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Channels
A channel relieves the CPU of the burden of
communicating directly with I/O devices and
permits data processing to proceed concurrently
with I/O operations. A channel connects with the
CPU, with main storage, and with control units.
A channel may be an independent unit, complete
with the necessary logical and internal-storage
capabilities, or it may time-share CPU facilities and
be physically integrated with the CPU. In either
case, the functions performed by a channel are
identical. The maximum data-transfer rate may
differ, however, depending on the implementation.
There are three types of channels: byte-
multiplexer, block-multiplexer, and selector
channels.
Input / Output Devices and Control Units
Input/ output devices include such equipment as
card· readers and punches, magnetic-tape units,
direct-access storage, displays, keyboards, printers,
teleprocessing devices, communications controllers,
and sensor-based equipment. Many I/O devices
2-4
IBM 4300 Processors Principles of Operation
function with an external medium, such as punched
cards or magnetic tape. Some I/O devices handle
only electrical signals, such as those found in
sensor-based networks. In either case, I/O-device
operation is regulated by a control unit. In all
cases, the control-unit function provides the logical
and buffering capabilities necessary to operate the
associated I/O device. From the programming
point of view, most control-unit functions merge
with I/O-device functions. The control-unit
function may be housed with the I/O device or in
the CPU, or a separate control unit may be used.
Operator Facilities
The operator facilities provide the functions
necessary for operator control of the machine.
Associated with the operator facilities may be an
operator-console device, which may also be used as
an I/O device for communicating with the program.
The main functions provided by the operator
facilities are system reset, clearing, initial program
loading, start, stop, alter, and display.
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