System bus

The system bus is intended for realization of communication of the processor with external devices in the computer by means of special control devices - adapters or controllers. All the latter are connected to the system bus using standard connectors. Tires are divided into three categories according to their functional purpose: address, information and control, which differ in bit capacity, that is, the number of data passing through them. The type of device used is largely determined by the speed of the computer.

The system bus can work in the following basic standards: MCA, ISA, VESA, EISA, PCI. For a long time, the ISA bus was considered a definite standard in the field of personal computers. It was developed on the basis of the eight-bit system bus IBM PC XT and IBM PC. It provided eight interrupt lines for pairing with external devices, as well as four lines for accessing memory directly.

The work of the system bus and microprocessor was carried out at a frequency of 4.77 MHz. And the information transfer rate could be about 4.5 MB per second. The next generation of computers already used a sixteen-bit bus, which, thanks to 24-address lines, allowed direct access to RAM, while its volume was 16 MB.

Sixteen hardware interrupts were used in this bus instead of eight, and the number of channels for direct access to information was eight, not four. Now the bus works asynchronously with a microprocessor at a frequency of 6 MHz, and this has caused an increase in the transfer rate of up to 16 MB per second. Now it already provided an opportunity to work with low-speed devices, but could not ensure the effective operation of modern devices. This affected the invention of new types of system buses.

In 1987, the MSA system bus was developed, which became the first with high performance. It differed in that its operating speed was 10 MHz, and the bus itself became 32-bit, which increased the transmission speed to 20 MB per second. However, due to the incompatibility of the tires, there was no possibility of using controllers intended for the ISA bus, because of which the architecture was not widely used.

The EISA system bus was developed in 1989, it became an extended version of the ISA. Its connectors allowed you to insert not only your own controllers, but also those for ISA. It operated at a frequency of 8-10 MHz, with a bit capacity of 32, which allows sending up to 4 GB, achieving a data rate of 33 MB per second. The disadvantage of this bus is the low speed of information exchange when processing graphics, images, as well as the relatively high price of controllers.

The PCI bus was developed for a new Pentium processor, but it can also be used on other platforms. It allows you to connect up to ten different devices. This bus uses data transmission of 32 or 64 bits, and the transmission rate was 132 and 264 MB per second.

Now the motherboards are connected to other devices via the AGP bus, which allows the graphics card to use the PC's RAM. It was able to cope with modern graphics, which should move around the monitor at a high speed, which is difficult to handle PCI. When using PCI, it turned out to be inappropriate to increase the memory on the video adapter because of the limited speed and bandwidth of the bus. The frequency of the system bus AGP allows you to exchange information between video memory and RAM directly, which can not be achieved by using other standards of these devices.