Programming Note
If a device becomes not ready upon completion of a
command, the ending interruption condition can be
cleared by TEST I/O without generation of unit
check due to the not-ready state, but any subsequent
TEST I/O issued to the device causes a unit-check
indication.
In order that sense indications set in conjunction
with unit check are preserved by the device until
requested by a sense command, some devices inhibit
certain functions until a command other than test I/O or no-operation is received. Furthermore, any
command other than sense, test I/O, or no­
operation causes the device to reset any sense in­
formation. To avoid degradation of the device and
its control unit and to avoid inadvertent resetting of
the sense information, a sense command should be
issued immediately to any device signaling unit
check. Unit Exception Unit exception is caused when the I/O device de­
tects a condition that usually does not occur. Unit exception includes conditions such as recognition of
a tape mark and does not necessarily indicate an
error. It has only one meaning for any particular
command and type of device.
The unit-exception condition cal\ be generated
only when the device is executing an I/O operation,
or when the device is involved with some activity
associated with an I/O operation and the condition
is of immedicate significance to the program. If the
device detects during the initiation sequence that the
operation cannot be executed, unit exception is pre­
sented to the channel and appears without channel
end, control unit end, or device end. Such unit sta­
tus indicates that no action has been taken at the
device in response to the command. If the condition
precluding normal execution of the operation occurs
after the execution has been started, unit exception
is accompanied by channel end, control unit end, or
device end, depending on when !he condition was
detected. Any unusual conditions associated with an
operation, but detected after device end has been
cleared, are indicated by signaling unit exception
with attention.
A command does not cause unit exception when
the device responds to the command during the ini­
tial selection with busy status.
Concluding an operation with the unit-exception
indication causes command chaining to be sup­
pressed.
Channel Status Conditions
The following conditions are detected and indicated
by the channel. Except for the conditions caused by
equipment malfunctioning, they can occur only while
the subchannel is involved with the execution of an I/O operation.
Program-Controlled Interruption
The program-controlled interruption condition is
generated when the channel fetches a CCW with the
program-controlled interruption (PCl) flag on. The
interruption due to the PCI flag takes place as soon
as possible after the CCW takes control of the opera­
tion but may be delayed an unpredictable amount of
time because of masking of the channel or other
activity in the system.
Detection of the PCI condition 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 re­
quested or offered by the I/O device. Incorrect
length is indicated for one of the following 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 main 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 main-storage
areas were exhausted. The count in the CSW is
zero.
Short Block on Input: The number of bytes trans­
ferred during a read, read-backward, or sense opera­
tion 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 trans­
ferred to the device. The count in the CSW is not zero.
The incorrect-length indication is suppressed
when the current CCW has the SLI flag and does
not have the CD flag. The indication does not occur
for immediate operations and for operations rejected
during the initiation sequence.
Presence of the incorrect-length condition sup­
presses command chaining unless the SLI flag in the Input/Output Operations 233

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.