Системная шина компьютера: история и эволюция.

Single computer bus that connects the major components of a computer system

A system bus is a single computer bus that connects the major components of a computer system,
combining the functions of a data bus to carry information, an address bus to determine where it should be sent or read from, and a control bus to determine its operation. The technique was developed to reduce costs and improve modularity, and although popular in the 1970s and 1980s, more modern computers use a variety of separate buses adapted to more specific needs. The system level bus (as distinct from a CPU's internal datapath busses) connects the CPU to memory and I/O devices. Typically a system level bus is designed for use as a backplane.

Many of the computers were based on the First Draft of a Report on the EDVAC report published in 1945. In what became known as the Von Neumann architecture, a central control unit and arithmetic logic unit (ALU, which he called the central arithmetic part) were combined with computer memory and input and output functions to form a stored program computer. The Report presented a general organization and theoretical model of the computer, however, not the implementation of that model. Soon designs integrated the control unit and ALU into what became known as the central processing unit (CPU). Computers in the 1950s and 1960s were generally constructed in an ad hoc fashion. For example, the CPU, memory, and input/output units were each one or more cabinets connected by cables. Engineers used the common techniques of standardized bundles of wires and extended the concept as backplanes were used to hold printed circuit boards in these early machines. The name "bus" was already used for "bus bars" that carried electrical power to the various parts of electric machines, including early mechanical calculators. The advent of integrated circuits vastly reduced the size of each computer unit, and buses became more standardized. Standard modules could be interconnected in more uniform ways and were easier to develop and maintain.

Intel has used the term Dual Independent Bus (DIB) for two different purposes. The first one came when Intel changed from a single local bus to the DIB, using the external front side bus to the main system memory and I/O devices, and the internal back side bus to the L2 CPU cache. This was introduced in the Pentium Pro in 1995. In 2005 and 2006 Intel introduced the 8500 and 5000 chipsets, where DIB referred to the two front side buses on a chipset, which doubles the system bandwidth compared to having just one FSB shared by all the CPUs. However, the information needed to guarantee the cache coherence of shared data located in different caches have to be sent in broadcast (snooped) to check the other FSB's CPUs' cache state, reducing the available bandwidth. To reduce the coherency traffic, a snoop filter was included in the higher end chipsets, in order to have cache state information available on chipset. In 2007 Intel extended the idea of multiple buses in the 7300 chipset with four independent FSBs, calling it dedicated high speed interconnects (DHSI). The system bus approach is obsolete in the modern personal and server computers, which instead use higher performance interconnection technologies such as HyperTransport and Intel QuickPath Interconnect, while the system bus architecture continued to be used on simpler embedded microprocessors. The systems bus can even be internal to a single integrated circuit, producing a system on a chip. Examples include AMBA, CoreConnect, and Wishbone.

Direct Media Interface is an example of a system bus (besides directly accessed PCIE lanes) implemented by Intel and known since at least 2004. It's primarily used to access memory mapped I/O devices and communicate CPU to the chipset.