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no device is installed, the absent device appears in
the not-ready state.
Input/ output devices accessible through more
than one channel have a distinct address for each
path of communications. This address identifies
the channel and the control unit. For sets of
devices connected to two or more control units, the
portion of the address identifying the device on the
control unit is fixed, and does not depend on the
path of communications.
The assignment of channel and device addresses
is arbitrary, subject to the rules described and any
model-dependent restrictions. The assignment is
made at the time of installation, and the addresses
normally remain fixed thereafter.
States of the Input/Output System
The state of the I/O system identified by an I/O
address depends on the collective state of the
channel, subchannel, and I/O device. Each of
these components of the I/O system can have up to
four states, as far as the response to an I/O
instruction is concerned. These states are listed in
the figure "Input/Output System States." The
name of the state is followed by its abbreviation
and a brief definition.
A channel, subchannel, or I/O device that is
available, interruption-pending, or working is called
"operational." A channel, subchannel, or I/O
device that is interruption-pending, working, or
not-operational is called "not available."
In a multiplexer channel, the channel and
subchannel are easily distinguishable and, if the
channel is operational, any combination of channel
and sub channel states is possible. Since the
selector channel can have only one subchannel, the
channel and subchannel are functionally coupled,
and certain states of the channel are related to
those of the subchannel. In particular, the working
state can occur only concurrently 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, does not affect the state
of the subchannel. Thus, the subchannel may be
available when the channel has an interruption
condition pending. Consistent distinction between
the subchannel and channel permits selector and
multiplexer channels to be covered uniformly by a
single description.
The device referred to in the figure
"Input/Output-System States" includes both the
device proper and its control unit. For some types
of devices, such as magnetic-tape units, the working
and the interruption-pending states can be caused
by activity in the addressed device or control unit.
A "not available
11
shared control unit imposes its
Name
Abbreviation and Definition
Channel
Available
Interruption pending
Working
Not operational
Subchannel
Available
Interruption pending
Working
Not operational
liD
Device
Avai lable
Interruption pending
Working
Not operational
Input/Output-System States
A
None of the following states
I
Interruption condition immedi-
ately available from channel
W
Channel operating in burst mode
N
Channel not operational
A
I
W
N
None of the following states
Information for CSW avai lable in
subchannel
Subchannel executing an operation
Subchannel not operational
A
None of the following states
I
Interruption condition in device
W
Device executing an operation
N
Device not operational
12-8
IBM 4300 Processors Principles of Operation
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