The system bus is a pathway composed of cables and connectors used to carry data between a computer microprocessor and the main memory. The bus provides a communication path for the data and control signals moving between the major components of the computer system. The system bus works by combining the functions of the three main buses: namely, the data, address and control buses. Each of the three buses has its separate characteristics and responsibilities.
The system bus connects the CPU with the main memory and, in some systems, with the level 2 (L2) cache. Other buses, such as the IO buses, branch off from the system bus to provide a communication channel between the CPU and the other peripherals.
The system bus combines the functions of the three main buses, which are as follows:
The control bus carries the control, timing and coordination signals to manage the various functions across the system.
The address bus is used to specify memory locations for the data being transferred.
The data bus, which is a bidirectional path, carries the actual data between the processor, the memory and the peripherals.
The design of the system bus varies from system to system and can be specific to a particular computer design or may be based on an industry standard. One advantage of using the industry standard is the ease of upgrading the computer using standard components such as the memory and IO devices from independent manufacturers.
System bus characteristics are dependent on the needs of the processor, the speed, and the word length of the data and instructions. The size of a bus, also known as its width, determines how much data can be transferred at a time and indicates the number of available wires. A 32-bit bus, for example, refers to 32 parallel wires or connectors that can simultaneously transmit 32 bits.
The design and dimensions of the system bus are based on the specific processor technology of the motherboard. This, in effect, affects the speed of the motherboard, with faster system buses requiring that the other components on the system be equally fast for the best performance.