The Netwide Assembler: NASM

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Chapter 4: The NASM Preprocessor

NASM contains a powerful macro processor, which supports conditional assembly, multi-level file inclusion, two forms of macro (single-line and multi-line), and a `context stack' mechanism for extra macro power. Preprocessor directives all begin with a % sign.

4.1 Single-Line Macros

4.1.1 The Normal Way: %define

Single-line macros are defined using the %define preprocessor directive. The definitions work in a similar way to C; so you can do things like

%define ctrl 0x1F & 
%define param(a,b) ((a)+(a)*(b)) 
          mov byte [param(2,ebx)], ctrl 'D'

which will expand to

          mov byte [(2)+(2)*(ebx)], 0x1F & 'D'

When the expansion of a single-line macro contains tokens which invoke another macro, the expansion is performed at invocation time, not at definition time. Thus the code

%define a(x) 1+b(x) 
%define b(x) 2*x 
          mov ax,a(8)

will evaluate in the expected way to mov ax,1+2*8, even though the macro b wasn't defined at the time of definition of a.

Macros defined with %define are case sensitive: after %define foo bar, only foo will expand to bar: Foo or FOO will not. By using %idefine instead of %define (the `i' stands for `insensitive') you can define all the case variants of a macro at once, so that %idefine foo bar would cause foo, Foo, FOO, fOO and so on all to expand to bar.

There is a mechanism which detects when a macro call has occurred as a result of a previous expansion of the same macro, to guard against circular references and infinite loops. If this happens, the preprocessor will only expand the first occurrence of the macro. Hence, if you code

%define a(x) 1+a(x) 
          mov ax,a(3)

the macro a(3) will expand once, becoming 1+a(3), and will then expand no further. This behaviour can be useful: see section 8.1 for an example of its use.

You can overload single-line macros: if you write

%define foo(x) 1+x 
%define foo(x,y) 1+x*y

the preprocessor will be able to handle both types of macro call, by counting the parameters you pass; so foo(3) will become 1+3 whereas foo(ebx,2) will become 1+ebx*2. However, if you define

%define foo bar

then no other definition of foo will be accepted: a macro with no parameters prohibits the definition of the same name as a macro with parameters, and vice versa.

This doesn't prevent single-line macros being redefined: you can perfectly well define a macro with

%define foo bar

and then re-define it later in the same source file with

%define foo baz

Then everywhere the macro foo is invoked, it will be expanded according to the most recent definition. This is particularly useful when defining single-line macros with %assign (see section 4.1.3).

You can pre-define single-line macros using the `-d' option on the NASM command line: see section 2.1.8.

4.1.2 Undefining macros: %undef

Single-line macros can be removed with the %undef command. For example, the following sequence:

%define foo bar 
%undef foo 
		mov eax, foo

will expand to the instruction mov eax, foo, since after %undef the macro foo is no longer defined.

Macros that would otherwise be pre-defined can be undefined on the command-line using the `-u' option on the NASM command line: see section 2.1.9.

4.1.3 Preprocessor Variables: %assign

An alternative way to define single-line macros is by means of the %assign command (and its case sensitivecase-insensitive counterpart %iassign, which differs from %assign in exactly the same way that %idefine differs from %define).

%assign is used to define single-line macros which take no parameters and have a numeric value. This value can be specified in the form of an expression, and it will be evaluated once, when the %assign directive is processed.

Like %define, macros defined using %assign can be re-defined later, so you can do things like

%assign i i+1

to increment the numeric value of a macro.

%assign is useful for controlling the termination of %rep preprocessor loops: see section 4.4 for an example of this. Another use for %assign is given in section 7.4 and section 8.1.

The expression passed to %assign is a critical expression (see section 3.7), and must also evaluate to a pure number (rather than a relocatable reference such as a code or data address, or anything involving a register).

4.2 Multi-Line Macros: %macro

Multi-line macros are much more like the type of macro seen in MASM and TASM: a multi-line macro definition in NASM looks something like this.

