Systems Reference Library IBM System/360 Principles of Operation

as conditions for the execution of the I/O instruction.
Each composite state is identified in the following discussion
by three alphabetic characters; the first character
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 component
is not significant for the execution of the instruction.
Available (AAA): The addressed channel, subchannel,
control unit, and I/O device are operational, are
not engaged in the execution of any previously initiated
operations, and do not contain any pending interruption
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 addressed
sub channel and channel are available. These
situations are possible:
1. The device is executing an operation after signaling
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 channelend
condition, such as backspacing file on a magnetic
tape unit.
3. The device or control unit is executing an operation
on another sub channel or channel.
4. The device or control unit contains the deviceend,
control-unit-end, or attention condition or, on
the selector channel, the channel-end condition associated
with an operation terminated by HALT I/O.
Device Not Operational (AAN): The addressed I/O
device is not operational. A device appears not operational
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 notoperational
state is indicated also when the addressed
device is not installed on the control unit. The addressed
subchannel and channel are available.
For devices such as magnetic tape units, the device
appears operational as long as the control unit associated
with the addressed device is operational. If
the device is not installed or has been logically removed
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 interruption
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 interruption
pending state.
On the selector channel the existence of an interruption
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 subchannel
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 operation.
All devices sharing the currently operating control
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 operational.
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 addressed
channel has established which device will
cause the next I/O interruption from this channel.
The state where the channel contains a pending interruption
condition is distinguished only by the instruction
TEST CHANNEL. This instruction does not
cause the subchannel and I/O device to be interrogated.
The other I/O instructions consider the channel
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