Computer Bus: What Is It?

In computing, a bus is defined as a set of physical connections (for example, cables, printed circuits, etc.) that can be shared by multiple hardware components in order to communicate with one another. The purpose of buses is <boldWto reduce the number of pathways needed for communication</bold> between the components, by carrying out all communications over a single data channel. This is why the metaphor of a data highway is sometimes used.

schematic diagram of a bus


If only two hardware components communicate over the line, it is called a hardware port, (such as a port or parallel port).

There are different types of computer buses. A bus is characterized by the amount of information that can be transmitted at once. This amount, expressed in bits, corresponds to the number of physical lines over which data is sent simultaneously. A 32-wire ribbon cable can transmit 32 bits in parallel. The term width is used to refer to the number of bits that a bus can transmit at once.

Additionally, the bus speed is defined by its frequency (expressed in Hertz), the number of data packets sent or received per second. Each time that data is sent or received is called a cycle.

It is possible to find the maximum transfer speed of the bus, or the amount of data that it can transport per unit of time, by multiplying its width by its frequency. For example, a bus with a width of 16 bits and a frequency of 133 MHz has a transfer speed equal to:

16 * 133.106 = 2128*106 bit/s, 
or 2128*106/8 = 266*106 bytes/s
or 266*106 /1000 = 266*103 KB/s
or 259.7*103 /1000 = 266 MB/s


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, it transports memory addresses that the processor wants to access in order to read or write data. It is a unidirectional bus.
  • The data bus: it transfers instructions coming from or going to the processor. It is a bidirectional bus.
  • The control bus, or the command bus: it transports orders and synchronization signals coming from the control unit and traveling to all other hardware components. It is a bidirectional bus, as it also transmits response signals from the hardware.

The Primary Buses

There are generally two buses within a computer.

  • The internal bus: sometimes called the front-side bus, or FSB for short, it allows the processor to communicate with the system's central memory (the RAM).
  • The expansion bus: sometimes called the input/output bus, it allows various motherboard components (namely, USB, serial, and [contents/415-serial-port-and-parallel-port parallel ports], cards inserted in PCI connectors, hard drives, CD-ROM and CD-RW drives, etc.) to communicate with one another. However, it is mainly used to add new devices using what it’s called expansion slots connected to the input/output bus.

Motherboard Chipset

What is “chipset”? A chipset is the component that routes data between the computer's buses, so that all the components that make up the computer can communicate with each other. The chipset originally was made up of a large number of electronic chips, hence the name.

It generally has two components. The first is the NorthBridge (also called the memory controller) that is in charge of controlling transfers between the processor and the RAM; this is the reason why it is located physically near the processor. It is sometimes called the GMCH, for Graphic and Memory Controller Hub.

The second is the SouthBridge (also called the input/output controller or expansion controller) that handles communications between peripheral devices. It is also called the ICH (I/O Controller Hub). The term bridge is generally used to designate a component that connects two buses:

system architecture of a PC

It is noteworthy that, in order to communicate, two buses must have the same width. This explains why RAM modules sometimes have to be installed in pairs (for example, early Pentium chips, whose processor buses were 64-bit, required two memory modules each 32 bits wide).

Here is a table that gives the specifications for the most commonly used buses:

StandardBus width (bits)Bus speed (MHz)Bandwidth (MB/sec)
ISA 8-bit88.37.9
ISA 16-bit168.315.9
PCI 32-bit3233127.2
PCI 64-bit 2.16466508.6
AGP (x2 Mode)3266x2528
AGP (x4 Mode)3266x41056
AGP (x8 Mode)3266x82112
Serial ATA (S-ATA)1180
Serial ATA II (S-ATA2)2380
USB 2.0160
FireWire 21200
SCSI-2 - Fast81010
SCSI-2 - Wide161020
SCSI-2 - Fast Wide 32 bits321040
SCSI-3 - Ultra82020
SCSI-3 - Ultra Wide162040
SCSI-3 - Ultra 284040
SCSI-3 - Ultra 2 Wide164080
SCSI-3 - Ultra 160 (Ultra 3)1680160
SCSI-3 - Ultra 320 (Ultra 4)1680 DDR320
SCSI-3 - Ultra 640 (Ultra 5)1680 QDR640

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