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a program check is generated. When the first
CCW designated by the CAW contains an invalid
command code, the status portion of the CSW with
the program-check indication is stored during
execution of START I/O or START I/O FAST
RELEASE being executed as START I/O. When
the invalid code is detected during command
chaining or subsequent to the execution of START
I/O FAST RELEASE, the new operation is not
initiated, and an interruption condition is
generated. The command code is ignored during
data chaining, unless it specifies transfer in
channel.
Designation of Storage Area
The storage area associated with an I/O operation
is defined by one or more CCWs. A CCW defines
an area by specifying the address of the first byte
to be transferred and the number of consecutive
bytes contained in the area. The address of the
first byte appears in the data-address field of the
CCW. The number of bytes contained in the
storage area is specified in the count field.
In write, read, control, and sense operations,
storage locations are used in ascending order of
addresses. As information is transferred to or from
storage, the address from the address field is
incremented, and the count from the count field is
decremented. The read-backward operation places
data in storage in a descending order of addresses,
and both the count and the address are
decremented. When the count reaches zero, the
storage area defined by the CCW is exhausted.
\ Any storage location that is provided can be
used in the transfer of data to or from an I/O
device if the location is in a page that is in the
addressable or connected state and is not protected
against the type of reference. Similarly, a CCW
can be located in any part of storage if the location
is in a page that is in the addressable or connected
state and is not protected against a fetch-type
reference.
When the first CCW is designated by the CAW
as being at a storage location that is not provided,
the I/O operation is not initiated, and the status
portion of the CSW with the program-check
indication is stored during the execution of START
I/O or START I/O FAST RELEASE being
executed as START I/O. When, subsequently,
during the operation or chain of operations, the
channel refers to a storage location that is not
provided, an interruption condition indicating
program check is generated, and the device is
signaled to terminate the operation.
12-30
IBM 4300 Processors Principles of Operation
When the first CCW designated by the CAW is
in a disconnected page or in a location that is
protected against a fetch-type reference, theI/O
operation is not initiated, and the status portion of
the CSW with the protection-check indication is
stored during the execution of START 1/
a
or
START I/O FAST RELEASE being executed as
START I/O. When, subsequently, during the I/O
operation or chain of operations, the channel refers
to a disconnected page or a protected location, an
interruption condition indicating protection check is
generated, and the device is signaled to terminate
the operation.
During an output operation, the channel may
fetch data from storage before the time the I/O
device requests the data. Any number of bytes
specified by the current CCW may thus be
prefetched. When data chaining during an output
operation, and for some block-multiplexer channels
when data chaining during an input operation, the
channel may pre fetch the next CCW at any time
during the execution of the current CCW.
Prefetching may cause the channel to refer to
storage locations that are protected or not provided
or in disconnected pages. Such errors detected
during prefetching of data or CCWs do not affect
the execution of the operation and do not cause
error indications until the I/O operation actually
attempts to use the data or until the CCW takes
control.
If
the operation is concluded by the I/O
device or by HALT I/O, HALT DEVICE, or
CLEAR I/O before the invalid information is
needed, no program check or protection check is
generated.
The count field in the CCW can specify any
number of bytes from one to 65,535. Except for a
CCW specifying transfer in channel, which has no
count field, the count field may not contain the
value zero. Whenever the count field in the CCW
initially contains a zero, a program check is
generated. When this occurs in the first CCW
designated by the CAW, the operation is not
initiated, and the status portion of the CSW with
the program-check indication is stored during
execution of START I/O or START I/O FAST
RELEASE being executed as START I/O. When a
count of zero is detected during data chaining, the
1/
a
device is signaled to terminate the operation.
Detection of a count of zero during command
chaining or subsequent to the execution of START
1/
a
FAST RELEASE suppresses initiation of the
new operation and generates an interruption
condition.
(
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