Assembly Language Style Guidelines - Comments


Comments in an assembly language program generally come in two forms: endline comments and standalone comments[14]. As their names suggest, endline lines comments always occur at the end of a source statement and standalone comments sit on a line by themselves[15]. These two types of comments have distinct purposes, this section will explore their use and describe the attributes of a well-commented program.

6.1 What is a Bad Comment?

It is amazing how many programmers claim their code is well-commented. Were you to count characters between (or after) the comment delimiters, they might have a point. Consider, however, the following comment:

                mov   ax, 0     ;Set AX to zero.

Quite frankly, this comment is worse than no comment at all. It doesn't tell the reader anything the instruction itself doesn't tell and it requires the reader to take some of his or her precious time to figure out that the comment is worthless. If someone cannot tell that this instruction is setting AX to zero, they have no business reading an assembly language program. This brings up the first guideline of this section:

Choose an intended audience for your source code and write the comments to that audience. For assembly language source code, you can usually assume the target audience are those who know a reasonable amount of assembly language.

Don't explain the actions of an assembly language instruction in your code unless that instruction is doing something that isn't obvious (and most of the time you should consider changing the code sequence if it isn't obvious what is going on). Instead, explain how that instruction is helping to solve the problem at hand. The following is a much better comment for the instruction above:

                mov   ax, 0     ;AX is the resulting sum.  Initialize it.

Note that the comment does not say "Initialize it to zero." Although there would be nothing intrinsically wrong with saying this, the phrase "Initialize it" remains true no matter what value you assign to AX. This makes maintaining the code (and comment) much easier since you don't have to change the comment whenever you change the constant associated with the instruction.

Write your comments in such a way that minor changes to the instruction do not require that you change the corresponding comment.

Note: Although a trivial comment is bad (indeed, worse than no comment at all), the worst comment a program can have is one that is wrong. Consider the following statement:

                mov   ax, 1     ;Set AX to zero.

It is amazing how long a typical person will look at this code trying to figure out how on earth the program sets AX to zero when it's obvious it does not do this. People will always believe comments over code. If there is some ambiguity between the comments and the code, they will assume that the code is tricky and that the comments are correct. Only after exhausting all possible options is the average person likely to concede that the comment must be incorrect.

Enforced Rule:
Never allow incorrect comments in your program.

This is another reason not to put trivial comments like "Set AX to zero" in your code. As you modify the program, these are the comments most likely to become incorrect as you change the code and fail to keep the comments in sync. However, even some non-trivial comments can become incorrect via changes to the code. Therefore, always follow this rule:

Enforced Rule:
Always update all comments affected by a code change immediately after making the code change.

Undoubtedly you've heard the phrase "make sure you comment your code as though someone else wrote it for you; otherwise in six months you'll wish you had." This statement encompasses two concepts. First, don't ever think that your understanding of the current code will last. While working on a given section of a program you're probably investing considerable thought and study to figure out what's going on. Six months down the road, however, you will have forgotten much of what you figured out and the comments can go a long way to getting you back up to speed quickly. The second point this code makes is the implication that others read and write code too. You will have to read someone else's code, they will have to read yours. If you write the comments the way you would expect others to write it for you, chances are pretty good that your comments will work for them as well.

Never use racist, sexist, obscene, or other exceptionally politically incorrect language in your comments. Undoubtedly such language in your comments will come back to embarass you in the future. Furthermore, it's doubtful that such language would help someone better understand the program.

It's much easier to give examples of bad comments than it is to discuss good comments. The following list describes some of the worst possible comments you can put in a program (from worst up to barely tolerable):

  • The absolute worst comment you can put into a program is an incorrect comment. Consider the following assembly statement:

		mov ax, 10;  { Set AX to 11 }
  • It is amazing how many programmers will automatically assume the comment is correct and try to figure out how this code manages to set the variable "A" to the value 11 when the code so obviously sets it to 10.
  • The second worst comment you can place in a program is a comment that explains what a statement is doing. The typical example is something like "mov ax, 10; { Set 'A' to 10 }". Unlike the previous example, this comment is correct. But it is still worse than no comment at all because it is redundant and forces the reader to spend additional time reading the code (reading time is directly proportional to reading difficulty). This also makes it harder to maintain since slight changes to the code (e.g., "mov ax, 9") requires modifications to the comment that would not otherwise be required.
  • The third worst comment in a program is an irrelevant one. Telling a joke, for example, may seem cute, but it does little to improve the readability of a program; indeed, it offers a distraction that breaks concentration.
  • The fourth worst comment is no comment at all.
  • The fifth worst comment is a comment that is obsolete or out of date (though not incorrect). For example, comments at the beginning of the file may describe the current version of a module and who last worked on it. If the last programmer to modify the file did not update the comments, the comments are now out of date.

