What does an I/O bus do

What does an I/O bus do?

Answer:

I/O busses connect the CPU to all other components, except RAM. Data are moved on the
busses from one component to another, and data from other components to the CPU and
RAM. The I/O busses differ from the system bus in speed. Their speed will always be lower
than the system bus speed. Over the years, different I/O busses have been developed. On
modern Pentium PC's, you will find at least two significant busses, and one less significant:
*The ISA bus, which is an older low speed bus.
*The PCI bus, which is a new high speed bus.
* The USB bus (Universal Serial Bus), which is a new low speed bus.
As mentioned earlier, I/O busses are really derivatives from the system bus. On the system
board it ends in a controller chip, which forms a bridge to the two other busses.
The essential in modern PC's is fast busses. Let us compare the two primary I/O busses:

Bus	Transmission time	Data volume per transmission
ISA	375 ns	16 bit
PCI	30 ns	32 bit

Clearly, there is a vast difference between the capacity of the two busses.
All in all, the busses have a very central placement in the PC's data exchange. Actually, all
components except the CPU communicate with each other and with RAM via the different I/O
busses.

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