Page 96 - A Practical Guide from Design Planning to Manufacturing

72 Chapter Three

is added to the processor, there is more potential for logic errors. As clock
rates increase, circuit design requires more detailed simulations. The
production of new fabrication generations is inevitably more complex
than previous generations. Because of the short lifetime of most micro-
processors in the marketplace, all of this must happen under the pres-
sure of an unforgiving schedule. The general steps in processor design
are shown in Fig. 3-1.
A microprocessor, like any product, must begin with a plan, and the
plan must include not only a concept of what the product will be, but
also how it will be created. The concept would need to include the type
of applications to be run as well as goals for performance, power, and
cost. The planning will include estimates of design time, the size of the
design team, and the selection of a general design methodology.
Defining the architecture involves choosing what instructions
the processor will be able to execute and how these instructions will
be encoded. This will determine whether already existing software can
be used or whether software will need to be modified or completely rewrit-
ten. Because it determines the available software base, the choice of
architecture has a huge influence on what applications ultimately run on
the processor. In addition, the performance and capabilities of the proces-
sor are in part determined by the instruction set. Design planning and
defining an architecture is the design specification stage of the project,
since completing these steps allows the design implementation to begin.
Although the architecture of a processor determines the instructions
that can be executed, the microarchitecture determines the way in which

Design plan

Architecture
Design
specification uArchitecture

Logic
Validation
Behavioral
design Circuits
Layout
Physical
design Si debug

Production
Silicon ramp
Figure 3-1 Microprocessor design flow.