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device or the status bits received from the device
may be invalid. The channel forces correct parity
on invalid CSW fields. The validity of these fields
can be ascertained by inspecting the limited
channel logout.
When any I/O instruction cannot be executed
because of a pending logout which affects the
operational capability of the channel or subchannel,
a full CSW is stored. The fields in the CSW are all
set to zeros, with the exception of the
logout-pending bit and the channel-control-check
bit, which are set to ones.
Subchannel Key
A CSW stored to reflect the progress of an
operation at the subchannel contains the
subchannel key used in that operation. The
contents of this field are not affected by
programming errors detected by the channel or by
the situations causing termination of the operation.
12-56
IBM 4300 Processors Principles of Operation
CCW Address
When the CSW is formed to reflect the progress of
the I/O operation at the subchannel, the CCW
address is normally 8· higher than the address of the
last C CW used in the operation.
The figure "Contents of the CCW-Address Field
in the CSW" lists the contents of the CCW-address
field for all situations that can cause the CSW to be
stored. They are listed in order of priority; that is,
if two situations occur, the CSW appears as
indicated for the situation higher on the list. When
a CSW has been stored and the situation exists that
a command-retry request has been recognized but
the CCW has not been re-executed, the "last-used
CCW
+
8" is the CCW that is to be retried. When
a program check is caused by two CCWs in
sequence both of which specify transfer in channel,
the second CCW is the one considered invalid. In
the figure, the three cases of disconnected location
and the two cases of invalid key are all protection
checks.
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