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.239 Interruption Action.
Channel Status Word
Unit Status Conditions .
Attention. Status Modifier .
Control Unit End
Busy .
Channel End.
Device End .
Unit Check .
Unit Exception
Channel Status Conditions . Program-Controlled I ntHrruption Incorrect Length. Program Check . Protection Check
Channel Data Check.
Channel Control Check. Interface Control Check
Chaining Check .
Contents of Channel Status Word. Information Provided by Channel Status Word Protection Key .
Command Address
Count. Status. Channel Logout . I/O Communications Area.
The transfer of information to or from main storage,
other than to or from the central processing unit or
by means of the direct control path, is referred to as
an input or output operation. An input I output (II 0) operation involves the use of an 110 device.
Inputloutput devices perform 110 operations under
control of control units, which are attached to the
central processing unit (CPU) by means of channels.
This portion of the manual describes the program­
med control of 110 devices by the channels and by
the CPU. Formats are defined for the various types
of I/O control information. The formats apply to all 1/ 0 operations and are independent of the type of 1/0 device, its speed, and its mode of operation.
The formats described include provisions for
functions unique to some 110 device types, such as
an erase gap on a magnetic tape unit. The way in
which a device makes use of the format is defined in
the System Library (SL) or Systems Reference Li­
brary (SRL) publication for the particular device.
Attachlnent of Input/Output Devices
Input/()utput Devices
Inputloutput devices provide external storage and a
means of communication between data processing
systems or between a system and its environment.
186 System/370 Principles of Operation
Inputloutput devices include such equipment as card
readers, card punches, magnetic tape units, direct­
access-storage devices (disks and drums),
typewriter-keyboard devices, printers, teleprocessing
devices, and sensor-based .equipment.
Most types of 110 devices, such as printers, card
equipment, or tape devices, deal directly with exter­
nal media, and these devices are physically distin­
guishable and identifiable. Other types consist only
of electronic equipment and do not directly handle
physical recording media. The channel-to-channel
adapter, for example, provides a channel-to-channel
data transfer path, and the data never reaches a phys­
ical recording medium outside main storage. Similar­
ly, the IBM 2702 TransmIssion Control handles
transmission of information between the data
processing system and a remote station, and its input
and output are signals on a transmission line. Fur­
thermore, in this latter case, the 2702 may be time­
shared for concurrent operation of a number of re­
mote stations, and the 2702 is distinguished as a
particular 110 device only during the time period
associated with the operation on the corresponding
remote station.
An inputloutput device ordinarily is attached to
one control unit and is accessible from one channel.
Switching equipment is available to make some de­
vices accessible to two or more channels by switch-

ing devices between control units and control units
between channels. The time required for switching
occurs during device selection time and may be ig­
nored.
Control Units
A control unit provides the logical capabilities neces­
sary to operate and control an I/O device, and
adapts the characteristics of each device to the
standard form of control provided by the channel.
In most configurations, communication between
the control unit and the channel takes place over the I/O interface. The control unit accepts control sig­
nals from the channel, controls the timing of data
transfer over the I/O interface, and provides indica­
tions concerning the status of the device.
The I/O device attached to the control unit may
be designed to perform only certain limited opera­
tions, or it may perform many different operations.
A typical operation is moving the recording medium
and recording data. To accomplish these functions,
the device needs detailed signal sequences peculiar
to the type of device. The control unit decodes the
commands received from the channel, interprets
them for the particular type of device, and provides
the signal sequence required for execution of the
operation.
A control unit may be housed separately, or it
may be physically and logically integral with the I/O device or the CPU. In the case of most electrome­
chanical devices, a well-defined interface exists be­
tween the device and the control unit because of the
difference in the type of equipment the control unit
and the device contain. These electromechanical
devices often are of a type where only one device of
a group attached to a control unit is required to op­
erate at a time (magnetic tape units or disk-access
mechanisms, for example), and the control unit is
shared among a number of I/O devices. On the oth­
er hand, in some electronic I/O devices such as the
channel-to-channel adapter, the control unit does
not have an identity of its own.
From the user's point of view, most functions
performed by the control unit can be merged with
those performed by the I/O device. Therefore, this
manual normally does not make specific mention of
the control unit function; the execution of I/O oper­
ations is described as if the I/O devices communi­
cated directly with the channel. Reference is made to
the control unit only when emphasizing a function
performed by it or when sharing of the control unit
among a number of devices affects the execution of I/O operations.
Channels
The channel directs the flow of information between I/O devices and main storage. It relieves the CPU of
the task of communicating directly with the devices
and permits data processing to proceed concurrently
with I/O operations.
The channel provides a standard interface for
connecting different types of I/O devices to the CPU and to main storage. It accepts control infor­
mation from the CPU in the format supplied by the
program and changes it into a sequence of signals
acceptable to a control unit. After the operation with
the device has been initiated, the CPU is released for
other work, and the channel assembles or disassem­
bles data and synchronizes the transfer of data bytes
over the interface with main-storage cycles. To ac­
complish this, the channel maintains and updates an
address and a count that describe the destination or
source of data in main storage. Similarly, when an I/O device provides signals that should be brought
to the attention of the program, the channel trans­
forms the signals to information that can be used in
the CPU. The channel contains common facilities for the
control of I/O operations. When these facilities are
provided in the form of separate autonomous equip­
ment designed specifically to control I/O devices, I/O operations are completely overlapped with the
activity in the CPU. The only main-storage cycles
required during I/O operations in such channels are
those needed to transfer data and control informa­
tion to or from the final locations in main storage.
These cycles do not interfere with the CPU program,
except when both the CPU and the channel concur­
rently attempt to refer to the same main storage.
Alternatively, the system may use the facilities of
the CPU for controlling I/O devices. When the CPU and the channel, or the CPU, channel, and control
unit, share common facilities, I/O operations cause
interference to the CPU, varying in intensity from
occasional delay of a CPU cycle to a complete lock­
out of CPU activity. The intensity depends on the
extent of sharing and on the I/O data rate. The
sharing of the facilities, however, is accomplished
automatically, and the program is not affected by CPU delays, except for an increase in execution
time.
Modes of Operation
An I/O operation occurs in one of two modes: burst
or byte interleave.
In burst mode, the I/O device monopolizes the I/O interface and channel and stays logically con­
nected to the channel for the transfer of a burst of
information. No other device can communieate over
Input/Output Operations 187