080 - What is System Bus in PC

A bus is a common pathway through which information flows from one computer component to another. This pathway is used for communication purpose and it is established between two or more computer components.

The bus contains multiple wires (signal lines) that contain addressing information that describes the memory location of where the data is being sent or where it is being retrieved. Each wire in the bus carries a single bit of information, which means the more wires a bus has the more information it can address. 

For example, a computer with a 32-bit address bus can address 4 GB of memory, and a computer with a 36-bit bus can address 64 GB of memory. 

A bus is capable of being a parallel or serial bus and today all computers utilize two bus types, an internal bus or local bus and an external bus, also called the expansion bus. 

An internal bus enables communication between internal components such as a video card and memory. An external bus is capable of communicating with external components such as a USB or SCSI device.

A computer or device's bus speed is listed in MHz, e.g. 100 MHz FSB. The throughput of a bus is measured in bits per second or megabytes per second.


In reality, each bus is generally constituted of 50 to 100 distinct physical lines, divided into three subassemblies: 
The address bus
• (sometimes called the memory bus) transports memory addresses which the processor wants to access in order to read or write data. It is a unidirectional bus. 
The data bus
• transfers instructions coming from or going to the processor. It is a bidirectional bus. 
The control bus
• (or command bus) transports orders and synchonisation signals coming from the control unit and travelling to all other hardware components. It is a bidirectional bus, as it also transmits response signals from the hardware.

Summary of functions of buses in computers

1. Data sharing
• All types of buses found in a computer transfer data between the computer peripherals connected to it. 
The buses transfer or send data in either serial or parallel method of data transfer. This allows for the exchange of 1, 2, 4 or even 8 bytes of data at a time. (A byte is a group of 8 bits). 

Buses are classified depending on how many bits they can move at the same time, which means that we have 8-bit, 16-bit, 32-bit or even 64-bit buses. 
2. Addressing
• A bus has address lines, which match those of the processor. This allows data to be sent to or from specific memory locations. 
3. Power
• A bus supplies power to various peripherals connected to it. 
4. Timing
• The bus provides a system clock signal to synchronize the peripherals attached to it with the rest of the system.

Computers have two major types of buses

1. System bus:
• This is the bus that connects the CPU to main memory on the motherboard. The system bus is also called the front-side bus, memory bus, local bus, or host bus. 
2. A number of I/O Buses:
• (I/O is an acronym for input / output), connecting various peripheral devices to the CPU. These devices connect to the system bus via a ‘bridge’ implemented in the processors chipset. Other names for the I/O bus include “expansion bus", "external bus” or “host bus”.

When referring to a computer, the bus also known as the address bus, data bus, or local bus.

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