Hardware Reference
In-Depth Information
needed between the CPU and such a bus to match the two sides up, that is, to con-
vert the 3-bit encoded signal into separate signals that can drive the bus lines.
Bus design and operation are sufficiently complex subjects that a number of
entire topics have been written about them (Anderson et al., 2004, Solari and
Willse, 2004). The principal bus design issues are bus width, bus clocking, bus
arbitration, and bus operations. Each of these issues has a substantial impact on
the speed and bandwidth of the bus. We will now examine each of these in the
next four sections.
Bus width is the most obvious design parameter. The more address lines a bus
has, the more memory the CPU can address directly. If a bus has
n
address lines,
then a CPU can use it to address 2
n
different memory locations. To allow large
memories, buses need many address lines. That sounds simple enough.
The problem is that wide buses need more wires than narrow ones. They also
take up more physical space (e.g., on the motherboard) and need bigger con-
nectors. All of these factors make the bus more expensive. Thus, there is a trade-
off between maximum memory size and system cost. A system with a 64-line ad-
dress bus and 2
32
bytes of memory will cost more than one with 32 address lines
and the same 2
32
bytes of memory. The possibility of expansion later is not free.
The result of this observation is that many system designers tend to be short-
sighted, with unfortunate consequences later. The original IBM PC contained an
8088 CPU and a 20-bit address bus, as shown in Fig. 3-37(a). Having 20 bits al-
lowed the PC to address 1 MB of memory.
20-Bit address
20-Bit address
Control
20-Bit address
Control
4-Bit address
8088
80286
80386
Control
Control
4-Bit address
8-Bit address
Control
Control
(a)
(b)
(c)
Figure 3-37.
Growth of an address bus over time.
When the next CPU chip (the 80286) came out, Intel flet it had to increase the
address space to 16 MB, so four more bus lines were added (without disturbing the