%macro prologue 1 
          push ebp 
          mov ebp,esp 
          sub esp,%1 

This defines a C-like function prologue as a macro: so you would invoke the macro with a call such as

myfunc:   prologue 12

which would expand to the three lines of code

myfunc:   push ebp 
          mov ebp,esp 
          sub esp,12

The number 1 after the macro name in the %macro line defines the number of parameters the macro prologue expects to receive. The use of %1 inside the macro definition refers to the first parameter to the macro call. With a macro taking more than one parameter, subsequent parameters would be referred to as %2, %3 and so on.

Multi-line macros, like single-line macros, are case-sensitive, unless you define them using the alternative directive %imacro.

If you need to pass a comma as part of a parameter to a multi-line macro, you can do that by enclosing the entire parameter in braces. So you could code things like

%macro silly 2 
%2:       db %1 
          silly 'a', letter_a    ; letter_a:  db 'a' 
          silly 'ab', string_ab  ; string_ab: db 'ab' 
          silly {13,10}, crlf    ; crlf:      db 13,10

4.2.1 Overloading Multi-Line Macros

As with single-line macros, multi-line macros can be overloaded by defining the same macro name several times with different numbers of parameters. This time, no exception is made for macros with no parameters at all. So you could define

%macro prologue 0 
          push ebp 
          mov ebp,esp 

to define an alternative form of the function prologue which allocates no local stack space.

Sometimes, however, you might want to `overload' a machine instruction; for example, you might want to define

%macro push 2 
          push %1 
          push %2 

so that you could code

          push ebx               ; this line is not a macro call 
          push eax,ecx           ; but this one is

Ordinarily, NASM will give a warning for the first of the above two lines, since push is now defined to be a macro, and is being invoked with a number of parameters for which no definition has been given. The correct code will still be generated, but the assembler will give a warning. This warning can be disabled by the use of the -w-macro-params command-line option (see section 2.1.12).

4.2.2 Macro-Local Labels

NASM allows you to define labels within a multi-line macro definition in such a way as to make them local to the macro call: so calling the same macro multiple times will use a different label each time. You do this by prefixing %% to the label name. So you can invent an instruction which executes a RET if the Z flag is set by doing this:

%macro retz 0 
          jnz %%skip 

You can call this macro as many times as you want, and every time you call it NASM will make up a different `real' name to substitute for the label %%skip. The names NASM invents are of the form ..@2345.skip, where the number 2345 changes with every macro call. The ..@ prefix prevents macro-local labels from interfering with the local label mechanism, as described in section 3.8. You should avoid defining your own labels in this form (the ..@ prefix, then a number, then another period) in case they interfere with macro-local labels.

4.2.3 Greedy Macro Parameters

Occasionally it is useful to define a macro which lumps its entire command line into one parameter definition, possibly after extracting one or two smaller parameters from the front. An example might be a macro to write a text string to a file in MS-DOS, where you might want to be able to write

          writefile [filehandle],"hello, world",13,10

NASM allows you to define the last parameter of a macro to be greedy, meaning that if you invoke the macro with more parameters than it expects, all the spare parameters get lumped into the last defined one along with the separating commas. So if you code:

%macro writefile 2+ 
          jmp %%endstr 
%%str:    db %2 
%%endstr: mov dx,%%str 
          mov cx,%%endstr-%%str 
          mov bx,%1 
          mov ah,0x40 
          int 0x21 

then the example call to writefile above will work as expected: the text before the first comma, [filehandle], is used as the first macro parameter and expanded when %1 is referred to, and all the subsequent text is lumped into %2 and placed after the db.

The greedy nature of the macro is indicated to NASM by the use of the + sign after the parameter count on the %macro line.

If you define a greedy macro, you are effectively telling NASM how it should expand the macro given any number of parameters from the actual number specified up to infinity; in this case, for example, NASM now knows what to do when it sees a call to writefile with 2, 3, 4 or more parameters. NASM will take this into account when overloading macros, and will not allow you to define another form of writefile taking 4 parameters (for example).

