Input / Output Device Addressing
An I/O device and the associated access path are
designated by an I/O address. The I/O address is a
16-bit binary number and consists of two parts: a
channel address in the eight high-order bit positions
and a device address in the eight low-order bit posi­
tions.
The channel-address field provides for identifying
up to 256 channels. Channel 0 is a byte-multiplexer channel; channels numbered 1-255 may be either
multiplexer or selector channels.
The number and type of channels available, as
well as their address assignment, depend on the sys­
tem model and the particular installation.
The device address identifies the particu1ar I/O device and control unit on the designated channel.
The address identifies, for example, a particular
magnetic tape drive, disk-access mechanism, or
transmission line. Any number in the range 0-255 can be used as a device address, providing facilities
for addressing up to 256 devices per channel. An
exception is some byte-multiplexer channels that provide fewer than the maximum configuration of
subchannels and hence eliminate the corresponding
unassignable device addresses.
Devices that do not share a control unit with oth­
er devices may be assigned any device address in the
range 0-255, provided the address is not recognized
by any other control unit. Logically, such devices are
not distinguishable from their control unit, and both
are identified by the same address.
Devices sharing a control unit (for example, mag­
netic tape drives or disk-access mechanisms) are
assigned addresses within sets of contiguous num­
bers. The size of such a set is equal to the maximum
number of devices that can share the control unit, or
16, whichever is smaller. Furthermore, such a set
starts with an address in which the number of low­
order zeros is at least equal to the number of bit
positions required for specifying the set size. The
high-order bit positions of an address within such a set identify the control unit, and the low-order bit
positions designate the device on the control unit.
Control units designed to accommodate more than
16 devices may be assigned nonsequential sets of ad­
dresses, each set consisting of 16, or the number re­
quired to bring the total number of assigned addresses equal to the maximum number of devices attachable
to the control unit, whichever is smaller. The address­ ing facilities are added in increments of a set so that
the number of device addresses assigned to a control
unit does not exceed the number of devices attached
by more than 15. 192 System/370 Principles of Operation
The control unit does not respond to any address
outside its assigned set or sets. For example, if a
control unit is designed to control devices having
only bits 0000-1001 in the low-order positions of
the device address, it does not recognize addresses
containing 1010-1111 in these bit positions. On the
other hand, a control unit responds to all addresses
in the assigned set, regardless of whether the device
associated with the address is installed. For example,
the IBM 3830 Storage Control Model 2, with four
disk units installed, responds to all of the 16 address­
es within the set assigned to it. If no control unit
responds to an address, the I/O device appears not
operational. If a control unit responds to an address
for which no device is installed, the absent device
appears in the not-ready state.
Input/ output devices accessible through more
than one channel have a distinct address for each
path of communications. This address identifies the
channel and the control unit. For sets of devices
connected to two or more control units, the portion
of the address identifying the device on the control
unit is fixed, and does not depend on the path of
communications.
Except for the rules described, the assignment of
channel and device addresses is arbitrary. The as­
signment 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 collective state of the chan­
nel, subchannel, and I/O device. Each of these com­
ponents 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 abbre­
viation and a brief definition.
A portion of the 110 system that is available,
interruption-pending, or working is called "operational." A portion of the I/O system that is
interruption-pending, working, or not-operational is
called "not available."
In the case of a multiplexer channel, the channel
and subchannel are easily distinguishable and, if the
channel is operational, any combination of channel
and subchannel 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 sub­
channel. In particular, the working state can occur
only concurrently in both the channel and subchan­
nel and, whenever an interruption condition is pend-

Name Abbreviation and Definition Channel Available A None of the following states I nterruption pending
Working W
Interruption immediately available from channel Channel operating in burst mode
Not operational N Channel not operational Subchannel Available A None of the following states
Interruption pending
Working
Not operational I W
N
Information for CSW available in subchannel Subchannel executing an operation Subchannel not operational I/O Device Available Interruption pending
Working
A
W
None of the following 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 alphabetic characters; 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 IBM 2702 Transmission
Input/Output Operations 193