24 COMPUTER ENGINEERING

Length of Time to Design Versus Product Life. By allowing more time for design, a product can be designed in such a way that it is easier to enhance. On the other hand, if prospective customers, especially new customers, are faced with a choice between the competitor's available nonoptimum product and your unavailable optimum product, they may not be willing to wait.

Operating Environment Versus Cost. Here there are numerous tradeoffs even within a conventional environment. In each of the packaging dimensions (heat, humidity, altitude, dust, electromagnetic interface (EMI), etc.), there are similar tradeoffs that may appeal to unique markets or may simply translate to increased re liability in a given setting. The Norden 1 l/34M is an example of packaging to provide a PDP-l 1 for the aerospace environment.

The principles of computer design and the optimization efforts associated with those principles are parts of computer science and electrical engineering, the responsible disciplines. From computer science come many of the technical aspects (such as instruction set architecture), much of the theory (such as algorithms and computational complexity), and almost all of the software design (such as operating systems and language translators) applied in the

1. There is no such thing as ground.
2. Digital circuits are made from analog parts.
3. Prototype designs always work.
4. Asserted timing conditions are designed first; unasserted timing conditions are found later.
5. When all but one wire in a group of wires switch, that one will switch also.
6. When all but one gate in a module switches, that one will switch also.
7. Every little pico farad has a nano henry all its own.
8. Capacitors convert voltage glitches to current glitches (conservation of energy).
9. Interconnecting wires are probably transmission lines.
10. Synchronizing circuits may take forever to make a decision.
11. Worse-case tolerances never add - but when they do, they are found in the best customer's machine.
1 2. Diagnostics are highly efficient in finding solved problems.
13. Processing systems are only partially tested since it is impractical to simulate all possible machine states.
14. Murphy's Laws apply 95 percent of the time. The other 5 percent of the time is a coffee break.

practice of computer engineering. However, in their construction, computers are electrical; and the discipline that has fundamental responsibility is electrical engineering. Thus, discussion of the Practice of Design View concludes with Table 5, a set of maxims compiled by Don Vonada, an experienced DEC engineer. Many other engineers in many other companies have developed similar sets of maxims.

Another view is based on the work of Blaauw [1970]. He distinguishes between architecture, implementation, and realization as three separable levels in the construction of anything, including computer structures.

The architecture of a computer system de fines its functionality (behavior) as it appears to the machine level programmer and can be characterized by the instruction set processor (ISP). The implementation of a computer system is the actual hardware structure - the register transfer (RT) level behavior and data-flow organization. This also includes various algorithms for con trolling a machine as it interprets an architecture. Realization encompasses the actual