Systems Reference Library IBM System/360 Principles of Operation

the device on the control unit is fixed and does not depend
on the path of communications.
In models in which more than 128 sub channels are
available, the shared subchannels can optionally be replaced
by sets of unshared subchannels. When the
option is implemented, the additional unshared subchannels
are assigned sequential addresses starting at
128.
Except for the rules described, the assignment of device
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 device
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 operation
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 instruction,
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 interruption:
Privileged Operation: An I/O instruction is encountered
when the CPU is in the problem state. The instruction
: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 address
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 available,
that contains a pending interruption condition, or
that is working, is said to be operational. The states of
containing an interruption condition, working, or being
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 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 concurrently
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 sub channel, however, are not synonymous,
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 distinction
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 includes
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 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
on the basis of the states of the channel and
subchannel, the components further removed are not
interrogated. Thus, ten composite states are identified