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controlled-interruption (PCl) flag set to one. The
I/O interruption due to the PCI flag takes place as
soon as possible after the CCW takes control of the
operation but may be delayed an unpredictable
amount of time because I/O interruptions are
disallowed or because of other activity in the
system.
The interruption condition due to the PCI flag
does not affect the progress of the I/O operation.
Incorrect Length
Incorrect length occurs when the number of bytes
contained in the storage areas assigned for the I/O
operation is not equal to the number of bytes
requested or offered by the I/O device. Incorrect
length is indicated for one of the following reasons:
Long Block on Input: During a read, read-
backward, or sense operation, the device attempted
to transfer one or more bytes to storage after the
assigned storage areas were filled. The extra bytes
have not been placed in storage. The count in the
CSW is zero.
Long Block on Output: During a write or control
operation, the device requested one or more bytes
from the channel after the assigned storage areas
were exhausted. The count in the CSW is zero.
Short Block on Input: The number of bytes
transferred during a read, read-backward, or sense
operation is insufficient to fill the storage areas
assigned to the operation. The count in the CSW is
not zero.
Short Block on Output: The device terminated a
write or control operation before all information
contained in the assigned storage areas was
transferred to the device. The count in the CSW is
not zero.
Incorrect length is not indicated when the
current CCW has the SLI flag set to one and the
CD flag set to zero. The indication does not occur
for immediate operations and for operations
rejected during the initiation sequence.
When incorrect length occurs, command chaining
is suppressed, unless the SLI flag in the CCW is
one or unless the operation is immediate. See the
figure nChannel-Chaining Action n in this chapter
for the effect of the CD, CC, and SLI flags on the
indication of incorrect length.
Programming Note
The setting of incorrect length is unpredictable in
the CSW stored during CLEAR I/O.
Program Check
Program check occurs when programming errors are
detected by the channel. Program check can be
due to the following causes:
Invalid CCW-Address Specification: The CAW or
the transfer-in-channel command does not
designate the CCW on integral boundaries for
doublewords. The three rightmost bits of the CCW
address are not zeros.
CCW Location Not Provided: The channel has
attempted to fetch a CCW from a storage location
that is not provided. This may occur because the
program has specified in the CAW or in the
transfer-in-channel command a page address (bits
8-20) equal to or greater than the page-capacity
count (PCC), or because on chaining the channel
has attempted to fetch a CCW from a page with a
page address equal to PCC.
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
specifying transfer in channel contains the value
zero in bit positions 48-63.
Data Location Not Provided: The channel has
attempted to transfer data to or from a storage
location that is not provided. This may occur
because the program has specified in the CCW a
page address (bits 8-20) equal to or greater than
the page-capacity count (PCC) or because the
channel attempts during data transfer to access a
page with a page address equal to PCC.
Invalid CAW Format: 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 37-39.
Invalid Sequence: The first CCW designated by
the CAW specifies transfer in channel, or the
channel has fetched two successive CCWs both of
which specify transfer in channel.
Detection of program check during the initiation
of an operation causes execution of the operation
to be suppressed. When program check is detected
after the device has been started, the device is
Chapter 12. Input/Output Operations
12-53
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