KarbosGuide.com. Module 2c.1
About the I/O buses
On these pages, you can read about the important
system bus derivatives, the different I/O buses:
Introduction to the I/O buses
We have seen before, that the PC's buses are the
fundamental data "highways" on the system board.
The "first" bus is the
, which connects the
CPU with RAM. In older designs it was a local bus. In
newer designs this bus is called the front side bus
The typical local bus has a speed and width
depending on the type CPU installed on the
motherboard. Typically, the system bus will be 64 bits
wide and run at 66, 100 or 133 MHz. These high
speeds create electrical noises and other problems.
Therefore, the speed must be reduced for data
reaching the expansion cards and other more
Very few expansion cards can operate at more than
40 MHz. Then the electronics shut down. The chips
can just not react faster. Therefore, the PC has
Originally only one bus
However, the first PCs had only one bus, which was
common for the CPU, RAM and I/O components:
The older first and second generation CPUs ran at
relatively low clock frequencies, and all system
components could keep up with those speeds.
RAM on adapters
Among other things, that allowed additional RAM to be
installed in expansion slots in the PC, by installing an
adapter in a vacant expansion slot. An adapter, where
RAM was mounted:
This setup would be unthinkable today. However it is
truely a local bus. All units are united on one bus using
the same clock.
First in 1987, Compaq figured out how to separate
system bus from I/O bus, so they could run at different
speeds. This multi-bus architecture has been industry
standard ever since. Modern PCs also have more than
one I/O bus.
What does an I/O bus do?
I/O buses connect the CPU to all other components,
except RAM. Data are moved on the buses from one
component to another, and data from other
components to the CPU and RAM. The I/O buses
differ from the system bus in speed. Their speed will
always be lower than the system bus speed. Over the
years, different I/O buses have been developed. On
modern PCs, you will usually find four buses:
— The ISA bus, which is an old low speed bus, soon
to be excluded from the PC design.
p The PCI bus, which is a new high speed bus.
The USB bus (
Universal Serial Bus)
, which is a
new low speed bus.
— The AGP bus which solely is used for the graphics
As mentioned earlier, I/O buses are really extensions