tions with channel end. The control-unit-end and
device-end conditions can be accompanied by error
indications from the device.
Facilities are provided for the program to initiate
execution of a chain ofI/O operations with a single
STARTI/O or START I/O FAST RELEASE.
When the chaining flags in the current CCW specify
command chaining and no unusual conditions have
been detected in the operation, 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 means of the
same sequence of signals over theI/O interface as
the first command specified by ST ARTI/O or
ST ARTI/O FAST RELEASE. The ending signals
occurring at the concluding of an operation caused
by a CCW specifying command chaining are not
made available to the program when another opera
tion is initiated by the command chaining; the chan
nel continues execution of the channel program. If,
however, an unusual condition has been detected,
the ending signals cause suppression of command
chaining and a termination of the channel program.
Conditions that initiateI/O interruptions are
asynchronous to activity in theCPU, and more than
one condition can occur at the same time. The chan
nel and theCPU establish priority among the condi
tions so that only one interruption request is pro
cessed at a time. The conditions are preserved in theI/O devices or subchannels until accepted by the CPU. Execution of an I/O operation or chain of opera
tions thus involves up to four levels of participation:
1. Except for the effects caused by the integration
ofCPU and channel equipment, the CPU is
busy for the duration of execution of STARTI/O or START I/O FAST RELEASE, which
lasts at most until the addressedI/O device
responds to the first command.
2. The subchannel is busy with the execution
from the initiation of the operation at theI/O device until the channel-end condition for the
last operation of the command chain is accept
ed by theCPU. 3. The control unit may remain busy after the
subchannel has been released and may gener
ate the control-unit-end condition when it be
comes free.
4. TheI/O device is busy from the initiation of
the first operation until the device-end condi
tion associated with the operation is accepted
or cleared by theCPU. A pending device-end condition causes the associ
ated device to appear busy, but normally does not
affect the state of any other part of the system. A
pending control-unit-end condition normally blocks
communications through the control unit to any de
vice attached to it, and a pending channel-end condi
tion normally blocks all communications through the
sub channel.
Compatibility of Operation
The organization of theI/O system provides for a
uniform method of controllingI/O operations. The
capability of a channel, however, depends on its use
and on theCPU model to which it is attached.
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 de-
pends 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 maximum allowable rate, and the extent
of the reduction depends on the frequency at which
new CCWs are fetched and on the address resolu-
tion of the first byte in each new main-storage area.
Furthermore, since in most instances the channel
may share main storage with theCPU and other
channels, activity in the rest of the system affects the
accessibility of main storage and, hence, the instan
taneous load the channel can sustain.
In view of the dependence of channel capacity on
programming and on activity in the rest of the sys
tem, an evaluation of the ability of elements in a
specificI/O configuration to function concurrently
must be based on a consideration of both the data
rate and the way theI/O operations are programmed.
Two systems employing identical complements ofI/O devices may be able to execute certain programs
in common, but it is possible that other programs
requiring, for example, data chaining, may not run
on one of the systems because of the increased load
caused by the data chaining.
Control of Input/Output Devices I The CPU controls I/O operations by means of eight I/O instructions: START I/O, START I/O FAST RELEASE,TESTI/O,CLEARI/O,HALTI/O, I HALT DEVICE, TEST CHANNEL, and STORE CHANNELID.
The instruction TEST CHANNEL andSTORE CHANNEL ID address a channel; they do not ad I dress an I/O device. The other six I/O instructions
address a channel and a device on that channel.
Input/Output Operations 191
device-end conditions can be accompanied by error
indications from the device.
Facilities are provided for the program to initiate
execution of a chain of
START
When the chaining flags in the current CCW specify
command chaining and no unusual conditions have
been detected in the operation, 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 means of the
same sequence of signals over the
the first command specified by ST ART
ST ART
occurring at the concluding of an operation caused
by a CCW specifying command chaining are not
made available to the program when another opera
tion is initiated by the command chaining; the chan
nel continues execution of the channel program. If,
however, an unusual condition has been detected,
the ending signals cause suppression of command
chaining and a termination of the channel program.
Conditions that initiate
asynchronous to activity in the
one condition can occur at the same time. The chan
nel and the
tions so that only one interruption request is pro
cessed at a time. The conditions are preserved in the
tions thus involves up to four levels of participation:
1. Except for the effects caused by the integration
of
busy for the duration of execution of START
lasts at most until the addressed
responds to the first command.
2. The subchannel is busy with the execution
from the initiation of the operation at the
last operation of the command chain is accept
ed by the
subchannel has been released and may gener
ate the control-unit-end condition when it be
comes free.
4. The
the first operation until the device-end condi
tion associated with the operation is accepted
or cleared by the
ated device to appear busy, but normally does not
affect the state of any other part of the system. A
pending control-unit-end condition normally blocks
communications through the control unit to any de
vice attached to it, and a pending channel-end condi
tion normally blocks all communications through the
sub channel.
Compatibility of Operation
The organization of the
uniform method of controlling
capability of a channel, however, depends on its use
and on the
Channels are provided with different data-transfer
capabilities, and an
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 de-
pends also on the way the
The channel can sustain its highest data rate
when no data chaining is specified. Data chaining
reduces the maximum allowable rate, and the extent
of the reduction depends on the frequency at which
new CCWs are fetched and on the address resolu-
tion of the first byte in each new main-storage area.
Furthermore, since in most instances the channel
may share main storage with the
channels, activity in the rest of the system affects the
accessibility of main storage and, hence, the instan
taneous load the channel can sustain.
In view of the dependence of channel capacity on
programming and on activity in the rest of the sys
tem, an evaluation of the ability of elements in a
specific
must be based on a consideration of both the data
rate and the way the
Two systems employing identical complements of
in common, but it is possible that other programs
requiring, for example, data chaining, may not run
on one of the systems because of the increased load
caused by the data chaining.
Control of Input/
The instruction TEST CHANNEL and
address a channel and a device on that channel.
Input/Output Operations 191