The Bus

The CPU has to be able to send various data values, instructions, and information to all the devices and components inside your computer as well as the different peripherals and devices attached. If you look at the bottom of a motherboard you'll see a whole network of lines or electronic pathways that join the different components together. These electronic pathways are nothing more than tiny wires that carry information, data and different signals throughout the computer between the different components. This network of wires or electronic pathways is called the 'Bus'.

That's not that difficult to comprehend, but you've probably heard mention of the internal bus, the external bus, expansion bus, data bus, memory bus, PCI bus, ISA bus, address bus, control bus,… it really can get quite confusing.

A computer's bus can be divided into two different types, Internal and External.

The Internal Bus connects the different components inside the case: The CPU, system memory, and all other components on the motherboard. It's also referred to as the System Bus.

The External Bus connects the different external devices, peripherals, expansion slots, I/O ports and drive connections to the rest of the computer. In other words, the External Bus allows various devices to be added to the computer. It allows for the expansion of the computer's capabilities. It is generally slower than the system bus. Another name for the External Bus, is the Expansion Bus.

So now we know the bus is just a bunch of tiny wires (traces and electronic pathways). One bunch carries info around to the different components on the motherboard, and another bunch of wires connects these components to the various devices attached to the computer.

What kind of stuff travels on the bus? For one thing, data. Data has to be exchanged between devices. Some of the electronic pathways or wires of the Internal Bus or the External Bus are dedicated to moving data. These dedicated pathways are called the Data Bus.

Data is stored, manipulated and processed in system memory. System memory is like a vast sea of information full of fish (data). Your computer has to move information in and out of memory, and it has to keep track of which data is stored where. The computer knows where all the fishes are, but it has to transmit that information to the CPU and other devices. It has to keep a map of the different address locations in memory, and it has to be able to transmit and describe those memory locations to the other components so that they can access the data stored there. The info used to describe the memory locations travels along the address bus. The size, or width of the address bus directly corresponds to the number of address locations that can be accessed. This simply means that the more memory address locations that a processor can address, the more RAM it has the capability of using. It makes sense, right?

A 286 with a 16 bit address bus can access over 16 million locations, or 16 Mb of RAM. A 386 CPU with a 32 bit address bus can access up to 4 GB of RAM. Of course, at the present time, due to space and cost limitations associated with the average home computer, 4GB of RAM is not practical. But, the address bus could handle it if it wanted to! Another name for the address bus is the memory bus.