Page 97 - A Practical Guide from Design Planning to Manufacturing

Design Planning 73

they are executed. This means that architectural changes are visible to
the programmer as new instructions, but microarchitectural changes
are transparent to the programmer. The microarchitecture defines the
different functional units on the processor as well as the interactions and
division of work between them. This will determine the performance per
clock cycle and will have a strong effect on what clock rate is ultimately
achievable.
Logic design breaks the microarchitecture down into steps small
enough to prove that the processor will have the correct logical behav-
ior. To do this a computer simulation of the processor’s behavior is writ-
ten in a register transfer language (RTL). RTL languages, such as Verilog
and VHDL, are high-level programming languages created specifically
to simulate computer hardware. It is ironic that we could not hope to
design modern microprocessors without high-speed microprocessors to
simulate the design. The microarchitecture and logic design together
make up the behavioral design of the project.
Circuit design creates a transistor implementation of the logic spec-
ified by the RTL. The primary concerns at this step are simulating the
clock frequency and power of the design. This is the first step where the
real world behavior of transistors must be considered as well as how that
behavior changes with each fabrication generation.
Layout determines the positioning of the different layers of material
that make up the transistors and wires of the circuit design. The pri-
mary focus is on drawing the needed circuit in the smallest area that
still can be manufactured. Layout also has a large impact on the fre-
quency and reliability of the circuit. Together circuit design and layout
specify the physical design of the processor.
The completion of the physical design is called tapeout. In the past
upon completion of the layout, all the needed layers were copied onto a
magnetic tape to be sent to the fab, so manufacturing could begin. The
day the tape went to the fab was tapeout. Today the data is simply
copied over a computer network, but the term tapeout is still used to
describe the completion of the physical design.
After tapeout the first actual prototype chips are manufactured.
Another major milestone in the design of any processor is first silicon,
the day the first chips arrive from the fab. Until this day the entire
design exists as only computer simulations. Inevitably reality is not
exactly the same as the simulations predicted. Silicon debug is the
process of identifying bugs in prototype chips. Design changes are made
to correct any problems as well as improving performance, and new
prototypes are created. This continues until the design is fit to be sold,
and the product is released into the market.
After product release the production of the design begins in earnest.
However, it is common for the design to continue to be modified even