6.2 What is a Good Comment?

Steve McConnell provides a long list of suggestions for high-quality code. These suggestions include:

  • Use commenting styles that don't break down or discourage modification. Essentially, he's saying pick a commenting style that isn't so much work people refuse to use it. He gives an example of a block of comments surrounded by asterisks as being hard to maintain. This is a poor example since modern text editors will automatically "outline" the comments for you. Nevertheless, the basic idea is sound.
  • Comment as you go along. If you put commenting off until the last moment, then it seems like another task in the software development process always comes along and management is likely to discourage the completion of the commenting task in hopes of meeting new deadlines.
  • Avoid self-indulgent comments. Also, you should avoid sexist, profane, or other insulting remarks in your comments. Always remember, someone else will eventually read your code.
  • Avoid putting comments on the same physical line as the statement they describe. Such comments are very hard to maintain since there is very little room. McConnell suggests that endline comments are okay for variable declarations. For some this might be true but many variable declarations may require considerable explanation that simply won't fit at the end of a line. One exception to this rule is "maintenance notes." Comments that refer to a defect tracking entry in the defect database are okay (note that the CodeWright text editor provides a much better solution for this -- buttons that can bring up an external file). Of course, endline comments are marginally more useful in assembly language than in the HLLs that McConnell addresses, but the basic idea is sound.
  • Write comments that describe blocks of statements rather than individual statements. Comments covering single statements tend to discuss the mechanics of that statement rather than discussing what the program is doing.
  • Focus paragraph comments on the why rather than the how. Code should explain what the program is doing and why the programmer chose to do it that way rather than explain what each individual statement is doing.
  • Use comments to prepare the reader for what is to follow. Someone reading the comments should be able to have a good idea of what the following code does without actually looking at the code. Note that this rule also suggests that comments should always precede the code to which they apply.
  • Make every comment count. If the reader wastes time reading a comment of little value, the program is harder to read; period.
  • Document surprises and tricky code. Of course, the best solution is not to have any tricky code. In practice, you can't always achieve this goal. When you do need to restore to some tricky code, make sure you fully document what you've done.
  • Avoid abbreviations. While there may be an argument for abbreviating identifiers that appear in a program, no way does this apply to comments.
  • Keep comments close to the code they describe. The prologue to a program unit should give its name, describe the parameters, and provide a short description of the program. It should not go into details about the operation of the module itself. Internal comments should to that.
  • Comments should explain the parameters to a function, assertions about these parameters, whether they are input, output, or in/out parameters.
  • Comments should describe a routine's limitations, assumptions, and any side effects.
All comments will be high-quality comments that describe the actions of the surrounding code in a concise manner

6.3 Endline vs. Standalone Comments

Whenever a comment appears on a line by itself, always put the semicolon in column one. You may indent the text if this is appropriate or aesthetic.
Adjacent lines of comments should not have any interspersed blank lines. A blank comment line should, at least, contain a semicolon in column one.

The guidline above suggests that your code should look like this:

; This is a comment with a blank line between it and the next comment.
; This is another line with a comment on it.

Rather than like this:

; This is a comment with a blank line between it and the next comment.

; This is another line with a comment on it.

The semicolon appearing between the two statements suggest continuity that is not present when you remove the semicolon. If two blocks of comments are truly separate and whitespace between them is appropriate, you should consider separating them by a large number of blank lines to completely eliminate any possible association between the two.

Standalone comments are great for describing the actions of the code that immediately follows. So what are endline comments useful for? Endline comments can explain how a sequence of instructions are implimenting the algorithm described in a previous set of standalone comments. Consider the following code:

; Compute the transpose of a matrix using the algorithm:
;       for i := 0 to 3 do
;               for j := 0 to 3 do
;                       swap( a[i][j], b[j][i] );

                forlp   i, 0, 3

                forlp   j, 0, 3

                mov     bx, i           ;Compute address of a[i][j] using
                shl     bx, 2           ; row major ordering (i*4 + j)*2.
                add     bx, j
                add     bx, bx
                lea     bx, a[bx]
                push    bx              ;Push address of a[i][j] onto

                mov     bx, j           ;Compute address of b[j][i] using
                shl     bx, 2           ;row major ordering (j*4 + i)*2.
                add     bx, i
                add     bx, bx
                lea     bx, b[bx]
                push    bx              ;Push address of b[j][i] onto

                call    swap            ;Swap a[i][j] with b[j][i].


