Programming Note
Control operations for which the entire operation isspecified 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 isspecified in the SRL publication for the device.
Termination of Data Transfer
When the device accepts a command, the subchannel
is set up for data transfer. The subchannel is said to
be working during this period. Unless the channel de
tects equipment malfunctioning or, on the selector
channel, the operation is terminated by HALTI/O, the
working state lasts until the channel receives the chan
nel-end signal from the device. When no command
chaining isspecified or when chaining is suppressed
because of unusual conditions, the channel-end con
dition causes the operation at the subchannel to be ter
minated and an interruption condition to be generated.
The status bits in the associated csw indicate channel
end and the unusual conditions, if any. The device can
signal channel end at any time after initiation of the
operation, and the signal may occur before any data
have been transferred.
For operations not involving data transfer, the de
vice normally controls the timing of the channel-end
condition. The duration of data transfer operations
may be variable and may be controlled by the device
or the channel.
Excluding equipment errors and HALTI/O, the chan
nel signals the device to terminate data transfer when
ever any of the following conditions occurs:
The storage areasspecified for the operation are
exhausted orfilled. Program-check condition is detected.
Protection-check condition is detected.
Chaining-check condition is detected.
Thefirst of these conditions occurs when the channel
has stepped the count in the last ccw associated with
the operation to zero. A count of zero indicates that
the channel has transferred all informationspecified by the program. The other three conditions are due to
errors and cause premature termination of data trans
fer. In either case, the termination is signaled in re
sponse 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 on magnetic
tape), it terminates the operation and generates the
channel-end condition.
The device can control the duration of an operation
and the timing of channel end by blocking of data.On certain operations for which blocks are defined (such
as reading on 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 de
vice has been previously signaled to terminate data
transfer.
The channel suppresses initiation of anI/O operation
when the data address in thefirst ccw associated with
the operation exceeds the addressing capacity of the
model. Complete check for the validity of the data ad
dress is performed only as data are transferred to or
from main storage. When the initial data address in
the ccw is invalid, no data are transferred during the
operation, and the device is signaled to terminate the
operation in response to thefirst service request. On writing, devices such as magnetic tape units request
thefirst byte of data before any mechanical motion is
started and, if the initial data address is invalid, the
operation is terminated 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 are
transferred depends on the type of device and is specified in the SRL publication for the device.
When command chaining takes place, the subchan
nel appears in the working state from the time thefirst operation is initiated until the device signals the
channel-end condition of the last operation of the
chain.On the selector channel, the device executing
the operation stays connected to the channel and the
whole channel appears to be in the working state for
the duration of the execution of the chain of oper
ations.On the multiplexor channel an operation in the
burst mode causes the channel to appear to be in the
working state only for the duration of the transfer of
the 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 chan
nel can in the meantime communicate with other de
vices on the interface.
Any unusual conditions cause command chaining to
be suppressed and an interruption condition to be gen
erated. The unusual conditions 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 condition by the time the channel-end signal
for the operation is received, the chain is terminated
as if the operation during which the condition oc
curred were the last operation of the chain. The de
vice-end signal subsequently is processed as an inter
ruption 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 re-
Input/Output Operations 107
Control operations for which the entire operation is
immediate operations. Whether the control function is
executed as an immediate operation depends on the
operation and type of device and is
Termination of Data Transfer
When the device accepts a command, the subchannel
is set up for data transfer. The subchannel is said to
be working during this period. Unless the channel de
tects equipment malfunctioning or, on the selector
channel, the operation is terminated by HALT
working state lasts until the channel receives the chan
nel-end signal from the device. When no command
chaining is
because of unusual conditions, the channel-end con
dition causes the operation at the subchannel to be ter
minated and an interruption condition to be generated.
The status bits in the associated csw indicate channel
end and the unusual conditions, if any. The device can
signal channel end at any time after initiation of the
operation, and the signal may occur before any data
have been transferred.
For operations not involving data transfer, the de
vice normally controls the timing of the channel-end
condition. The duration of data transfer operations
may be variable and may be controlled by the device
or the channel.
Excluding equipment errors and HALT
nel signals the device to terminate data transfer when
ever any of the following conditions occurs:
The storage areas
exhausted or
Protection-check condition is detected.
Chaining-check condition is detected.
The
has stepped the count in the last ccw associated with
the operation to zero. A count of zero indicates that
the channel has transferred all information
errors and cause premature termination of data trans
fer. In either case, the termination is signaled in re
sponse to a service request from the device and causes
data transfer to cease. If the device has no blocks de
tape), it terminates the operation and generates the
channel-end condition.
The device can control the duration of an operation
and the timing of channel end by blocking of data.
as reading on 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 de
vice has been previously signaled to terminate data
transfer.
The channel suppresses initiation of an
when the data address in the
the operation exceeds the addressing capacity of the
model. Complete check for the validity of the data ad
dress is performed only as data are transferred to or
from main storage. When the initial data address in
the ccw is invalid, no data are transferred during the
operation, and the device is signaled to terminate the
operation in response to the
the
started and, if the initial data address is invalid, the
operation is terminated 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 are
transferred depends on the type of device and is speci
When command chaining takes place, the subchan
nel appears in the working state from the time the
channel-end condition of the last operation of the
chain.
the operation stays connected to the channel and the
whole channel appears to be in the working state for
the duration of the execution of the chain of oper
ations.
burst mode causes the channel to appear to be in the
working state only for the duration of the transfer of
the 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 chan
nel can in the meantime communicate with other de
vices on the interface.
Any unusual conditions cause command chaining to
be suppressed and an interruption condition to be gen
erated. The unusual conditions 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 condition by the time the channel-end signal
for the operation is received, the chain is terminated
as if the operation during which the condition oc
curred were the last operation of the chain. The de
vice-end signal subsequently is processed as an inter
ruption 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 re-
Input/Output Operations 107