Processor Types and Specifications - Upgrading and Repairing PCs - page 37

Another improvement in newer processors is the use of multiple separate buses for different tasks.
Traditional processor design had all the data going through a single bus, whereas newer processors
have separate physical buses for data to and from the chipset, memory, and graphics card slot(s).
Address Bus
The address bus is the set of wires that carry the addressing information used to describe the memory
location to which the data is being sent or from which the data is being retrieved. As with the data
bus, each wire in an address bus carries a single bit of information. This single bit is a single digit in
the address. The more wires (digits) used in calculating these addresses, the greater the total number
of address locations. The size (or width) of the address bus indicates the maximum amount of RAM a
chip can address.
The highway analogy in the previous section, “ Data I/O Bus , ” can show how the address bus fits in. If
the data bus is the highway and the size of the data bus is equivalent to the number of lanes, the
address bus relates to the house number or street address. The size of the address bus is equivalent to
the number of digits in the house address number. For example, if you live on a street in which the
address is limited to a two-digit (base 10) number, no more than 100 distinct addresses (00-99) can
exist for that street (10 2 ). Add another digit, and the number of available addresses increases to 1,000
(000-999), or 10 3 .
Computers use the binary (base 2) numbering system, so a two-digit number provides only four
unique addresses (00, 01, 10, and 11), calculated as 2 2 . A three-digit number provides only eight
addresses (000-111), which is 2 3 . For example, the 8086 and 8088 processors use a 20-bit address
bus that calculates a maximum of 2 20 , or 1,048,576 bytes (1MB), of address locations. Table 3.3
describes the memory-addressing capabilities of processors.
Table 3.3. Processor Physical Memory-Addressing Capabilities
The data bus and address bus are independent, and chip designers can use whatever size they want for
each. Usually, however, chips with larger data buses have larger address buses. The sizes of the
buses can provide important information about a chip's relative power, measured in two important
ways. The size of the data bus indicates the chip's information-moving capability, and the size of the
address bus tells you how much memory the chip can handle.
Internal Registers (Internal Data Bus)
The size of the internal registers indicates how much information the processor can operate on at one