Note that the block comments before this sequence explain, in high level terms, what the code is doing. The endline comments explain how the statement sequence implements the general algorithm. Note, however, that the endline comments do not explain what each statement is doing (at least at the machine level). Rather than claiming "add bx, bx" is multiplying the quantity in BX by two, this code assumes the reader can figure that out for themselves (any reasonable assembly programmer would know this). Once again, keep in mind your audience and write your comments for them.

6.4 Unfinished Code

Often it is the case that a programmer will write a section of code that (partially) accomplishes some task but needs further work to complete a feature set, make it more robust, or remove some known defect in the code. It is common for such programmers to place comments into the code like "This needs more work," "Kludge ahead," etc. The problem with these comments is that they are often forgotten. It isn't until the code fails in the field that the section of code associated with these comments is found and their problems corrected.

Ideally, one should never have to put such code into a program. Of course, ideally, programs never have any defects in them, either. Since such code inevitably finds its way into a program, it's best to have a policy in place to deal with it, hence this section.

Unfinished code comes in five general categories: non-functional code, partially functioning code, suspect code, code in need of enhancement, and code documentation. Non-functional code might be a stub or driver that needs to be replaced in the future with actual code or some code that has severe enough defects that it is useless except for some small special cases. This code is really bad, fortunately its severity prevents you from ignoring it. It is unlikely anyone would miss such a poorly constructed piece of code in early testing prior to release.

Partially functioning code is, perhaps, the biggest problem. This code works well enough to pass some simple tests yet contains serious defects that should be corrected. Moreover, these defects are known. Software often contains a large number of unknown defects; it's a shame to let some (prior) known defects ship with the product simply because a programmer forgot about a defect or couldn't find the defect later.

Suspect code is exactly that- code that is suspicious. The programmer may not be aware of a quantifiable problem but may suspect that a problem exists. Such code will need a later review in order to verify whether it is correct.

The fourth category, code in need of enhancement, is the least serious. For example, to expedite a release, a programmer might choose to use a simple algorithm rather than a complex, faster algorithm. S/he could make a comment in the code like "This linear search should be replaced by a hash table lookup in a future version of the software." Although it might not be absolutely necessary to correct such a problem, it would be nice to know about such problems so they can be dealt with in the future.

The fifth category, documentation, refers to changes made to software that will affect the corresponding documentation (user guide, design document, etc.). The documentation department can search for these defects to bring existing documentation in line with the current code.

This standard defines a mechanism for dealing with these five classes of problems. Any occurrence of unfinished code will be preceded by a comment that takes one of the following forms (where "_" denotes a single space):


It is important to use all lower case and verify the correct spelling so it is easy to find these comments using a text editor search or a tool like grep. Obviously, a separate comment explaining the situation must follow these comments in the source code.


; #defect#suspect ;
; #defect#enhancement ;
; #defect#documentation ;

Notice the use of comment delimiters (the semicolon) on both sides even though assembly language, doesn't require them.

Enforced Rule:
If a module contains some defects that cannot be immediately removed because of time or other constraints, the program will insert a standardized comment before the code so that it is easy to locate such problems in the future. The five standardized comments are ";_#defect#severe_;, ";_#defect#functional_;", ";_#defect#suspect_;", ";_#defect#enhancement_;", and ";_#defect#documentation_;" where "_" denotes a single space. The spelling and spacing should be exact so it is easy to search for these strings in the source tree.

6.5 Cross References in Code to Other Documents

In many instances a section of code might be intrinsically tied to some other document. For example, you might refer the reader to the user document or the design document within your comments in a program. This document proposes a standard way to do this so that it is relatively easy to locate cross references appearing in source code. The technique is similar to that for defect reporting, except the comments take the form:

		;  text #link#location text ;

"Text" is optional and represents arbitrary text (although it is really intended for embedding html commands to provide hyperlinks to the specified document). "Location" describes the document and section where the associated information can be found.


; #link#User's Guide Section 3.1 ;
; #link#Program Design Document, Page 5 ;
; #link#Funcs.pas module, "xyz" function ;
; <A HREF="DesignDoc.html#xyzfunc"> #link#xyzfunc
</a> ;
If a module contains some cross references to other documents, there should be a comment that takes the form "; text #link#location text ;" that provides the reference to that other document. In this comment "text" represents some optional text (typically reserved for html tags) and "location" is some descriptive text that describes the document (and a position in that document) related to the current section of code in the program.

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