functional areas to one another. This is how the
functional areas communicate with each other. A bus
is a parallel data communication path over which
information is transferred a byte or word at a time. The
buses contain logic that the CPU controls. The items
controlled are the transfer of data, instructions, and
commands between the functional areas of the
computer: CPU, memory, and I/O. The type of
information is generally similar on all computers; only
the names or terminology of the bus types differs. The
name of the bus or its operation usually implies the type
of signal it carries or method of operation.
The direction of signal flow for the different buses
is indicated on figures in the computer’s technical
manuals. The direction may be unidirectional or
bidirectional depending on the type of bus and type of
computer. Consult the computer’s technical manual for
details. After becoming familiar with the basic
functions and operations of buses, you’ll see that
regardless of the names, their basic concepts are
consistent throughout the computer. They provide
avenues for information to be exchanged inside the
computer.
BUS TYPES
The preferred method for data/information transfer
between system components is by a common data bus.
Where point-to-point data transfer is required, the
digital format is the preferred method. General
Requirements for Electronic Equipment Specifications,
MIL-STD-2036 series, provides a list of the industry
accepted standard internal data buses. They include
the standard and the interface as follows:
IEEE 696 —IEEE Standard 696 Interface
Devices, S-100
IEEE 896. l —IEEE Standard Backplane Bus
Specification for Multiprocessor Architecture,
Future Bus
IEEE 961 —Standard for an 8-bit
Microcomputer Bus System, STD Bus
IEEE 1014 —Standard for a Versatile Backplane
Bus, VMEbus
IEEE 1196 —Standard for a Simple 32-Bit
Backplane Bus, NuBus
IEEE 1296 —Standard for a High-performance
Synchronous 32-Bit Bus, Multibus II
All computers use three types of basic buses. The
name of the bus is generally determined by the type of
signal it is carrying or the method of operation. We
group the buses into three areas as you see them in their
most common uses. They are as follows:
Control (also called timing and control bus),
address, and data (also called a memory bus)
buses
Instruction (I), Operand (O), Input/Output
Memory (I/O MEM) or Input/Output Controller
(IOC), and Computer Interconnection System
(CIS)
Time multiplexed bus
Control Bus
The control bus is used by the CPU to direct and
monitor the actions of the other functional areas of the
computer. It is used to transmit a variety of individual
signals (read, write, interrupt, acknowledge, and so
forth) necessary to control and coordinate the
operations of the computer. The individual signals
transmitted over the control bus and their functions are
covered in the appropriate functional area description.
Address Bus
The address bus consists of all the signals necessary
to define any of the possible memory address locations
within the computer, or for modular memories any of
the possible memory address locations within a module.
An address is defined as a label, symbol, or other set of
characters used to designate a location or register where
information is stored. Before data or instructions can
be written into or read from memory by the CPU or I/O
sections, an address must be transmitted to memory
over the address bus.
Data Bus
The bidirectional data bus, sometimes called the
memory bus, handles the transfer of all data and
instructions between functional areas of the computer.
The bidirectional data bus can only transmit in one
direction at a time. The data bus is used to transfer
instructions from memory to the CPU for execution. It
carries data (operands) to and from the CPU and
memory as required by instruction translation. The data
bus is also used to transfer data between memory and
the I/O section during input/output operations. The
information on the data bus is either written into
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