The system bus is designed to implement communication between the processor and external devices in the computer using special control devices - adapters or controllers. All the latter are connected to the system bus using standard connectors. Tires are usually divided into three categories according to their functional purpose: address, information and control, which differ in bit depth, 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 certain 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 lines of interrupt for interfacing with external devices, as well as four lines for accessing memory directly.
The system bus and microprocessor were operated at a frequency of 4.77 MHz. And the information transfer rate could be approximately 4.5 MB per second. In the next generation of computers, a sixteen-bit bus was already used, which, thanks to 24-address lines, allowed direct access to RAM, while its volume was 16 MB.
Sixteen hardware interrupts were already used in this bus instead of eight, and the number of channels for direct access to information was already eight, not four. Now the bus works asynchronously with a microprocessor at a frequency of 6 MHz, and this has led to an increase in the transmission speed to 16 MB per second. Now it already provided the opportunity to work with low-speed devices, but could not ensure the effective functioning of modern devices. This influenced the invention of new types of system buses.
In 1987, the MCA system bus was developed, which was the first with high performance. It was distinguished by the fact that its operating speed was 10 MHz, and the bus itself had already become 32-bit, which increased the transmission speed to 20 MB per second. However, due to the incompatibility of the buses, it was not possible to use controllers designed for the ISA bus, which is why the architecture did not find wide application.
The EISA system bus was developed in 1989 and has become an extended version of ISA. Its connectors allowed to insert not only their own controllers, but also those for ISA. It worked with a frequency of 8-10 MHz, while its capacity is 32, which allows you to send up to 4 GB, reaching a data exchange speed 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 the controllers.
The PCI bus was developed for the new Pentium processor, but can also be used on other platforms. It allows you to connect up to ten different devices. This bus uses 32 or 64 bit data transfer, and the transfer rate was 132 and 264 MB per second.
Now motherboards are connected to other devices via the AGP bus, which allows the graphics card to use the RAM of a personal computer. She was able to cope with modern graphics, which must move around the monitor at high speed, which is difficult to cope with PCI. When using PCI, it turned out to be impractical to increase the memory on the video adapter due to the limited speed and bandwidth of the bus. The AGP system bus frequency allows the exchange of information between video memory and RAM directly, which cannot be achieved using other standards of these devices.