The address bus carries the address of the piece of memory or I/O device to be read from or written to.
It is a unidirectional bus, which is to say that data travels only one way; from the CPU to memory.
The number of lines on the bus determines the number of addressable memory elements. For example an 8 bit bus can represent 2 to the power of 8 unique addresses. This equates to 256 unique memory addresses. A 16 bit bus can address 65536 unique addresses and so on.
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