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The transfer of information to or from main
storage, other than to or from the central
processing unit, is referred to as an input or output
operation. An input/output (I/O) operation
involves the use of an I/O device. Input/output
devices perform I/O operations under control of
control units, which are attached to the central
processing unit (CPU) by means of channels.
This portion of the publication describes the
programmed control of I/O 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 I/O operations and are
independent of the type of I/O device, its speed,
and its mode of operation.
The formats described inclu<;le provisions for
functions unique to some I/O 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) publication for the
particular device.
All main-storage references for I/O operations
are references to virtual storage. Unless indicated
otherwise, "storage" means virtual storage, and
"address" means virtual address. The terms "I/O
address," "channel address," and "device address"
are never abbreviated to "address" in this
publication.
Attachment of Input/Output Devices
Input / Output Devices
Input/ output devices provide external storage and a
means of communication between data-processing
systems or between a system and its environment.
Input/ output devices include such equipment as
card readers, card punches, magnetic-tape units,
direct-access-storage devices (disks and drums),
display units, typewriter-keyboard devices, printers,
teleprocessing devices, and sensor-based equipment.
Most types of I/O devices, such as printers, card
equipment, or tape devices, deal directly with
external media, and these devices are physically
distinguishable 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 physical recording medium
outside main storage. Similarly, a transmission-
control unit handles transmission of information
between the data-processing system and a remote
station, and its input and output are signals on a
transmission line. An I/O device may
12-2
IBM 4300 Processors Principles of Operation
be physically distinct equipment, or it may
time-share equipment with other I/O devices.
An input/output device ordinarily is attached to
one control unit and is accessible from one channel.
Switching equipment is available to make some
devices accessible to two or more channels by·
switching devices between control units and control
units between channels. The time required for
switching occurs during device-selection time and
may be ignored.
Control Units
A control unit provides the logical capabilities
necessary 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.
The control unit accepts control signals from the
channel, controls the timing of data transfer, and
provides indications concerning the status of the
device.
The 1/
a
device attached to the control unit may
be designed to perform only certain limited
operations, 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 most electromechanical
devices, a well-defined interface exists between 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
operate 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 other 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 programmer's point of view, most
functions performed by the control unit can be
merged with those performed by the I/O device.
Therefore, this publication normally does not make
specific mention of the control unit function; the
execution of I/O operations is described as if the
I/O devices communicated directly with the
channel. Reference is made to the control unit
only when emphasizing a function performed by it
(
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