CCW is on or unless the condition occurs in an im­
mediate operation. See the table in the chaining
section of this manual for the effect of the CD, CC,
and SLI flags on the indication of incorrect length.
Programming Note
The setting of the incorrect-length indication is un­
predictable in the CSW stored during CLEAR I/O. Program Check Program check occurs when programming errors are
detected by the channel. The condition can be due
to the following causes:
Invalid CCW Address Specification: The CA W or
the transfer-in-channel command does not designate
the CCW on integral boundaries for doublewords.
The three low-order bits of the CCW address are
not zero.
Invalid CCW Address: The channel has attempted
to fetch a CCW from a main-storage location which
is not available to the channel. An invalid CCW
address ean occur in the channel because the pro­
gram has specified an invalid address in the CAW or
in the transfer-in-channel command or because on
chaining the channel attempts to fetch a CCW from
an unavailable location.
Invalid Command Code: The command code in the
first CCW designated by the CAW or in a CCW
fetched on command chaining has four low-order
zeros. The command code is not tested for validity
during data chaining.
Invalid Count: A CCW other than a CCW specify­
ing transfer in channel contains the value zero in bit
positions 48-63.
Invalid IDA W Address Specification: Channel indi­
rect data addressing is specified, and the data ad­
dress does not designate the first IDAW on an inte­
gral word boundary.
Invalid IDA W Address: The channel has attempted
to fetch an IDA W from a main-storage location
which is not available to the channel. An invalid
ID A W address can occur in the channel because the
program has specified an invalid address in a CCW
that specifies indirect data addressing or because the
channel, IOn sequentially fetching IDA Ws, attempts
to fetch from an unavailable location.
234 System/370 Principles of Operation Invalid Data Address: The channel has attempted to
transfer data to or from a main-storage location
which is not available to the channel. An invalid
data address can occur in the channel because the
program has specified an invalid address in the
CCW, or in an IDA W, or because the channel, on
sequentially accessing storage, attempts to access an
unavailable location.
Invalid IDA W Specification: Bits 0-7 of the IDA W
are not all zeros, or the second or subsequent IDA W
does not specify the beginning or, for read-backward
operations, the ending byte of a 2,048-byte block.
Invalid CA W Fonnat: The CAW does not contain
zeros in bit positions 4-7.
Invalid CCW Format: A CCW other than a CCW
specifying transfer in channel does not contain zeros
in bit positions 38-39.
Invalid Sequence: The first CCW designated by the
CAW specifies transfer in channel or the channel
has fetched two successive CCW s both of which
specify transfer in channel.
Detection of the program-check condition during
the initiation of an operation causes execution of the
operation to be suppressed. When the condition is
detected after the device has been started, the device
is signaled to conclude the operation the next time it
requests or offers a byte of data. The program­
check condition causes command chaining to be
suppressed.
Protection Check
Protection check occurs when the channel attempts a
storage access that is prohibited by the protection
mechanism. Protection applies to the fetching of
CCWs, IDAWs, and output data, and to the storing
of input data. Storage accesses associated with each 1/0 operation are performed using the key provided
in the CAW associated with that operation.
When the protection-check condition occurs dur­
ing the fetching of a CCW that specifies the initia­
tion of an I/O operation, or occurs during the fetch­
ing of the first IDA W, the operation is not initiated.
When protection check is detected after the device
has been started, the device is signaled to conclude
the operation the next time it requests or offers a
byte of data. The condition causes command chain­
ing to be suppressed.

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