Of course, the above macro could have been implemented as a non-greedy macro, in which case the call to it would have had to look like

          writefile [filehandle], {"hello, world",13,10}

NASM provides both mechanisms for putting commas in macro parameters, and you choose which one you prefer for each macro definition.

See section 5.2.1 for a better way to write the above macro.

4.2.4 Default Macro Parameters

NASM also allows you to define a multi-line macro with a range of allowable parameter counts. If you do this, you can specify defaults for omitted parameters. So, for example:

%macro die 0-1 "Painful program death has occurred." 
          writefile 2,%1 
          mov ax,0x4c01 
          int 0x21 

This macro (which makes use of the writefile macro defined in section 4.2.3) can be called with an explicit error message, which it will display on the error output stream before exiting, or it can be called with no parameters, in which case it will use the default error message supplied in the macro definition.

In general, you supply a minimum and maximum number of parameters for a macro of this type; the minimum number of parameters are then required in the macro call, and then you provide defaults for the optional ones. So if a macro definition began with the line

%macro foobar 1-3 eax,[ebx+2]

then it could be called with between one and three parameters, and %1 would always be taken from the macro call. %2, if not specified by the macro call, would default to eax, and %3 if not specified would default to [ebx+2].

You may omit parameter defaults from the macro definition, in which case the parameter default is taken to be blank. This can be useful for macros which can take a variable number of parameters, since the %0 token (see section 4.2.5) allows you to determine how many parameters were really passed to the macro call.

This defaulting mechanism can be combined with the greedy-parameter mechanism; so the die macro above could be made more powerful, and more useful, by changing the first line of the definition to

%macro die 0-1+ "Painful program death has occurred.",13,10

The maximum parameter count can be infinite, denoted by *. In this case, of course, it is impossible to provide a full set of default parameters. Examples of this usage are shown in section 4.2.6.

4.2.5 %0: Macro Parameter Counter

For a macro which can take a variable number of parameters, the parameter reference %0 will return a numeric constant giving the number of parameters passed to the macro. This can be used as an argument to %rep (see section 4.4) in order to iterate through all the parameters of a macro. Examples are given in section 4.2.6.

4.2.6 %rotate: Rotating Macro Parameters

Unix shell programmers will be familiar with the shift shell command, which allows the arguments passed to a shell script (referenced as $1, $2 and so on) to be moved left by one place, so that the argument previously referenced as $2 becomes available as $1, and the argument previously referenced as $1 is no longer available at all.

NASM provides a similar mechanism, in the form of %rotate. As its name suggests, it differs from the Unix shift in that no parameters are lost: parameters rotated off the left end of the argument list reappear on the right, and vice versa.

%rotate is invoked with a single numeric argument (which may be an expression). The macro parameters are rotated to the left by that many places. If the argument to %rotate is negative, the macro parameters are rotated to the right.

So a pair of macros to save and restore a set of registers might work as follows:

%macro multipush 1-* 
%rep %0 
          push %1 
%rotate 1 

This macro invokes the PUSH instruction on each of its arguments in turn, from left to right. It begins by pushing its first argument, %1, then invokes %rotate to move all the arguments one place to the left, so that the original second argument is now available as %1. Repeating this procedure as many times as there were arguments (achieved by supplying %0 as the argument to %rep) causes each argument in turn to be pushed.

Note also the use of * as the maximum parameter count, indicating that there is no upper limit on the number of parameters you may supply to the multipush macro.

It would be convenient, when using this macro, to have a POP equivalent, which didn't require the arguments to be given in reverse order. Ideally, you would write the multipush macro call, then cut-and-paste the line to where the pop needed to be done, and change the name of the called macro to multipop, and the macro would take care of popping the registers in the opposite order from the one in which they were pushed.

This can be done by the following definition:

%macro multipop 1-* 
%rep %0 
%rotate -1 
          pop %1 

This macro begins by rotating its arguments one place to the right, so that the original last argument appears as %1. This is then popped, and the arguments are rotated right again, so the second-to-last argument becomes %1. Thus the arguments are iterated through in reverse order.

