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variable and may be controlled by the device or the
channel.
Excluding equipment errors, CLEAR I/O,
HALT DEVICE, and HALT I/O, the channel
signals the device to conclude data transfer
whenever any of the following events occurs:
1. The storage areas specified for the operation
are exhausted or filled.
2. A program check is detected ..
3. A protection check is detected.
4. A chaining check is detected.
The first event occurs when the channel has
stepped the count to zero in the last CCW
associated with the operation. A count of zero
indicates that the channel has transferred all
information specified by the program. The other
three events are due to errors and cause premature
conclusion of data transfer. In every case, the
conclusion is signaled in response to a service
request from the device and causes data transfer to
cease.
If
the device has no blocks defined for the
operation (such as writing from magnetic tape), it
concludes the operation and generates channel end.
The device can control the duration of an
operation and the timing of channel end. On
certain operations for which blocks are defined
(such as reading from magnetic tape), the device
does not provide the channel-end signal until the
end of the block is reached, regardless of whether
or not the device has been previously signaled to
conclude data transfer.
If
the initial data address in the CCW refers to a
storage location that is not provided or to a
disconnected or protected page, no data is
transferred during the operation, and the device is
signaled to conclude the operation in response to
the first service request. On writing, devices such
as magnetic-tape units request the first byte of data
before any mechanical motion is started and, if the
initial data address refers to a storage location that
is not provided or to a disconnected or protected
page, the operation is concluded before the
recording medium has been advanced. However,
since the operation has been initiated, the device
provides channel end, and an interruption condition
is generated. Whether a block at the device is
advanced when no data is transferred depends on
the type of device and is specified in the SL
publication for the device.
When command chaining takes place, the
subchannel is in the working state from the time
the first operation is initiated until the device
signals channel end for the last operation of the
12-42
IBM 4300 Processors Principles of Operation
chain. On the selector channel, the device
executing the operation stays connected to the
channel and the whole channel is in the working
state during the entire execution of the chain of
operations. On the multiplexer channel, an
operation in the burst mode causes the channel to
be in the working state only while transferring a
burst of data.
If
channel end and device end do
not occur concurrently, the device disconnects from
the channel after providing channel end, and the
channel can in the meantime communicate with
other devices.
Any unusual situations cause command chaining
to be suppressed and an interruption condition to
be generated. The unusual situations can be
detected by either the channel or the device, and
the device can provide the indications with channel
end, control-unit end, or device end. When the
channel is aware of the unusual situation by the
time the channel-end signal for the operation is
received, the chain is ended as if the operation
during which the situation occurred were the last
operation of the chain. The device-end· signal
subsequently is processed as an interruption
condition. When the device signals unit check or
unit exception with control-unit end or device end,
the sub channel terminates the working state upon
receipt of the signal from· the device. The
channel-end indication in this case is not made
available to the program.
Termination
by
HALT I/O or HALT DEVICE
The instructions HALT I/O and HALT DEVICE
cause the current operation at the addressed
channel or sub channel to be immediately
terminated. The method of termination differs
from that used upon exhaustion of count or upon
detection of programming errors to the extent that
termination by HALT I/O or HALT DEVICE is
not necessarily contingent on the receipt of a
service request from the device.
When HALT I/O is issued to a channel
operating in burst mode, the channel issues the halt
signal to the device currently operating with the
channel, regardless of the device address specified
with the HALT I/O instruction.
If
the channel is
involved in the data-transfer portion of an
operation, data transfer is immediately terminated,
and the device is disconnected from the channel.
If
HALT I/O is addressed to a selector channel
executing a chain of operations and the device has
already provided channel end for the current
operation, the instruction causes the device to be
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