An internal channel from the CPU to memory across which the addresses of data (not the data) are transmitted. The number of lines (wires) in the address bus determines the amount of memory that can be directly addressed as each line carries one bit of the address. For example, a 20-line address bus represents the binary number 1,048,576 and reaches that number of memory bytes (the size of the address bus in the IBM PC in 1981). A computer with a 32-bit address bus can directly address 4GB of physical memory, while one with 36 bits can address 64GB.