The main responsibility of the system bus is the transfer of information between the base
microprocessor and other electronic components of the computer. For this bus
also addressing devices and an exchange of
official signals. Thus, simple system bus can provide
as a set of signal lines, united for their intended purpose (data
addresses management). Transmission of information on tire manages one of the connected
to the devices or specially selected for this node, called the arbitrator
tire.
The system bus IBM PC and IBM PC / XT was designed for simultaneous transmission
only 8 bits of information, as used in computer microprocessor 18088
has 8 lines of data. In addition, the system bus included 20 targeted lines
which limited the address space limit of 1 MB. To work with
external devices in the bus were also 4 line of hardware
interrupt (IRQ) and 4 from the requirement for external devices direct access
in memory (DMA, Direct Memory Access). To connect card expansion
use special 62-pin connectors. Note that the system bus and
microprocessor from one of synchronizing clock generator with a frequency of 4.77
MHz. Thus, in theory, the data transfer rate can reach more
4.5 MB / sec.