Computer Bus

What is a computer bus

In informatics, it is called bus (bus) the set of physical connections (cables, printed circuit tracks, etc.) that can be used together by the various material elements in order to execute a communication.

Busbars aim to reduce the number of routes required for the communication of the various components, bringing together communications through a single data channel. This is the reason why the ‘data freeway’ metaphor is used to refer to buses:

If the line serves only for the communication of two material components, there is talk of material door (serial port, parallel port, etc.).

How a bus works

This volume, expressed in bits, corresponds to the number of physical lines on which the data is sent simultaneously. A 32-wire coverage thus allows 32 bits to be transmitted in parallel. There is talk of width, to designate the number of bits that a channel can transmit simultaneously.

On the other hand, the speed of the bus is also defined by its frequency (expressed in Hertz), that is, the number of data packets sent or received, per second. We speak of a cycle to designate each sending or receiving of data. In this way, it is possible to know the velocity maximum channel (or maximum transfer rate), that is, the amount of data it can carry per unit of time, multiplying its width by its frequency. A bus with a width of 16 bits, clocked at a frequency of 133 MHz has a speed equal to:

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

How are the bus subsets

Generally, each bus consists of 50 to 100 distinct physical lines, classified into three functional subsets: the address bus (also called an addressing bus or memory bus) is unidirectional and carries the memory addresses that the processor wants to access to read or write data; O data bus it is bidirectional and conveys instructions coming from or going to the processor; it’s the control bus (also called the order bus) is bidirectional and carries orders and synchronization signals from the control unit and going to the set of material components.

What are the main types of bus

On a computer, we generally distinguish two main buses: the system bus (also called the internal bus) and the extension bus. The first allows the processor to communicate with the central memory of the system (RAM memory) while the second (sometimes called the input / output bus) allows the various components of the motherboard (USB, serial, parallel, cards connected to the PCI connectors, hard drives, CD-ROM readers and writers) to communicate with each other and the addition of new devices thanks to the extension connectors (called slots) connected to the input / output bus.

What is a chipset

It’s called chipset the element in charge of pinning the information between the various buses of the computer so that all of them can communicate. Originally, the chipset it consisted of a large number of electronic components, which explains its name. Currently, most of them are composed of two elements: the NorthBridge (also called memory controller), which is in charge of controlling the exchanges between the processor and the living memory and is located close to the processor, and the SouthBridge (also called an input / output or extension controller), which manages communications with the input / output peripherals. The south bridge is also called ICH (I / O Hub controller).

We generally call a bridge an interconnection element between two buses:

It is interesting to note that, in order to communicate with each other, two buses must be the same width. This explains why RAM memory modules must often be paired on certain systems. For example, in the first Pentium, whose processor bus width was 64 bits, it was necessary to install memory modules with a width of 32 bits per pair.

See the table below which summarizes the characteristics of the main buses:

Standard	Bus width (bits)	Bus speed (MHz)	Bandwidth (MB / s)
ISA 8-bit	8	8.3	7.9
16-bit ISA	16	8.3	15.9
EISA	32	8.3	31.8
VLB	32	33	127.2
32-bit PCI	32	33	127.2
PCI 64-bit 2.1	64	66	508.6
AGP	32	66	254.3
AGP (x2 Mode)	32	66×2	528
AGP (x4 Mode)	32	66×4	1056
AGP (x8 Mode)	32	66×8	2112
ATA33	16	33	33
ATA100	16	50	100
ATA133	16	66	133
Serial ATA (S-ATA)	1		180
Serial ATA II (S-ATA2)	2		380
USB	1		1.5
USB 2.0	1		60
Firewire	1		100
Firewire 2	1		200
SCSI-1	8	4.77	5
SCSI-2 – Fast	8	10	10
SCSI-2 – Wide	16	10	20
SCSI-2 – 32-bit Fast Wide	32	10	40
SCSI-3 – Ultra	8	20	20
SCSI-3 – Ultra Wide	16	20	40
SCSI-3 – Ultra 2	8	40	40
SCSI-3 – Ultra 2 Wide	16	40	80
SCSI-3 – Ultra 160 (Ultra 3)	16	80	160
SCSI-3 – Ultra 320 (Ultra 4)	16	80 DDR	320
SCSI-3 – Ultra 640 (Ultra 5)	16	80 QDR	640