What is BUS?
Bus: The
electrically conducting path along which data is transmitted inside any
digital electronic device. A bus consists of a set of parallel
conductors, which may be conventional wires, copper tracks on a PRINTED
CIRCUIT BOARD, or microscopic aluminum trails on the surface of a
silicon chip. Each wire carries just one bit, so the number of wires
determines the largest data WORD the bus can transmit: a bus with eight
wires can carry only 8-bit data words, and hence defines the device as
an 8-bit device. A bus normally has a single word memory
circuit called a LATCH attached to either end, which briefly stores the
word being transmitted and ensures that each bit has settled to its
intended state before its value is transmitted.
The bus helps the various parts of the PC
communicate. If there was no bus, you would have an unwieldy number of
wires connecting every part to every other part. It would be like having
separate wiring for every light bulb and socket in your house. There
are a variety of buses found inside the computer. The data bus allows data to travel back and forth between the microprocessor (CPU) and memory (RAM).
The address bus carries information about the location of data in
memory. The control bus carries the control signals that make sure
everything is flowing smoothly from place to place. If your computer has
expansion slots, there's an expansion bus. Messages and information pass between your computer and the add-in boards you
plug in over the expansion bus. Although this is a bit confusing, these
different buses are sometimes together called simply "the bus." A user
can think of the computer's "bus" as one unit made up of three parts:
data, address, and control, even though the three electrical pathways do
not run along each other (and therefore don't really form a single
"unit") within the computer.
There
are different sizes, or widths of data buses found in computers today. A
data bus' width is measured by the number of bits that can travel on it
at once. The
speed at which its bus can transmit words, that is, its bus BANDWIDTH,
crucially determines the speed of any digital device. One way to make a
bus faster is to increase its width; for example a 16-bit bus can
transmit two 8-bit words at once, 'side-by-side', and so carries 8-bit
data twice as fast as an 8-bit bus can. A computer's CPU will typically
contain several buses, often of differing widths, that connect its
various subunits. It is common for modern CPUs to use on-chip buses that
are wider than the bus they use to communicate with external devices
such as memory, and the speed difference between on- and off-chip
operations must then be bridged by keeping a reservoir of temporary data
in a CACHE. For example many of the Pentium class of processors use 256
bits for their fastest on-chip buses, but only 64 bits for external
links.
An
8-bit bus carries data along 8 parallel lines. A 16-bit bus, also
called ISA (Industry Standard Architecture), carries data along 16
lines. A 32-bit bus, classified as EISA (Enhanced Industry Standard
Architecture) or MCA (Micro Channel Architecture), can carry data along
32 lines.
The
speed at which buses conduct signals is measured in megahertz (Mhz).
Typical PCs today run at speeds between 20 and 65Mhz. Also see CPU,
Expansion Card, Memory, Motherboard, RAM, ROM, and System Unit.
How Does It Work?
A
bus transfers electrical signals from one place to another. An actual
bus appears as an endless amount of etched copper circuits on the
motherboard's surface. The bus is connected to the CPU through the Bus
Interface Unit.
Data
travels between the CPU and memory along the data bus. The location
(address) of that data is carried along the address bus. A clock signal
which keeps everything in synch travels along the control bus.
The
clock acts like a traffic light for all the PC's components; the "green
light" goes on with each clock tick. A PC's clock can "tick" anywhere
from 20 to 65 million times per second, which makes it seem like a
computer is really fast. But since each task (such as saving a file) is
made up of several programmed instructions, and each of those
instructions takes several clock cycles to carry out, a person sometimes
has to sit and wait for the computer to catch up.
Chapter : Introduction to Computer
