channel end, control end, or device end. Such unit
status indicates that no action has bcen 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 de­
vice end, depending on when the condition was de­
tected.
Termination of an operation with the unit-exception
indication causes command chaining to be suppressed. Channel Status Conditions The following conditions are detected and indicated by
the channel. Except for the conditions caused by equip­
ment ma1functioning, 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 gen­
erated when the channel fetches a ccw with the pro­ gram-controlled-interruption (PCI) flag on. The inter­
ruption due to the PC! flag takes place as soon as
possible after fetching the ccw 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 con­
tained in the storage areas assigned for the I/O oper­
ation 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: l.Jong blocl( 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. IJong 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 hackward, or sense oper­
ation 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 con­ tained in the assigned storage areas was transferred to
the device. The count in the csw is not zero.
The incorrect-length indication is suppressed when
112
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 ini­
tiation sequence.
Presence of the incorrect-length condition suppresses
command chaining unless the SLI flag in the ccw is on
or unless the condition occurs in an immediate oper­
ation.
The following table lists the effect of the incorrect­
length condition for all combinations of the CD, CC, and SLI flags. It indicates for the two types of oper­
ations when the operation at the sub channel is termi­
nated (stop) and when the command chaining takes
place. The entry "incorrect length" (IL) means that
the indication is made available to the program; a
double hyphen (--) means that the indication is sup­
pressed. For all entries, the current operation is as­
sumed to have caused the incorrect-length condition.
FLAGS ACTION AND INDICATION
CD CC SLI HEGULAB OPEBA TION IMMEDIATE OPEHATION 0 0 0 Stop,IL Stop, -- 0 0 1 Stop, --Stop, -- 0 1 0 Stop,IL Chain command 0 1 1 Chain command Chain command
1 0 0 Stop,IL Stop, --
I 0 1 Stop,IL Stop, --
I 1 0 Stop,IL Stop, --
I 1 1 Stop,IL Stop, --
Program Check
Program check occurs when programming errors are
detected hy the channel. The condition can be due to
the following causes:
Invalid C CW Address Specification: The CAW or the
transfer-in-channel command does not designate the
ccw on integral boundaries for double words. 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 location outside the main stor­
age of the particular installation. An invalid ccw ad­ dress can occur in the channel because the program
has specified an invalid address in the CAW or in the
transfer-in-channel command or because on chaining
the channel has stepped the address above the highest
available 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 com­
mand code is not tested for validity during data chain­
ing.
Invalid Count: A ccw other than a ccw specifying
transfer in channel contains the value zero in bit po­ sitions 48-63.
Invalid Data Address: The channel has attempted
to transfer data to or from a location outside the main
storage of the particular installation. An invalid data

address can occur in the channel because the program
has specified an invalid address in the ccw or because
the channel has stepped the address above the highest
available address or, on reading backward, below zero.
Invalid Key: The CAW contains a nonzero storage
protection key in a model not having the protection
feature installed.
Invalid CAW Format: The CAW does not contain
zeros in bit positions 4-7.
Invalid CCW Format: A ccw other than a ccw spe­
cifying transfer in channel does not contain zeros in
hit positions 37-39.
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 de­
tected after the device has been started, the device is
signaled to terminate the operation. The program­
check condition causes command chaining to be sup­
pressed.
Protection Check Protection check occurs when the channel attempts to
place data in a portion of main storage that is protect­
ed for the current operation on the subchannel. The
protection key associated with the I/O operation does
not match the key of the addressed main-storage lo­
cation, and neither of the keys is zero.
Detection of the protection-check condition causes
the device to be signaled to terminate the operation;
command chaining is suppressed.
The protection-check condition can be generated
only on models having the protection feature installed.
Channel Data Check Channel data check is caused by data errors detected
in the channel or in main storage. The condition covers
all data transferred to or from an I/O device, including
sense and control information. It includes any parity
errors detected on I/O data in main storage, in the
channel, or as received from the device over the I/O interface.
The channel attempts to force correct parity on data
placed in main storage. On output operations, the par­
ity of data sent to the device is not changed. Parity errors on data cause command chaining to be
suppressed and, depending on model, may cause the
current operation to be terminated. When the channel
and the CPU share common equipment, parity errors on
data may cause malfunction reset to be performed. The
recovery procedure in the channel and the subsequent
state of the subchannel upon a malfunction reset de­
pend on the model.
Channel Control Check Channel control check is caused by any machine mal­
functioning affecting channel controls. The condition
includes parity errors on ccw and data addresses and
parity errors on the contents of the ccw. Conditions
responsible for channel control check usually cause the
contents of the csw to be invalid and conflicting.
The csw as generated by the channel has correct
parity. The channel either forces correct parity on the
csw fields or sets the invalid fields to zero.
Detection of the channel-control-check condition
causes the current operation, if any, to be immediately
terminated and may cause the channel to perform the
malfunction-reset function. The recovery procedure in
the channel and the subsequent state of the subchannel
upon a malfunction reset depend upon the model.
Interface Control Check Interface control check is caused by any invalid signal
on the I/O interface. The condition is detected by the
channel and usually 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 or indicated the busy condition with­
out providing any other status bits.
4. A signal from a device occurred at an invalid
time or had invalid duration. Detection· of the interface-control-check condition
causes the current operation, if any, to be immediately
terminated and may cause the channel to perform the
maHunction-reset function. The recovery procedure in
the channel and the subsequent state of the subchan­
nel upon a malfunction reset depends on the model.
Chaining Check Chaining check is caused by channel overrun during
data chaining on input operations. The condition oc­
curs when the I/O data rate is too high for the particu­
lar resolution of data addresses. Chaining check cannot
occur on output operations.
Detection of the chaining-check condition causes the 110 device to be signaled to terminate the operation.
It causes command chaining to be suppressed.
Input/Output Operations 113