Computer Bus - What Is It?

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In computing, a bus is defined as a set of physical connections (i.e. cables, printed circuits, etc.), which can be shared by multiple hardware components in order to communicate with one another.

The purpose of buses is to reduce the number of "pathways" needed for communication 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).



Characteristics

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.

This way, 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

Architecture

In reality, each bus is generally constituted of 50 to 100 distinct physical lines, divided into three subassemblies. The first is the address bus (sometimes called the memory bus), which transports memory addresses that the processor wants to access in order to read or write data. It is a unidirectional bus.

The second is the data bus, which transfers instructions coming from or going to the processor. It is a bidirectional bus.

The third one is the control bus, or the command bus, which 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 first is the internal bus (sometimes called the front-side bus, or FSB for short). The internal bus allows the processor to communicate with the system's central memory (the RAM).

The second is the expansion bus (sometimes called the input/output bus), which allows various motherboard components (i.e. 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 that are called expansion slots connected to the input/output bus.

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), which is in charge of controlling transfers between the processor and the RAM, which is why it is located physically near the processor. It is sometimes called the GMCH, for Graphic and Memory Controller Hub</ital>.

The second is the <bold>SouthBridge
(also called the input/output controller or expansion controller), which 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
EISA328.331.8
VLB3233127.2
PCI 32-bit3233127.2
PCI 64-bit 2.16466508.6
AGP3266254.3
AGP (x2 Mode)3266x2528
AGP (x4 Mode)3266x41056
AGP (x8 Mode)3266x82112
ATA33163333
ATA1001650100
ATA1331666133
Serial ATA (S-ATA)1180
Serial ATA II (S-ATA2)2380
USB11.5
USB 2.0160
FireWire1100
FireWire 21200
SCSI-184.775
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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Jean-François Pillou - Founder of CCM
Better known as Jeff, Jean-François Pillou is the founder of CommentCaMarche.net. He is also CEO of CCM Benchmark and digital director at the Figaro Group.

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Latest update on June 17, 2018 at 06:17 PM by Olivia Long.

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