Termination of the I/O operation normally is indi
cated by two conditions: channel end and device end.
The channel-end condition indicates that theI/O de
vice has received or provided all information associ
ated with the operation and no longer needs channel
facilities. The device-end signal indicates that theI/O device has terminated execution of the operation.
The device-end condition can occur concurrently with
the channel-end condition or later.Operations that tie up the control unit after releasing
channel facilities may, under certain conditions, cause
a third type of signal. This signal, called control unit
end, may occur only after channel end and indicates
that the control unit is available for initiation of an
other operation.
The conditions signaling the termination of anI/O operation can be brought to the attention of the pro
gram byI/O interruptions or, when the channel is
masked,by programmed interrogation of the I/O de
vice. In either case, these conditions cause storing the
csw, which contains additional information concern
ing the execution of the operation. At the time the
channel-end condition is generated, the channel pro
vides an address and a count that indicate the extent
of main storage used. Both the channel and the device
can provide indications of unusual conditions. The
device-end and control-unit-end conditions can be ac
companied by error indications from the device.
Facilities are provided for the program to initiate
execution of a chain of commands with a singleSTART I/O. When command chaining is specified, the receipt
of the device-end signal causes the channel to fetch
a new ccw and to initiate a new command at the
device. A chained command is initiated by mcans of
the same sequence of signals over theI/O interface as
the first command specified bySTART I/O. The condi
tions signaling the termination of an operation are
not made available to the program when command
chaining occurs.
Conditions that initiateI/O interruptions are asyn
chronous to the activity in theCPU, and more than one
condition can occur at the same time. The channel and
theCPU establish priority among the conditions so that
only one interruption request is processed at a time.
The conditions are preserved in theI/O devices and
subchannels until accepted by theCPU. Execution of an I/O operation or chain of operations
thus involves up to four levels of participation. Except
for the effects of shared equipment, theCPU is tied up
for the duration of execution ofSTART I/O, which
86
lasts at most until the addressedI/O device responds
to the first command. The subchannel is busy with the
execution from the time the operation is initiated at
theI/O device until the channel-end condition for the
last operation of the command chain is accepted by
thcCPU. The control unit may remain busy after the
subchannel has been released and may generate the
control-unit-end condition when it becomes free. Final
ly, theI/O device is busy from the initiation of the
first command until the device-end condition associ
ated with the last operation is cleared. A pending
device-end condition causes the associated device to
appear busy, but does not affect the state of any other
part of the system. A pending control unit end blocks
communications through the control unit to any device
attached to it, while a pending channel end normally
blocks all communications through the subchannel.Compatibility of Operation
The organization of theI/O system provides for a uni
form method of controllingI/O operations. The capac
ity of a channel, however, depends on its use and on
the model to which it belongs. Channels are provided
with different data-transfer capabilities, and anI/O device designed to transfer data only at a specific rate
(a magnetic tape unit or a disk storage for example)
can operate only on a channel that can accommodate
at least this data rate.
The data rate a channel can accommodate depends
also on the way theI/O operation is programmed. The
channel can sustain its highest data rate when no data
chaining is specified. Data chaining reduces the maxi
mum allowable rate, and the extent of the reduction
depcnds on the frequency at which new ccw's are
fetched and on the address resolution of the first byte
in the new area. Furthermore, since the channel may
share main storage with theCPU and other channels,
activity in the rest of the system affects the accessi
bility of main storage and, hence, the instantaneous
load the channel can sustain.
In view of the dependence of channel capacity on
programming and on activity in therest of the system,
an evaluation of the ability of a specificI/O configu
ration to function concurrently must be based on a
consideration of both the data rate and the way theI/O operations are programmed. Two systems employ
ing identical complements ofI/O devices may be able
to executc certain programs in common, but it is pos
sible that other programs requiring, for example, data
chaining, may not run on one of the systems.
cated by two conditions: channel end and device end.
The channel-end condition indicates that the
vice has received or provided all information associ
ated with the operation and no longer needs channel
facilities. The device-end signal indicates that the
The device-end condition can occur concurrently with
the channel-end condition or later.
channel facilities may, under certain conditions, cause
a third type of signal. This signal, called control unit
end, may occur only after channel end and indicates
that the control unit is available for initiation of an
other operation.
The conditions signaling the termination of an
gram by
masked,
vice. In either case, these conditions cause storing the
csw, which contains additional information concern
ing the execution of the operation. At the time the
channel-end condition is generated, the channel pro
vides an address and a count that indicate the extent
of main storage used. Both the channel and the device
can provide indications of unusual conditions. The
device-end and control-unit-end conditions can be ac
companied by error indications from the device.
Facilities are provided for the program to initiate
execution of a chain of commands with a single
of the device-end signal causes the channel to fetch
a new ccw and to initiate a new command at the
device. A chained command is initiated by mcans of
the same sequence of signals over the
the first command specified by
tions signaling the termination of an operation are
not made available to the program when command
chaining occurs.
Conditions that initiate
chronous to the activity in the
condition can occur at the same time. The channel and
the
only one interruption request is processed at a time.
The conditions are preserved in the
subchannels until accepted by the
thus involves up to four levels of participation. Except
for the effects of shared equipment, the
for the duration of execution of
86
lasts at most until the addressed
to the first command. The subchannel is busy with the
execution from the time the operation is initiated at
the
last operation of the command chain is accepted by
thc
subchannel has been released and may generate the
control-unit-end condition when it becomes free. Final
ly, the
first command until the device-end condition associ
ated with the last operation is cleared. A pending
device-end condition causes the associated device to
appear busy, but does not affect the state of any other
part of the system. A pending control unit end blocks
communications through the control unit to any device
attached to it, while a pending channel end normally
blocks all communications through the subchannel.
The organization of the
form method of controlling
ity of a channel, however, depends on its use and on
the model to which it belongs. Channels are provided
with different data-transfer capabilities, and an
(a magnetic tape unit or a disk storage for example)
can operate only on a channel that can accommodate
at least this data rate.
The data rate a channel can accommodate depends
also on the way the
channel can sustain its highest data rate when no data
chaining is specified. Data chaining reduces the maxi
mum allowable rate, and the extent of the reduction
depcnds on the frequency at which new ccw's are
fetched and on the address resolution of the first byte
in the new area. Furthermore, since the channel may
share main storage with the
activity in the rest of the system affects the accessi
bility of main storage and, hence, the instantaneous
load the channel can sustain.
In view of the dependence of channel capacity on
programming and on activity in the
an evaluation of the ability of a specific
ration to function concurrently must be based on a
consideration of both the data rate and the way the
ing identical complements of
to executc certain programs in common, but it is pos
sible that other programs requiring, for example, data
chaining, may not run on one of the systems.