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Address bus - Wikipedia, the free encyclopedia

1 of 2

http://en.wikipedia.org/wiki/Address_bus

Address bus
From Wikipedia, the free encyclopedia

An address bus is a computer bus (a series of lines connecting two or
more devices) that is used to specify a physical address. When a
processor or DMA-enabled device needs to read or write to a memory
location, it specifies that memory location on the address bus (the value
to be read or written is sent on the data bus). The width of the address
bus determines the amount of memory a system can address. For
example, a system with a 32-bit address bus can address 232 (4,294,967,296)
memory locations. If each memory address holds one byte, the
addressable memory space is 4 GB.

Implementation
Early processors used a wire for each bit of the address width. For
example, a 16-bit address bus had 16 physical wires making up the bus. As
the buses became wider, this approach became expensive in terms of the
number of chip pins and board traces. Beginning with the Mostek 4096
DRAM, address multiplexing implemented with multiplexers became
common. In a multiplexed address scheme, the address is sent in two
equal parts on alternate bus cycles. This halves the number of address
bus signals required to connect to the memory. For example, a 32-bit
address bus can be implemented by using 16 lines and sending the first
half of the memory address, immediately followed by the second half.

Examples
Accessing an individual byte frequently requires reading or writing the
full bus width (a word) at once. In these instances the least significant
bits of the address bus may not even be implemented - it is instead the
responsibility of the controlling device to isolate the individual byte
required from the complete word transmitted. This is the case, for
instance, with the VESA Local Bus which lacks the two least significant
bits, limiting this bus to aligned 32-bit transfers.
Historically, there were also some examples of computers which were
only able to address words.
08-Nov-14 7:24 AM

Address bus - Wikipedia, the free encyclopedia

2 of 2

http://en.wikipedia.org/wiki/Address_bus

See also
Bus error
Memory address
Memory bus
Retrieved from "http://en.wikipedia.org/w/index.php?title=Address_bus&
oldid=618573478"
Categories: Computer buses
This page was last modified on 26 July 2014 at 18:38.
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08-Nov-14 7:24 AM


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