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Action in Channel upon Exhaustion of Count
Flags in
or
Current
Receipt of Channel End
CCW
Regular Operation
CD CC SLI Immediate Operation
I
II
III
0
0
0
End, NIL
Stop, IL
End, NIL
End, IL
0
0
1
End, NIL
Stop, NIL
End, NIL
End, NIL
0
1
0
Chain Command
Stop, IL
Chain command End, IL
0
1
1
Chain Command
Chain command Chain command Chain command
1
0
0
End, NIL
Chain Data
*
End, IL
1
0
1
End, NIL
Chain Data
*
End, IL
1
1
0
End, NIL
Chain Data
*
End, IL
1
1
1
End, NIL
Chain Data
*
End, IL
Explanation:
II
III
End
Stop
IL
NIL
Count exhausted, end of block at device not reached.
Count exhausted and channel end from device.
Count not exhausted and channel end from device.
The operation
is
terminated.
If
the
operation
is
immediate
and
has
been
specified
by
the first CCW
associated with a START I/O, a condition code 1 is set,
and the status portion of the CSW is stored as part
of
the
execut i on of the START I/O. I n a
11
other cases, an
interruption condition is generated in the
subchannel.
The
device is signaled to terminate data transfer, but
the subchannel
remains
in
the
working
state
until
channel
end
is received; at this time an interruption
condition is generated in the subchannel.
Incorrect length is
indicated
with
the
interruption
condition.
Incorrect length is not indicated.
Chain command The
channel
performs command chaining upon receipt of
device end.
Chain data
The channel immediately fetches a new CCW for the
same
operation.
The srtuation where the residual count is zero but data
chaining is indicated
at
the time the device provides
channel end cannot validly
occur.
When data chaining
is indicated, the channel fetches
the
new
CCW
after
transferring the last byte of data
designated
by
the
current
CCW
but
before
the device provides the next
request for data or status transfer.
As a result,
the
channel recognizes the channel end from the device only
after
it has fetched the new CCW, which cannot contain
a count of zero unless a
programming
error
has
been
made.
Channel· Chaining Action
Data Chaining
During data chaining, the new CCW fetched by the
channel defines a new storage area for the original
12-32
IBM 4300 Processors Principles of Operation
I/O operation. Execution of the operation at the
I/O device is not affected. When all data
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