is designed to control devices having
only the values 0000 to 1001 in the
rightmost bit positions of the device
address, it does not recognize device
addresses containing 1010 to 1111 in
these bit positions. On the other hand,
a control unit responds to all device
addresses in the assigned set for which the corresponding I/O devices are ready,
or are not ready but can be made ready
by means of an ordinary manual inter­
vention. A control unit mayor may not
respond to an address within the
assigned set when the corresponding
device is not installed or has been
logically removed from the control unit.
If a control unit responds to a device
address for which no I/O device is
installed or the device has been
logically removed from the control unit,
the absent device appears in the not­
ready state. If no control unit
responds to the device address, the I/O device appears not operational.
Input/output devices accessible through
more than one channel in a channel set
have a distinct I/O address for each
path of communications. This I/O address identifies the channel and the
control unit. For sets of devices shar­
ing a control unit or connected to two
or more control units, the portion of
the I/O address identifying the device
on the control unit is fixed and does
not depend on the path of
communications.
The assignment of I/O addresses is arbi­
trary, 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 collec-
tive state of the channel, subchannel,
and I/O device. Each of these compo­
nents 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 abbrevi­
ation 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 distinguisha­
ble and, if the channel is operational,
any combination of channel and subchan­ nel 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 concur­
rently 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, may not
affect the state of the subchannel.
Thus, the subchannel may as a function
of the I/O instruction, be available
when the channel is interruption-pending
or has an interruption condition pending
at a device. A consistent distinction
between the subchannel and channel
permits selector and multiplexer chan­
nels to be covered uniformly by a single
description. Chapter 13. Input/Output Operations 13-9
Name Abbreviation and Definition Channel Available
Interruption pending
Working
Not operational
A
I
W N None of the following states
Interruption condition immedi­
ately available from channel Channel operating in burst mode Channel not operational
Subchannel
Available
Interruption pending
A
I
None of the following states
Information for CSW available in
subchannel
Working
Not operational
W N Subchannel executing an operation
Subchannel not operational
I/O Device
Available
Interruption pending
Working
Not operational
A
I
W N None of the following states
Interruption condition in device Device executing an operation
Device not operational
Input/Output-System States
The I/O device referred to in the figure
"Input/Output-System states" includes
both the I/O device proper and its
control unit. For some types of I/O devices, such as magnetic-tape units,
the working and the interruption-pending
states can be caused by activity in the
addressed I/O device or control unit. A
"not available" shared control unit
imposes its state on all devices
attached to the control unit. The
states of the I/O 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 chan­
nel or subchannel, the components
further removed are not interrogated.
Thus, 10 composite states may be distin­
guished 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
are operational, are not engaged in the
execution of any previously initiated
operations, and do not contain any pend­
ing interruption conditions.
Because of internal activity, some
block-multiplexer channels may at times 13-10 System/370 Principles of Operation 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.
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. When the control unit
responds to the selection of a not-ready
device, the device is said to be opera­
tional and therefore in the available
state even though unit check is indi­
cated. (See the section "Unit Check" in
this chapter.)
Interruption Pending in Device (AAI) or
Device Working (AAW): 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 opera­
tion, 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 opera­
tion, such as backspacing file on a
Previous Page Next Page