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state on all devices attached to the control unit.
The states of the devices are not related to those of
the channel and subchannel.
When the response to an I/O instruction is
determined by the state of the channel or
subchannel, the components further removed are
not interrogated. Thus, 10 composite states may be
distinguished as conditions for the execution of I/O
instructions. Each composite state is identified by
three letters. The first letter specifies the state of
the channel, the second letter specifies the state of
the subchannel, and the third letter specifies the
state of the device. Each letter may be A, I, W, or
N, denoting the state of the component. The letter
X 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
~re
operational, are not engaged in the execution of
any previously initiated operations, and do not
contain any pending interruption conditions.
Because of internal activity, some block-
multiplexer channels may at times appear to be
working even though they are not engaged in the
execution of a previously initiated operation and do
not contain any interruption condition. This will
result in a WXX state instead of the AAA state.
Interruption Pending in Device (AAI) or Device
Working (AA W): The addressed channel and
subchannel are available. The addressed control
unit or I/O device is executing a previously
initiated operation or contains an interruption
condition. These situations are possible:
1. The device is executing an operation, such as
rewinding magnetic tape or seeking on a disk
file, after signaling channel end.
2. The control unit associated with the device is
executing an operation, such as backspacing file
on a magnetic-tape unit, after signaling channel
end.
3. The device or control unit is executing an
operation 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 terminated 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 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 disconnected from the
system. The not-operational state is indicated also
when the control unit is provided 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. (See also the section
"Input/Output Device Addressing" in this
chapter.)
If
the addressed device is not installed or has
been logically removed from the control unit, but
the associated control unit is operational and the
address has been assigned to the control unit, the
device is said to be not ready. When an instruction
is addressed to a device in the not-ready state, the
control unit responds to the selection and indicates
unit check whenever the not-ready state precludes
a successful execution of the operation. (See the
section "Unit Check" in this chapter.)
Interruption Pending in Sub channel (AIX): The
addressed channel is available. An interruption
condition is pending in the addressed subchannel.
The subchannel is able to provide information for a
CSW. The interruption information indicates status
associated with the addressed device or another
device on the sub channel. The state of the
addressed device is not significant, except when
TEST I/O is addressed to the device associated
with the interruption condition, in which case the
CSW contains status information provided by the
device.
The state AIX does not occur on the selector
channel. On the selector channel, the existence of
an interruption condition in the subchannel
immediately causes the channel to assign to this
condition the highest priority for I/O interruptions
and, hence, leads to the state IIX.
Subchannel Working (A WX): The addressed
channel is available. The addressed subchannel is
executing a previously initiated operation or chain
of operations and has not yet received channel end
for the last operation. The state of the addressed
device is not significant, except when HALT I/O
or HALT DEVICE is issued. During the execution
of HALT I/O and HALT DEVICE; the state of
the device may be interrogated and will then be
indicated in either the CSW or the condition code.
The subchannel-working state does not occur on
the selector channel since all operations on the
Chapter 12. Input/Output Operations
12-9
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