The motherboard is a circuit board that connects the CPU to the memory and all the other hardware. The CPU sits on the motherboard (also called the logic board).

Buses are circuits on the motherboard that connect the CPU to other components. There are many buses on the motherboard. A bus moves instructions and data around the system. The speed of a bus is measured in megahertz (MHz). The faster the bus, the faster data is communicated. The speed of the motherboard is defined by the bus speed.

Buses are limited by their width in bits. They are usually 8, 16 or 32-bits wide. This tells us how many bits can be sent by the bus at any one time, eg a 32-bit bus can send 32 bits at once.

The bus that connects the CPU to the memory is called the front-side bus (FSB) or system bus. CPU cores share Level 2 and Level 3 cache across the FSB. They will usually connect to Level 2 cache through the back-side bus (BSB). The BSB is much faster than the FSB.

The FSB contains two types of bus:

  • Address bus - this sends information about where data needs to go by sending an address to the memory. The address bus only sends data in one direction - from the CPU to RAM.
  • Data bus - this sends data to the memory or receives data from the memory. Data can flow both ways along the data bus.
On small CPUs, the data bus and address bus are sometimes combined into one bus. This is called multiplexing.