the device on the control unit is fixed and does not de­
pend on the path of communications.
In models in which more than 128 sub channels are
available, the shared subchannels can optionally be re­
placed by sets of unshared subchannels. When the
option is implemented, the additional unshared sub­
channels are assigned sequential addresses starting at
128.
Except for the rules described, the assignment of de­
vice addresses is arbitrary. The assignment is made at
the time of installation and normally is fixed.
Programmill1g Notes
Shared subchannels are used with devices, such as
magnetic tape units and disk access mechanisms, that
share a control unit. For such devices, the sharing of
the subchannel does not restrict the concurrency of I/O operations since the control unit permits only one de­
vice to be involved in a date transfer operation at a
time.
The program can refer to a shared sub channel by
addresses 0-7 or by one of the addresses assigned to
the subchannel. No restrictions are imposed on the use
of a shared subchannel. If the subchannel is available,
the addressed device is selected, and the specified op­
eration is performed, regardless of the control unit to
which the device is attached. Instruction Exception Handling Before the channel is signaled to execute an I/O in­
struction, the instruction is tested for validity by the CPU. Exceptional conditions detected at this time cause
a program interruption. When the interruption occurs,
the current psw is stored as the old psw and is replaced
by a new psw. The interruption code in the old psw
identifies the cause of the interruption.
The following exception may cause a program inter­
ruption: Privileged Operation: An I/O instruction is encoun­
tered when the CPU is in the problem state. The in­
struction :is suppressed before the channel has been
signaled to execute it. The csw, the condition code in
the psw, and the state of the addressed sub channel
and I/O device remain unchanged. States of the Input/Output System The state of the I/O system identified by an I/O ad­
dress depends on the collective state of the channel,
sub channel, 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 following table. The name of
the state is followed by its abbreviation and a brief
definition.
88 I/O DEVICE Available
Interruption pending
Working
Not operational SUDCHANNEL Available
Interruption pending
Working
Not operational CHANNEL Available
Interruption pending
Working
Not operational ADBREV DEFINITION
A None of the following states
I Interruption condition pending in
W Device executing an operation
N Device not operational ADDREV DEFINITION
A None of the following states
I Information for CSW available in
subehannel
W Subchannel executing an operation
N Subchannel not operational ADDREV DEFINITION
A None of the following states
I Interruption immediately available
from channel
W Channel operating in hurst mode
N Channel not operational
A channel, subchannel, or I/O device that is avail­
able, that contains a pending interruption condition, or
that is working, is said to be operational. The states of
containing an interruption condition, working, or be­
ing not operational are collectively referred to as "not available." In the case of the multiplexor channel, the channel
and subchannel are easily distinguishable and, if the
channel is operational, any combination of channel and
subchanncl states are possible. Since the selector chan­
nel 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 concur­
rently in both the channel and subchannel and, when­
ever an interruption condition is pending in the sub­
channel, the channel also is in the same state. The
channel and sub channel, however, are not synony­
mous, and an interruption condition not associated
with data transfer, such as attention or device end,
does not affect the state of the subchannel. Thus, the
subchanne.1 may be available when the channel has
an interruption condition pending. Consistent dis­
tinction between the subchannel and channel permits
both types of channels to be covered uniformly by a
single description.
The device referred to in the preceding table in­
cludes 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 shared control unit imposes its state on all de­
vices attached to the control unit. The states of the de­
vices are not related to those of the channel and sub­
channel.
When the response to an I/O instruction is deter­
mined on the basis of the states of the channel and
subchannel, the components further removed are not
interrogated. Thus, ten composite states are identified

as conditions for the execution of the I/O instruction.
Each composite state is identified in the following dis­
cussion by three alphabetic characters; the first char­
acter position identifies the state of the channel, the
second identifies the state of the subchannel, and the
third refers to the state of the device. Each character
position can contain A, I ,w, or N, denoting the state
of the component. The symbol x in place of a letter
indicates that the state of the corresponding compo­
nent is not significant for the execution of the in­
struction.
Available (AAA): The addressed channel, subchan­
nel, control unit, and I/O device are operational, are
not engaged in the execution of any previously initi­
ated operations, and do not contain any pending in­
terruption conditions.
Interruption Pending in Device (AAI) or Device
Working (AA W): The addressed I/O device or control
unit is executing a previously initiated operation or
contains a pending interruption condition. The ad­
dressed sub channel and channel are available. These
situations are possible:
1. The device is executing an operation after sig­
naling the channel-end condition, such as rewinding
tape or seeking on a disk file.
2. The control unit associated with the device is
executing an operation after signaling the channel­
end condition, such as backspacing file on a magnetic
tape unit.
3. The device or control unit is executing an opera­
tion on another sub channel or channel.
4. The device or control unit contains the device­
end, control-unit-end, or attention condition or, on
the selector channel, the channel-end condition as­
sociated with an operation terminated by HALT I/O. Device Not Operational (AAN): The addressed I/O device is not operational. A device appears not opera­
tional when no control unit recognizes the address.
This occurs when the control unit is not provided in
the system, when power is off in the control unit, or
when the control unit has been logically switched off
the I/O interface. For some types of devices, the not­
operational state is indicated also when the addressed
device is not installed on the control unit. The ad­
dressed subchannel and channel are available.
For devices such as magnetic tape units, the device
appears operational as long as the control unit asso­
ciated with the addressed device is operational. If
the device is not installed or has been logically re­
moved from the control unit, selection of the device
for TEST I/O or a command other than sense causes
the unit-check indication.
Interruption Pending in Subchannel (AIX): An in­
terruption condition is pending in the addressed sub-
channel because of the termination of the portion of
the operation involving the use of channel facilities.
The subchannel has information for a complete csw.
The interruption condition can indicate termination
of an operation at the addressed I/O device or at
another device on the subchannel. In the case of the
multiplexor channel, the channel is available. The
state of the addressed device is not significant, except
when TEST I/O is addressed to the device associated
with the terminated operation. The device associated
with the terminated operation normally is in the in­
terruption pending state. On the selector channel the existence of an inter­
ruption condition in the sub channel immediately
causes the channel to assign to this condition the
highest priority for I/O interruptions and, hence, leads
to the state IIX. Sub channel Working (AWX): The addressed sub­
channel is executing a previously initiated operation
or chain of operations in the multiplex mode and has
not yet reached the channel end for the last opera­
tion. All devices sharing the currently operating con­
trol unit appear in the working state but, for shared
subchannels, the states of devices not attached to the
control unit are not known. The addressed channel is
available.
The sub channel-working state does not occur on the
selector channel since all operations on the selector
channel are executed in the burst mode and cause the
channel to be in the working state ( wwx) .
Subchannel Not Operational (ANX): The addressed
sub channel on the multiplexor channel is not opera­
tional. A subchannel is not operational when it is not
provided in the system. The channel is available.
This state cannot occur on the selector channel.
Interruption Pending in Channel (IXX): The ad­
dressed channel has established which device will
cause the next I/O interruption from this channel.
The state where the channel contains a pending in­
terruption condition is distinguished only by the in­
struction TEST CHANNEL. This instruction does not
cause the subchannel and I/O device to be interro­
gated. The other I/O instructions consider the chan­
nel available when it contains a pending interruption
condition.
Channel Working (WXX): The addressed channel
is operating in the burst mode. In the case of the
multiplexor channel, a burst of bytes is currently
being handled. In the case of the selector channel, an
operation or a chain of operations is currently being
executed, and the channel end for the last operation
has not yet been reached. The states of the addressed
device and, in the case of the multiplexor channel,
of the sub channel are not significant.
Input/Output Operations 89