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[ The PC Guide | Systems and Components Reference Guide | Motherboard and System Devices | System Buses | System Bus Functions and Features ]

The Local Bus Concept

The switch from character-based applications to graphics-based ones began in earnest at the start of the 90s, with the rapid growth in popularity of the Windows operating system. The increase in the amount of information that must be moved between the processor, memory, video and hard disks when using a graphical operating system compared to a text-based one is tremendous. A complete, standard screen of monochrome text is just 4,000 bytes of information (2,000 bytes for the characters, and 2,000 bytes for screen attributes). However, a standard 256-color Windows screen requires over 300,000 bytes, an increase of about 15,000%! (Out of interest, the highest-end resolution and color depth generally used today, 1600x1200 at 16 million colors, requires 5.8 million bytes of information per screen!).

The transformation of the software world from text to graphics also meant much larger programs and more storage requirements. From an I/O standpoint, much more I/O bandwidth was needed to handle the additional data going to and from the video card and the increasingly larger and faster hard disks. By this time, Intel had also moved on to the 80486 processor that provided many times the performance of earlier CPUs. The ISA bus, still running at the same speed and bus width that it did on the IBM AT, was finally and totally outmatched by these increasing demands and became a major bottleneck to improving system performance. Increasing the speed of the processor accomplished little if it was always waiting for the slow system bus to transmit data.

The solution was to create a new, faster bus, that would augment the ISA bus and be used especially for high-bandwidth devices such as video cards. This new bus would be put on (or near) the processor's much faster memory bus, to let it run at or near the external speed of the processor, and to allow data to flow between these devices and the processor without having to go through the much slower ISA bus. By placing these devices "local" to the processor, the local bus was born.

The first local bus was the VESA local bus. The current local bus of choice on modern computers is the Peripheral Component Interconnect or PCI bus.

Next: System Bus Types


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