System Bus - Description

Description

To provide even more modularity with reduced cost, memory and I/O buses (and the required control and power buses) were sometimes combined into a single unified system bus. Modularity and cost became important as computers became small enough to fit in a single cabinet (and customers expected similar price reductions). Digital Equipment Corporation (DEC) further reduced cost for mass-produced minicomputers, and memory-mapped I/O into the memory bus, so that the devices appeared to be memory locations. This was implemented in the Unibus of the PDP-11 around 1969, eliminating the need for a separate I/O bus. Even computers such as the PDP-8 without memory-mapped I/O were soon implemented with a system bus, which allowed modules to be plugged into any slot. Some authors called this a new streamlined "model" of computer architecture.

Many early microcomputers (with a CPU generally on a single integrated circuit) were built with a single system bus, starting with the S-100 bus in the Altair 8800 computer system in about 1975. The IBM PC used the Industry Standard Architecture (ISA) bus as its system bus in 1981. The passive backplanes of early models were replaced with the standard of putting the CPU on a motherboard, with only optional daughterboards or expansion cards in system bus slots.

The Multibus became a standard of the Institute of Electrical and Electronics Engineers as IEEE standard 796 in 1983. Sun Microsystems developed the SBus in 1989 to support smaller expansion cards. The easiest way to implement symmetric multiprocessing was to plug in more than one CPU into the shared system bus, which was used through the 1980s. However, the shared bus quickly became the bottleneck and more sophisticated connection techniques were explored.