The address bus is part of the system bus of computers. It is used to transport the computing tasks and to address the positions in the working memory. The width of the address bus or the number of address lines determines the addressing possibilities.
With an address width of 20 bits, as with the first central processing units( CPU) of the 80x86 series, an addressing range of 1 million addresses results, which corresponds to a memory area of 1 megabyte( MB). With an address width of 24 bits, the addressing range increases to 16 million, respectively 16 MB memory, and with a width of 32 bits, as with the Pentium Pro, `4 * 10^9` addresses can be addressed. This corresponds to a memory volume of 4 gigabytes( GB) of memory. And with a 64 bit address bus width, there are over `10^18` addressing possibilities.