is designed to control devices having
only the values0000 to 1001 in the
rightmost bit positions of the device
address, it does not recognize device
addresses containing1010 to 1111 in
these bit positions.On the other hand,
a control unit responds to all device
addresses in the assigned set for whichthe corresponding I/O devices are ready,
or are not ready but can be made ready
by means of an ordinary manual inter
vention. A control unit mayor may not
respond to an address within the
assigned set when the corresponding
device is not installed or has been
logically removed from the control unit.
If a control unit responds to a device
address for which noI/O device is
installed or the device has been
logically removed from the control unit,
the absent device appears in the not
ready state. If no control unit
responds to the device address, theI/O device appears not operational.
Input/output devices accessible through
more than one channel in a channel set
have a distinctI/O address for each
path of communications. ThisI/O address identifies the channel and the
control unit. For sets of devices shar
ing a control unit or connected to two
or more control units, the portion of
theI/O address identifying the device
on the control unit is fixed and does
not depend on the path of
communications.
The assignment ofI/O addresses is arbi
trary, subject to the rules described
and any model-dependent restrictions.
The assignment is made at the time of
installation, andthe addresses normally
remain fixed thereafter.
STATESOF THE INPUT/OUTPUT SYSTEM The state of the I/O system identified
by anI/O address depends on the collec-
tive state of the channel, subchannel,
andI/O device. Each of these compo
nents of theI/O system can have up to
four states, as far as the response to
anI/O instruction is concerned. These
states are listed in the figure
"Input/Output-System States." The name
of the state is followed by its abbrevi
ation and a brief definition.
A channel, subchannel, orI/O device
that is available, interruption-pending,
or working is called "operational." A
channel, subchannel, orI/O device that
is interruption-pending, working, or
not-operational is called "not
available."
In a multiplexer channel, the channel
and subchannel are easily distinguisha
ble and, if the channel is operational,
any combination of channel and subchannel states is possible. Since the selector channel can have only one
subchannel, the channel and subchannelare functionally coupled, and certain
states ofthe channel are related to
those of the subchannel. In particular,
the working state can occur only concur
rently in both the channel and
subchannel and, whenever an interruption
condition is pending in the subchannel,
the channel also is in the same state.
The channel and subchannel, however, are
not synonymous, and an interruption
condition not associated with data
transfer, such as attention, may not
affect the state of the subchannel.
Thus, the subchannel may as a function
of theI/O instruction, be available
when the channel is interruption-pending
or has an interruption condition pending
at a device. A consistent distinction
between the subchannel and channel
permits selector and multiplexer chan
nels to be covered uniformly by a single
description.Chapter 13. Input/Output Operations 13-9
only the values
rightmost bit positions of the device
address, it does not recognize device
addresses containing
these bit positions.
a control unit responds to all device
addresses in the assigned set for which
or are not ready but can be made ready
by means of an ordinary manual inter
vention. A control unit mayor may not
respond to an address within the
assigned set when the corresponding
device is not installed or has been
logically removed from the control unit.
If a control unit responds to a device
address for which no
installed or the device has been
logically removed from the control unit,
the absent device appears in the not
ready state. If no control unit
responds to the device address, the
Input/output devices accessible through
more than one channel in a channel set
have a distinct
path of communications. This
control unit. For sets of devices shar
ing a control unit or connected to two
or more control units, the portion of
the
on the control unit is fixed and does
not depend on the path of
communications.
The assignment of
trary, subject to the rules described
and any model-dependent restrictions.
The assignment is made at the time of
installation, and
remain fixed thereafter.
STATES
by an
tive state of the channel, subchannel,
and
nents of the
four states, as far as the response to
an
states are listed in the figure
"Input/Output-System States." The name
of the state is followed by its abbrevi
ation and a brief definition.
A channel, subchannel, or
that is available, interruption-pending,
or working is called "operational." A
channel, subchannel, or
is interruption-pending, working, or
not-operational is called "not
available."
In a multiplexer channel, the channel
and subchannel are easily distinguisha
ble and, if the channel is operational,
any combination of channel and subchan
subchannel, the channel and subchannel
states of
those of the subchannel. In particular,
the working state can occur only concur
rently in both the channel and
subchannel and, whenever an interruption
condition is pending in the subchannel,
the channel also is in the same state.
The channel and subchannel, however, are
not synonymous, and an interruption
condition not associated with data
transfer, such as attention, may not
affect the state of the subchannel.
Thus, the subchannel may as a function
of the
when the channel is interruption-pending
or has an interruption condition pending
at a device. A consistent distinction
between the subchannel and channel
permits selector and multiplexer chan
nels to be covered uniformly by a single
description.