Name Abbreviation and Definition Channel Available A None of the following states I nterruption pending
Working W
Interruption immediatelyavailable from channel Channel operating in burst mode
Not operational NChannel not operational Subchannel Available A None of the following states
Interruption pending
Working
Not operationalI W
N
Information forCSW available in subchannel Subchannel executing an operation Subchannel not operational I/O Device Available Interruption pending
Working
A
W
None of thefollowing states
Interruption condition pending in device
Device executing an operation
Not operational N
Device not operational
Input/Output System States
ing in the subchannel, the channel also is in the same
state. The channel and subchannel, however, are not
synonymous, and an interruption condition not asso-,
ciated with data transfer, such as attention, does not
affect the state of the sub channel. Thus, the sub
channel may be available when the channel has an
interruption condition pending. Consistent distinc
tion between the subchannel and channel permits
selector and multiplexer channels to be covered uni
formly 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"not available" shared con
trol unit imposes its state on all devices attached to
the control unit. The states of the devices 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 identi
fied as conditions for the execution of the I/O in
struction. Each composite state is identified in the
following discussion by three alphabeticcharacters; the first character position identifies the state of the
channel, the second identifies the state of the sub
channel, and the third refers to the state of the de
vice. 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 component is not significant for the
execution of the instruction.
Available (AAA): The addressed channel, sub chan
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
interruption conditions.
Interruption Pending in Device (AAI) or Device
Working (AA W): The addressed channel and sub
channel are available. The addressed control unit or
I/O device is executing a previously initiated opera
tion or contains a pending interruption condition.
These situations are possible:
1. The device is executing an operation, such as
rewinding tape or seeking on a disk file, after
signaling the channel-end condition.
2. The control unit associated with the device is
executing an operation, such as backspacing
file on a magnetic tape unit, after signaling the
channel-end condition.
3. The device or control unit is executing an oper
ation on another subchannel or channel.
4. The device or control unit contains the device
end, control-unit-end, or attention condition or
a channel-end condition associated with a ter
minated operation.
Device Not Operational (AAN): The addressed
channel and sub channel are available. The addressed
I/O device is not operational. A device appears not
operational when no control unit recognizes the ad
dress. This occurs when the control unit is not pro
vided 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. The not-operational
state is indicated also when the control unit is pro
vided and is designed to attach the device, but the
device has not been installed and the address has not
been assigned to the control unit (for example, the
second set of lines on the IBM2702 Transmission
Input/Output Operations 193
Working W
Interruption immediately
Not operational N
Interruption pending
Working
Not operational
N
Information for
Working
A
W
None of the
Interruption condition pending in device
Device executing an operation
Not operational N
Device not operational
Input/Output System States
ing in the subchannel, the channel also is in the same
state. The channel and subchannel, however, are not
synonymous, and an interruption condition not asso-,
ciated with data transfer, such as attention, does not
affect the state of the sub channel. Thus, the sub
channel may be available when the channel has an
interruption condition pending. Consistent distinc
tion between the subchannel and channel permits
selector and multiplexer channels to be covered uni
formly 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
trol unit imposes its state on all devices attached to
the control unit. The states of the devices 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 identi
fied as conditions for the execution of the I/O in
struction. Each composite state is identified in the
following discussion by three alphabetic
channel, the second identifies the state of the sub
channel, and the third refers to the state of the de
vice. 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 component is not significant for the
execution of the instruction.
Available (AAA): The addressed channel, sub chan
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
interruption conditions.
Interruption Pending in Device (AAI) or Device
Working (AA W): The addressed channel and sub
channel are available. The addressed control unit or
I/O device is executing a previously initiated opera
tion or contains a pending interruption condition.
These situations are possible:
1. The device is executing an operation, such as
rewinding tape or seeking on a disk file, after
signaling the channel-end condition.
2. The control unit associated with the device is
executing an operation, such as backspacing
file on a magnetic tape unit, after signaling the
channel-end condition.
3. The device or control unit is executing an oper
ation on another subchannel or channel.
4. The device or control unit contains the device
end, control-unit-end, or attention condition or
a channel-end condition associated with a ter
minated operation.
Device Not Operational (AAN): The addressed
channel and sub channel are available. The addressed
I/O device is not operational. A device appears not
operational when no control unit recognizes the ad
dress. This occurs when the control unit is not pro
vided 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. The not-operational
state is indicated also when the control unit is pro
vided and is designed to attach the device, but the
device has not been installed and the address has not
been assigned to the control unit (for example, the
second set of lines on the IBM
Input/Output Operations 193