BuSeS

A bus is a common digital pathway between resources and devices. In a PC, there are two major types: the system bus and peripheral bus. The system bus, also known as the "front side bus" or "local bus," is the internal path from the CPU to memory and is split into address bus and data bus subsets. Addresses are sent over the address lines to signal a memory location, and data are transferred over the data lines to that location.

System buses transfer data in parallel. In a 32-bit bus, data are sent over 32 wires simultaneously. A 64-bit us uses 64 wires.


• Three Main Bus Architectures

ISA (Industry Standard Architecture): Pronounced "i-suh" ISA stems from the original computer. It was an 8-bit bus originally known as the PC bus and then the XT bus. It was later extended to 16 bits and became the AT bus and eventually the ISA bus.

MCA (Micro Channel Architecture): A 32-bit bus used in IBM P/S series and other IBM models. This architecture allows multi processing that allows several processors to work simultaneously. Micro channel architecture is not compatible with PC bus architecture.

EISA ( Extended Industry Standard Architecture): Pronounced "eesa" this bus was a 32-bit extension of ISA created by major vendors to counter IBM's Micro Channel. EISA slots accepted both EISA and ISA cards, but clock speed was still at the slow ISA rate. EISA was used in servers but later abandoned for PCI.



• Local Buses

A local bus is a computer bus that connects directly, or almost directly, from the CPU to one or more slots on the expansion bus. The significance of direct connection to the CPU is avoiding the bottleneck created by the expansion bus, thus providing fast throughput. There are several local buses built into various types of computers to increase the speed of data transfer. Local buses for expanded memory and video boards are the most common. There are two local-bus systems available today. That is:


VL-Bus Local Bus (VESA Local Bus): VL-Bus specification was introduced by the VESA (Video Electronics Standards Association). the VESA Local Bus was very commonplace on 486 motherboards. Probably a majority of 486-based systems had a VESA Local Bus video card, although early 486 systems never had VLB slots, as VLB debuted years after the introduction of the 486 processor. By 1996, the Pentium (driven by Intel's Triton chipset and PCI architecture) had eliminated the 80486 market and the VESA Local Bus with it. Many of the last 80486 motherboards made have PCI slots in addition to (or completely replacing) the VLB slots.



PCI (Peripheral Component Interconnect): The PCI bus, available in 32- and 64-bit versions, is the most popular bus architecture. It is used in PCs as well as many other platforms. In 2002, PCI Express was introduced, providing greatly enhanced speeds. Typical PCI cards used in PCs include: network cards, sound cards, modems, extra ports such as USB or serial, TV tuner cards and disk controllers. Historically video cards were typically PCI devices, but growing bandwidth requirements soon outgrew the capabilities of PCI. PCI video cards remain available for supporting extra monitors and upgrading PCs that do not have any AGP or PCI Express slots.
Many devices traditionally provided on expansion cards are now commonly integrated onto the motherboard itself, meaning that modern PCs often have no cards fitted. However, PCI is still used for certain specialized cards; although many tasks traditionally performed by expansion cards may now be performed equally well by USB devices.