Hardware Reference
In-Depth Information
4 bytes of information at a time. Taking this further, a 64-bit data bus is like having an
eight-lane highway moving data in and out of the chip.
After 64-bit-wide buses were reached, chip designers found that they couldn't increase
speed further, because it was too hard to synchronize all 64 bits. It was discovered that by
going back to fewer lanes, it was possible to increase the speed of the bits (that is, shorten
the cycle time) such that even greater bandwidths were possible. Because of this, many
newer processors have only 4-bit or 16-bit-wide data buses, yet they have higher band-
widths than the 64-bit buses they replaced.
Another improvement in newer processors is the use of multiple separate buses for differ-
enttasks.Traditionalprocessordesignhadallthedatagoingthroughasinglebus,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 re-
trieved. As with the data bus, each wire in an address bus carries a single bit of inform-
ation. This single bit is a single digit in the address. The more wires (digits) used in cal-
culating 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
.
Computersusethebinary(base2)numberingsystem,soatwo-digitnumberprovidesonly
fouruniqueaddresses(00,01,10,and11),calculatedas2
2
.Athree-digitnumberprovides
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
