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.
or by TEST
command chaining, the storing can be caused by the
HALT
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
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.
114
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
tion, control unit end, and device end (and channel
end when it occurs after termination of an operation
on the selector channel by HALT
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
stored.
Errors detected during execution of
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
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
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
check.
