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Processor / Memory Address Bus
The address bus is the set of lines that carry information about where in memory the data is to be transferred to or from. No actual data is carried on this bus, rather memory addresses, which control the location that data is either read from or written to, are sent here. The speed of the address bus is the same as the data bus it is matched to.
The width of the address bus controls the addressability of the processor, which is how much system memory the processor can read or write to. Continuing the highway analogy, the address bus carries the information about exit numbers for the cars to use. The wider the address bus, the more digits the exit number could have, and the more exits that could be supported on the highway. The width of the address and data buses aren't linked; you can have a highway with many lanes but few exits, or vice versa. Usually though, newer processors have both wider data and address buses.
Address bus size is not something that is thought of very often, because it has no direct impact on performance. Processors usually can address far more physical memory than most people will ever use, and in fact the system chipset or motherboard factors usually place much tighter restrictions on maximum system memory than the processor does. For example, a Pentium can theoretically address 4 GB of system memory, but most normal motherboards won't take even one quarter that amount.