Page 62 - A Practical Guide from Design Planning to Manufacturing

38 Chapter Two

Before beginning to design a processor, we must consider what other
components are needed to create a finished product and how these com-
ponents will communicate with the processor. There must be a main
memory store that will hold instructions and data as well as results
while the computer is running. Permanent storage will require a hard
drive or other nonvolatile memory. Getting data into the system requires
input devices like a keyboard, mouse, disk drives, or other peripherals.
Getting results out of the system requires output devices like a monitor,
audio output, or printer.
The list of available components is always changing, so most proces-
sors rely on a chipset of two or more separate computer chips to manage
communications between the processor and other components. Different
chipsets can allow the same processor to work with very different com-
ponents to make a very different product. The motherboard is the cir-
cuit board that physically connects the components. Much of the
performance difference between computers is a result of differences in
processors, but without the right chipset or motherboard, the processor
may become starved for data and performance limited by other computer
components.
The chipset and motherboard are crucial to performance and are
typically the only components designed specifically for a particular
processor or family of processors. All the other components are
designed independently of the processor as long as they communicate
by one of the bus standards supported by the chipset and motherboard.
For this reason, this chapter leaves out many details about the imple-
mentation of the components. Hard drives, CD drives, computer printers,
and other peripherals are complex systems in their own right (many
of which use their own processors), but from the perspective of the
main processor all that matters is what bus standards are used to
communicate.

Bus Standards
Most computer components are concerned with storing data or moving
that data into or out of the microprocessor. The movement of data within
the computer is accomplished by a series of buses. A bus is simply a col-
lection of wires connecting two or more chips. Two chips must support
the same bus standard to communicate successfully. Bus standards
include both physical and electrical specifications.
The physical specification includes how many wires are in the bus, the
maximum length of the wires, and the physical connections to the bus.
Using more physical wires makes it possible to transmit more data in
parallel but also makes the bus more expensive. Current bus standards
use as few as 1 and as many as 128 wires to transmit data. In addition