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generated, and the subchannel is not available until
the condition is cleared. The reasons for the
rejection are indicated to the program by means of
the corresponding status bits in the CSW. The
not-operational state of the I/O device, which
during the execution of ST ART I/O and sometimes
during the execution of START I/O FAST
RELEASE causes condition code 3 to be set,
instead causes the interface-control-check bit to be
set to one. The new operation at the I/O device is
not started.
When START I/O FAST RELEASE is executed
by a channel independent of the addressed device,
tests for most program-specified information, for
control-unit and device availability, for control-unit
and device status, and for most errors are
performed subsequent to the execution of START
I/O FAST RELEASE. Some situations which
would have caused a condition code 1 or 3 to be
set had the instruction been START
1/
a
instead
cause an interruption condition to be generated.
The CSW, when stored, indicates that the
interruption condition is a deferred condition code
1 or 3.
Immediate Operations
Some control commands cause the
1/
a
device to
signal channel end immediately upon receipt of the
command code. An I/O operation causing channel
end to be signaled during the initiation sequence is
called an immediate operation.
When the first CCW designated by the CAW
during a START I/O or START I/O FAST
RELEASE executed as a START 1/
a
initiates an
immediate operation with command chaining not
indicated and command retry not occurring, no
interruption condition is generated. In this case,
channel end is brought to the attention of the
program by causing START I/O or START I/O
FAST RELEASE to store the CSW status portion.
The subchannel is immediately made available to
the program. The 1/
a
operation, however, is
initiated, and, if channel end is not accompanied by
device end, the device remains busy. Device end,
when subsequently provided by the device, causes
an interruption condition to be generated.
An immediate operation initiated by the first
CCW deSignated by the CAW during a START
1/
a
FAST RELEASE executed independent of the
addressed device appears to the program as a
nonimmediate command. That is, any status
generated by the device for the immediate
command, or for a subsequent command if
command chaining occurs, causes an interruption
condition to be generated.
When command chaining is specified after an
immediate operation and no unusual situations have
been detected during the execution, or when
command retry occurs for an immediate operation,
neither ST ART I/O nor ST ART I/O FAST
RELEASE causes the immediate storing of CSW
status. The subsequent commands in the chain are
handled normally, and channel end for the last
operation generates an interruption condition even
if the device provides the signal immediately upon
receipt of the command code.
Whenever immediate completion of an
1/
a
operation is signaled, no data has been transferred
to or from the device.
Since a count of zero is not valid, any CCW
specifying an immediate operation must contain a
nonzero count. When an immediate operation is
executed, however, incorrect length is not indicated
to the program, and command chaining is
performed when so specified.
Programming Note
Control operations for which the entire operation is
specified in the command code may be executed as
immediate operations. Whether the control
function is executed as an immediate operation
depends on the operation and type of device and is
specified in the SL publication for the device.
Conclusion of Data Transfer
When the device accepts a command, the
subchannel is set up for data transfer. The
subchannel is in the working state during this
period. Unless the channel detects equipment
malfunctioning or the operation is concluded by
CLEAR I/O, or, on the selector channel, the
operation is concluded by CLEAR I/O, HALT
I/O, or HALT DEVICE, the working state lasts
until the channel receives the channel-end signal
from the device. When no command chaining is
specified or when chaining is suppressed because of
unusual situations, channel end causes the
operation at the subchannel to be terminated and
an interruption condition to be generated. The
status bits in the associated CSW indicate channel
end and any unusual situations. The device can
signal channel end at any time after initiation of
the operation, and the signal may occur before any
data has been transferred.
For operations not involving data transfer, the
device normally controls the timing of channel end.
The duration of data-transfer operations may be
Chapter 12. Input/Output Operations
12-41
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