4.2.7 Concatenating Macro Parameters

NASM can concatenate macro parameters on to other text surrounding them. This allows you to declare a family of symbols, for example, in a macro definition. If, for example, you wanted to generate a table of key codes along with offsets into the table, you could code something like

%macro keytab_entry 2 
keypos%1 equ $-keytab 
          db %2 
          keytab_entry F1,128+1 
          keytab_entry F2,128+2 
          keytab_entry Return,13

which would expand to

keyposF1 equ $-keytab 
          db 128+1 
keyposF2 equ $-keytab 
          db 128+2 
keyposReturn equ $-keytab 
          db 13

You can just as easily concatenate text on to the other end of a macro parameter, by writing %1foo.

If you need to append a digit to a macro parameter, for example defining labels foo1 and foo2 when passed the parameter foo, you can't code %11 because that would be taken as the eleventh macro parameter. Instead, you must code %{1}1, which will separate the first 1 (giving the number of the macro parameter) from the second (literal text to be concatenated to the parameter).

This concatenation can also be applied to other preprocessor in-line objects, such as macro-local labels (section 4.2.2) and context-local labels (section 4.6.2). In all cases, ambiguities in syntax can be resolved by enclosing everything after the % sign and before the literal text in braces: so %{%foo}bar concatenates the text bar to the end of the real name of the macro-local label %%foo. (This is unnecessary, since the form NASM uses for the real names of macro-local labels means that the two usages %{%foo}bar and %%foobar would both expand to the same thing anyway; nevertheless, the capability is there.)

4.2.8 Condition Codes as Macro Parameters

NASM can give special treatment to a macro parameter which contains a condition code. For a start, you can refer to the macro parameter %1 by means of the alternative syntax %+1, which informs NASM that this macro parameter is supposed to contain a condition code, and will cause the preprocessor to report an error message if the macro is called with a parameter which is not a valid condition code.

Far more usefully, though, you can refer to the macro parameter by means of %-1, which NASM will expand as the inverse condition code. So the retz macro defined in section 4.2.2 can be replaced by a general conditional-return macro like this:

%macro retc 1 
          j%-1 %%skip 

This macro can now be invoked using calls like retc ne, which will cause the conditional-jump instruction in the macro expansion to come out as JE, or retc po which will make the jump a JPE.

The %+1 macro-parameter reference is quite happy to interpret the arguments CXZ and ECXZ as valid condition codes; however, %-1 will report an error if passed either of these, because no inverse condition code exists.

4.2.9 Disabling Listing Expansion

When NASM is generating a listing file from your program, it will generally expand multi-line macros by means of writing the macro call and then listing each line of the expansion. This allows you to see which instructions in the macro expansion are generating what code; however, for some macros this clutters the listing up unnecessarily.

NASM therefore provides the .nolist qualifier, which you can include in a macro definition to inhibit the expansion of the macro in the listing file. The .nolist qualifier comes directly after the number of parameters, like this:

%macro foo 1.nolist

Or like this:

%macro bar 1-5+.nolist a,b,c,d,e,f,g,h

4.3 Conditional Assembly

Similarly to the C preprocessor, NASM allows sections of a source file to be assembled only if certain conditions are met. The general syntax of this feature looks like this:

; some code which only appears if <condition> is met 
; only appears if <condition> is not met but <condition2> is 
; this appears if neither <condition> nor <condition2> was met 

The %else clause is optional, as is the %elif clause. You can have more than one %elif clause as well.

4.3.1 %ifdef: Testing Single-Line Macro Existence

Beginning a conditional-assembly block with the line %ifdef MACRO will assemble the subsequent code if, and only if, a single-line macro called MACRO is defined. If not, then the %elif and %else blocks (if any) will be processed instead.

For example, when debugging a program, you might want to write code such as

          ; perform some function 
%ifdef DEBUG 
          writefile 2,"Function performed successfully",13,10 
          ; go and do something else

Then you could use the command-line option -dDEBUG to create a version of the program which produced debugging messages, and remove the option to generate the final release version of the program.

You can test for a macro not being defined by using %ifndef instead of %ifdef. You can also test for macro definitions in %elif blocks by using %elifdef and %elifndef.

