pertain to the rejected command.
Except for conditions caused by equipment malfunc­
tioning, concluding at the initiation time can occur of attention, unit check, unit exception, or
program check, and causes both the channel-end and
device-end bits in the CSW to be turned off.
A CSW associated with conditions occurring after
the operation at the sub channel has been concluded
contains zeros in the protection key, command ad­
dress, and count fields, provided the conditions are
not cleared during START I/O or START I/O FAST RELEASE and provided the logout-pending
condition is not indicated. These conditions include
attention, control unit end, and device end (and
channel end when it occurs after the concluding of
an operation on the selector channel by HALT I/O or HALT DEVICE).
When the above conditions, 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 pro­
tection key, command address, and count fields in
location 64 are preserved. Similarly, only the status
236 System/370 Principles of Operation
bits of the CSW are changed when the command is
rejected or the operation at the subchannel is con­
cluded during the execution of ST ART 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 oper­
ation do not affect the validity of the CSW unless
the channel-control-check or interface-control-check
conditions are indicated. Channel control check
indicates that equipment errors have been detected
which can cause any part of the CSW, as well as the
address in the PSW identifying the I/O device, to be
invalid. Interface control check indicates that the
address identifying the device or the status bits re­
ceived from the device may be invalid. The channel
forces correct parity on invalid CSW fields.
When any I/O instruction cannot be executed
because of a pending logout condition which affects
the operational capability of the channel or subchan­
nel, a full CSW is stored. The fields in the CSW are
all set to zeros, with the exception of the logout­
pending bit and the channel-control-check bit, which
are set to ones.
Protection Key
A CSW stored to reflect the progress of an operation
at the subchannel contains the protection key used
in that operation. The contents of this field are not
affected by programming errors detected by the
channel or by the condition causing termination of
the operation.
Command Address
When the CSW is formed to reflect the progress of
the I/O operation at the subchannel, the command
address is normally eight higher than the address of
the last CCW used in the operation.
The following table lists the contents of the com­
mand address field for all conditions that can cause
the CSW to be stored. The conditions are listed in
order of priority; that is, if two conditions are indi­
cated or occur, the CSW appears as indicated for the
condition higher on the list. The programming er­
rors listed in the table refer to conditions included in
program check. When a CSW has been stored and
the situation exists that a command retry request has
been recognized but the CCW has not been re­
executed, "last-used CCW + 8" is the CCW that is
to be retried.
Count
The residual count, in conjunction with the original
count specified in the last CCW used, indicates the
number of bytes transferred to or from the area des­
ignated by the CCW. When an input operation is