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
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
Content of Channel Status Word
The content of the csw depends on the condition caus­
ing the storing of the csw and on the programming
method by which the information is obtained. The
status portion always identifies the condition that
caused storing of the csw. The protection key, com­
mand address, and count fields may contain informa­
tion 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 termi­
nation 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. Except for conditions associated with
command chaining, the storing can be caused by the PCI or channel-end condition, by the execution of
HALT I/O on the selector channel, or by equipment
malfunction. The contents of the csw are related to the
current values of the corresponding quantities, al­
though the count is unpredictable after programming
errors and after an interruption due to the PCI flag.
A csw stored upon the execution of a chain of oper­
ation pertains to the last operation the channel exe­
cuted or attempted to initiate. Information concerning
the preceding operations is not preserved and is not
made available to the program.
When an unusual condition causes command chain­
ing to be suppressed, the premature termination of the
chain is not explicitly indicated in the csw. A csw
associated with a termination due to a condition occur­
ing 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 indication is not made
available to the program, and the channel provides the
current protection key, command address, and count,
as well as the unusual indication, with the control­
unit-end or device-end bit in the csw. The command
address and count fields pertain to the operation that
was executed.
When the execution of a chain of commands is ter­
minated by an error detected during initiation of a
new operation, the command address and count fields
pertain to the rejected command. Termination at initi­
ation time can occur because of attention, unit check,
unit exception, program check, or equipment mal­
functioning and causes both the channel-end and de­
vice-end bits in the csw to be off.
A csw associated with conditions occurring after the
opcration at the subchannel has been terminated con­
tains zeros in the protection key, command address,
and count fields, provided the conditions are not
cleared by START I/O. These conditions include atten­
tion, control unit end, and device end (and channel
end when it occurs after termination of an operation
on the selector channel by HALT I/O). When the above conditions are cleared by START I/O, only the status portion of the csw is stored, and the
original contents of the protection key, command ad­
dress, and count fields in location 64 are preserved.
Similarly, only the status bits of the csw are changed
when the command is rejected or the operation at the
subchannel is terminated during the execution of START I/O or whenever HALT I/O causes csw status to be
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 con­
ditions 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 received from the device may
he invalid. The channel forces correct parity on invalid
csw fields.
Protection Key
A csw stored to reflect the progress of an operation at
the su bchannel contains the protection key used in
that operation. The content of this field is not affected
by programming errors detected by the channel or by
the condition causing termination of the operation.
Models in which the protection feature is not im­
plemented cause an all-zero key to be stored.
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 indicated or
occur, the csw appears as indicated for the condition
higher on the list. The programming errors listed in
the table refer to conditions included in program
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