The address bus is a collection of wires that carry the addressing information that describes the memory location to which data is being transmitted or retrieved. Each wire on an address bus, like the data bus, carries a single bit of data. A single digit in the address is represented by this single bit. The overall number of address locations increases as the number of wires (digits) used in generating these addresses increases. The maximum amount of RAM a chip may access is controlled by the address bus size (or width).

 

The address bus can be visualized using the highway analogy from the previous section, "Data I/O Bus." The address bus corresponds to the home number or street address if the data bus is a highway and the size of the data bus is equal to the number of lanes. The amount of digits in the house address number equals the size of the address bus. For example, if you reside on a street with a two-digit (base 10) address, there can be no more than 100 different addresses (00–99) for that street (10 2 ). By inserting another digit, the total number of addresses allowed increases to 1,000 (000–999), or 10 3.

Because computers use the binary (base 2) numbering system, a two-digit number can only have four different addresses (00, 01, 10, and 11), which may be computed as 2 2. Only eight addresses are provided via a three-digit number (000–111), which is 2 3. The 8086 and 8088 CPUs, for example, have a 20-bit address bus that can compute up to 220. On the other hand, the number of address locations is 1,048,576 bytes (1MB). The memory-addressing capabilities of CPUs are listed in Table 3.3.

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