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Simple question: What is the role of a bus bridge?

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it lets the bus cross the river? –  quack quixote Jun 14 '10 at 15:27
    
To provide service between stations when the trains aren't running? –  Spiff Jun 14 '10 at 15:51
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2 Answers

In computer terminology, a Bus Bridge connects two different buses together - and we are not talking about yellow Buses here.

Computers have evolved different standards for interfacing with peripherals, etc. Common Bus Bridges you see are PCI to PCMCIA adapters that are actually bus bridges. or PCI-E to ExpressCard bus bridges.

Basically, a Bus Bridge allows you to interface two different Bus types - boring, but true answer.

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Read "what every programmer should know about memory", within section 2 the role of bridges are explained:

Over the years the personal computers and smaller servers standardized on a chipset with two parts: the Northbridge and Southbridge.

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All CPUs (two in the previous example, but there can be more) are connected via a common bus (the Front Side Bus, FSB) to the Northbridge. The Northbridge contains, among other things, the memory controller, and its implementation determines the type of RAM chips used for the computer. Different types of RAM, such as DRAM, Rambus, and SDRAM, require different memory controllers.

To reach all other system devices, the Northbridge must communicate with the Southbridge. The Southbridge, often referred to as the I/O bridge, handles communication with devices through a variety of different buses. Today the PCI, PCI Express, SATA, and USB buses are of most importance, but PATA, IEEE 1394, serial, and parallel ports are also supported by the Southbridge. Older systems had AGP slots which were attached to the Northbridge. This was done for performance reasons related to insufficiently fast connections between the Northbridge and Southbridge. However, today the PCI-E slots are all connected to the Southbridge.

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Please don't just link to answers. Copy and paste the answer here in quotes and then link to your source. In a year or two, when someone else is looking for the same answer, there's no guarantee that site will still be up. –  MDMarra Jun 15 '10 at 11:08
    
read that article and you will find, that the copyNpaste version would be rather long. –  akira Jun 15 '10 at 11:39
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I'd still add a brief summary, so that the answer is still useful in the event that the link breaks someday. –  Kara Marfia Jun 15 '10 at 13:04
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