2 Guiding Principles

This chapter discusses the principles of high-quality software. It is likely that your design will not be able to achieve all of these goals; if it does, your design is very good indeed. Note that some of these design goals contradict other design goals. Contradiction is part of the challenge of doing a good design.

Maintainability of software is of such importance that those principles are expanded separately in "Principles for Software Maintenance".

2.1 Software Design

The following is an alphabetical list of software design principles:


This entails designing objects and environments to be usable, with no modification, by the greatest number of people as possible, including people with varying educational and social backgrounds, as well as those with motor or sensory challenges.


Elements within a design should be aligned with one or more other elements. Alignment of related or like elements within a design reduces the perception of disorder and promotes understanding.


Chunking groups units of information into a small number of units (maximum of four plus or minus one) to help the efficient processing of information by shortterm memory, as well as to accommodate its limits.


This is a technique used for critical actions, inputs, or commands. Confirmations are primarily used to prevent unintended actions. Minimize errors in critical or irreversible operations with confirmations. If you overuse confirmations, expect that they will be ignored Avoid overusing confirmations to ensure that they remain unexpected and uncommon; otherwise, they may be ignored. Use a two-step operation for hardware confirmations and dialogs for software confirmations.


Express similar parts or concepts in similar ways to make a system to improve usability and learnability. Apply consistency to design and coding style, as well as user interfaces. Do not apply consistency to the point of compromising clarity or usability.


Design for the maintenance programmer or sustaining engineer for maintainability, or ease of maintenance. The design of the system should be self-explanatory.


Extensibility entails enhancing a system without violating the underlying structure. The most likely changes should cause the system the least trauma. For example, you know the BIOS is responsible for booting the system, so adding a new type of boot device should not cause trauma to the system. Be careful about the assumptions you make.


Design to help users avoid errors and reduce the negative consequences of errors any errors made. Recommended methods for achieving design forgiveness include affordances, reversibility of actions, and safety nets. Effectively designing for forgiveness results in a design needing minimal confirmations, warnings, and help.

High fan-in

Sharing a high number of routines that call a given routine produces high fan-in. Sharing entails designing a system to make good use of utility routines at the lower levels.

Horror Vacui

This is a Latin phrase for "fear of emptiness", which is the desire to fill empty spaces with information or objects. Research shows that as horror vacui increases, perceived value decreases. In programming, lines consisting solely of comment characters, with no actual comment, are good examples of horror vacui.

Intellectual manageability

Intellectual manageability is a primary goal in any system. It is essential to the overall system integrity and affects how easily programmers can initially build a system as well as maintain it later.

Interference Effects

When two or more perceptual (or cognitive) processes are in conflict, the competing mental processes slow down mental processing or make mental processing less accurate. Examples of this include violations of convention (a red OK light), information conflicts (a color name, GREEN, in a different color), and incorrect use of opposites.


Design the system so that it has no extra parts, i.e. "lean". If you add extra code, remember that it needs to be developed, reviewed, tested, maintained, understood, and taken into account when the code is modified. Also, future versions of the code may have to be backward compatible with the extra code.

Low complexity

Low complexity is part of intellectual manageability.

Minimal connectedness

Design so that you keep connections among subprograms to a minimum. This minimizes work during integration, testing, and maintenance. Use industry standards whenever possible. Make sure you are not reinventing something that already exists.

Minimize code size

Unlike many software engineering projects, EDK II firmware is targeted to be stored in a device that represents a tangible cost to the system. Thus, the minimization of code size is important to reduce the overall cost of a system.


Design the system so that it is easily moved to another environment. With EDK II, making sure the code will run on IA32, X64 and Intel(R) Itanium(R) processors (IPF) is an example of portability.


Design the system so that pieces of it can be reused in other systems. In EDK II, reusability also means designing code so that it can be used in various classes of platforms from embedded systems to massively parallel computers.

Standard techniques

Greater reliance on unique or exotic pieces makes a system harder to understand, and more intimidating for someone trying to understand it the first time. Using standardized, common approaches should be to give the whole system a familiar feeling. This standardization is one of the primary goals of this document.

Stratified design

In order to view the system at any single level and get a consistent view of it, you should attempt to keep the levels of decomposition layered. The OSI multi-layernetworking model is an example of stratified design.

2.2 Principles for Software Maintenance

2.2.1 Understand the problem before you fix it.

The best way to ruin a code base is to attempt to fix problems without a clear understanding of the problem itself. If your fix is out of context, the next fix in the routine will be four times more complicated. Triangulate the error with cases that should and should not cause the error. Keep at it until you understand the error.

2.2.2 Understand the program, not just the problem.

Understanding the context in which a problem occurs will increase your likelihood of solving the problem completely.

2.2.3 Fix the symptom AND the underlying problem.

Fix the symptom, but your focus should be on fixing the underlying problem. Without thoroughly understanding the problem, you will not fix the code. You will also feel very uncomfortable when a peer reviews your fix before you check it into the source base.

2.2.4 Change the code only for good reason.

Do not change code at random until it seems to work; that isn't effective. If you do this you are not learning anything and are just goofing around. You should have confidence that a change will work before making a change. Being wrong about a change should be rare and cause personal reevaluation.

2.2.5 Do not debug by superstition.

Don't blame every problem on the computer, bad data, or the effects of a full moon. You wrote the program; take responsibility for it.

2.2.6 Don't blame everyone else's code.

It is human nature to trust the code that you wrote and understand, and to distrust all other code. You must resist this tendency and root cause the problem systematically.

2.2.7 Don't use source control to debug problems.

You don't debug by randomly trying previous versions of code. You can use previous versions as a single test of your triangulated test cases. Bugs are not always introduced by changes. Bugs can lie dormant in a code base for long periods of time. By blindly rolling back changes, you could just be hiding a bug, versus fixing one.

2.2.8 Check your fix.

Run the triangulated test cases against your code. Have another set of eyes look at your change, preferably someone who is experienced in the code.

2.2.9 Look for similar errors.

Errors tend to occur in groups, so check to make sure a similar mistake was not made in other parts of the code.

2.2.10 Fix the code AND the comments.

When you have correct code but incorrect comments, you will confuse the next person in the code. Remember, programming is a team sport.

2.2.11 Fix the comments AND the documentation.

The next person to make a large change to code will thank you and might even not camp out in your cube for a week. This has been simplified in EDK II by embedding the documentation within the comments.

2.3 Additional Recommendations

  • If you have trouble debugging without violating these rules, please ask for help.

  • Learn to debug by debugging with someone who is good at it. There is (approximately) a 20-to-1 difference in the time it takes an experienced programmer to find the same set of errors as an inexperienced programmer.

  • Try to not focus only on the bug, but on the process and techniques the experienced programmer uses to find bugs.

  • When working with a more experienced debugger, your goal should be to improve your debugging skills.

  • If you rely too heavily on the debugging skills of others, your own expertise suffers.