Buses In Computer System - Computer Learning

Bus

Bus in  Computer system is an electronic components that is used to carry data or instructions between different components of computer system.

PCI Bus:

The Peripheral Component Interconnect (PCI) bus produced by Intel in 1992 provided direct access to system memory for any connected hardware devices.

This bus allowed multiple packets of information from different sources to travel down it simultaneously. Previously it was used to connect to the graphics card and this setup meant that information from the graphics card travelled through the bus along with any other information coming from a device connected to the PCI. When all the information arrived at the CPU, it had to wait in line to get time with the CPU. This system worked well for many years, but eventually the PCI bus could not keep up. The Internet and most software were more and more graphically oriented, and the demands of the graphics card needed priority over all other PCI devices.

This is now a legacy technology and is used for functions that do not require great quantities or speed in data transmissions. PCI slots were mainly used for network, graphics and sound cards.

PCI cards use 47 pins to attach to a PCI slot. Pins are thin metal feet that allow computer chips to be attached to a circuit board. AGP replaced the PCI as the standard way to connect a graphics card to the motherboard.

AGP Bus

In 1996, Intel introduced AGP as a more efficient way to deliver streaming video and real-time-rendered 3-D graphics that were becoming more prevalent in all aspects of computing to and from the CPU. Previously, the standard method of delivery was the PCI bus.

AGP is based on the design of the PCI bus; but unlike a bus, it provides a dedicated point-to-point connection from the graphics card to the CPU. With a clear path to the CPU and system memory, AGP provided a much faster, more efficient way for a computer to get the information it needed to render complex graphics.

An AGP, or accelerated graphics port, allowed the operating system to designate RAM for use by the graphics card on the fly.

AGP started with a bandwidth of 266 MB/s; AGP 2x -> 533 MB/s; AGP 4x -> 1066 MB/s; AGP 8x -> 2133 MB/s. However, AGP then became overtaken by the newer PCI-Express slots, which come in several denominations to make them the do-all, fit-all slot for every expansion board, not just graphics.

PCI Express 1.0 Bus:

PCI Express or PCIe began to appear in 2004 and eliminates the need for the AGP by accepting more data and supplying more power to video cards. By early 2006, most motherboards featured PCI x16 slots (4,000MB/s) in place of AGP 8x.

 The shorter ones (shown below) are xl PCIe slots and are common to all PCI Express slots. PCI-Express slots will also accept older PCI cards. To handle graphics and sound data faster, the PCIe slot can be expanded to x2, x4, x8, or, x16 slots, where the numbers represent multiples of the speed of an xl PCIe slot. Their ability to move data is indicated by the multiplier factor in their designations.

New computer applications, such as streaming video and photo editing, put new demands on PCs to move vast amounts of data ever quicker. Even the fastest of them, AGP 8x, which spewed out 2.133 GB/s, was not good enough for the demands of real-time-photorealistic animation that needs values for the colors of millions of pixels pushed through the circuits 60 times or more each second. The solution is a bus architecture that uses both parallel and serial transfers.