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Definition: address bus

An internal channel that transmits the RAM location of the data being processed or the instruction being fetched. The number of lines (wires) in the address bus determines the maximum amount of RAM that can be directly accessed by the CPU as each line carries one bit of the address. In 1981, the IBM PC had a limit of one megabyte of RAM because its address bus had 20 lines, and 20 bits represents the number 1,048,576 in binary.

A computer with a 32-bit address bus can directly address 4GB of physical memory, while one with 36 bits can address 64GB. See binary values, address, RAM and IBM PC.