4.3.2 %ifctx: Testing the Context Stack

The conditional-assembly construct %ifctx ctxname will cause the subsequent code to be assembled if and only if the top context on the preprocessor's context stack has the name ctxname. As with %ifdef, the inverse and %elif forms %ifnctx, %elifctx and %elifnctx are also supported.

For more details of the context stack, see section 4.6. For a sample use of %ifctx, see section 4.6.5.

4.3.3 %if: Testing Arbitrary Numeric Expressions

The conditional-assembly construct %if expr will cause the subsequent code to be assembled if and only if the value of the numeric expression expr is non-zero. An example of the use of this feature is in deciding when to break out of a %rep preprocessor loop: see section 4.4 for a detailed example.

The expression given to %if, and its counterpart %elif, is a critical expression (see section 3.7).

%if extends the normal NASM expression syntax, by providing a set of relational operators which are not normally available in expressions. The operators =, <, >, <=, >= and <> test equality, less-than, greater-than, less-or-equal, greater-or-equal and not-equal respectively. The C-like forms == and != are supported as alternative forms of = and <>. In addition, low-priority logical operators &&, ^^ and || are provided, supplying logical AND, logical XOR and logical OR. These work like the C logical operators (although C has no logical XOR), in that they always return either 0 or 1, and treat any non-zero input as 1 (so that ^^, for example, returns 1 if exactly one of its inputs is zero, and 0 otherwise). The relational operators also return 1 for true and 0 for false.

4.3.4 %ifidn and %ifidni: Testing Exact Text Identity

The construct %ifidn text1,text2 will cause the subsequent code to be assembled if and only if text1 and text2, after expanding single-line macros, are identical pieces of text. Differences in white space are not counted.

%ifidni is similar to %ifidn, but is case-insensitive.

For example, the following macro pushes a register or number on the stack, and allows you to treat IP as a real register:

%macro pushparam 1 
%ifidni %1,ip 
          call %%label 
          push %1 

Like most other %if constructs, %ifidn has a counterpart %elifidn, and negative forms %ifnidn and %elifnidn. Similarly, %ifidni has counterparts %elifidni, %ifnidni and %elifnidni.

4.3.5 %ifid, %ifnum, %ifstr: Testing Token Types

Some macros will want to perform different tasks depending on whether they are passed a number, a string, or an identifier. For example, a string output macro might want to be able to cope with being passed either a string constant or a pointer to an existing string.

The conditional assembly construct %ifid, taking one parameter (which may be blank), assembles the subsequent code if and only if the first token in the parameter exists and is an identifier. %ifnum works similarly, but tests for the token being a numeric constant; %ifstr tests for it being a string.

For example, the writefile macro defined in section 4.2.3 can be extended to take advantage of %ifstr in the following fashion:

%macro writefile 2-3+ 
%ifstr %2 
          jmp %%endstr 
%if %0 = 3 
%%str:	  db %2,%3 
%%str:	  db %2 
%%endstr: mov dx,%%str 
          mov cx,%%endstr-%%str 
	  mov dx,%2 
	  mov cx,%3 
          mov bx,%1 
          mov ah,0x40 
          int 0x21 

Then the writefile macro can cope with being called in either of the following two ways:

          writefile [file], strpointer, length 
          writefile [file], "hello", 13, 10

In the first, strpointer is used as the address of an already-declared string, and length is used as its length; in the second, a string is given to the macro, which therefore declares it itself and works out the address and length for itself.

Note the use of %if inside the %ifstr: this is to detect whether the macro was passed two arguments (so the string would be a single string constant, and db %2 would be adequate) or more (in which case, all but the first two would be lumped together into %3, and db %2,%3 would be required).

The usual %elifXXX, %ifnXXX and %elifnXXX versions exist for each of %ifid, %ifnum and %ifstr.

4.3.6 %error: Reporting User-Defined Errors

The preprocessor directive %error will cause NASM to report an error if it occurs in assembled code. So if other users are going to try to assemble your source files, you can ensure that they define the right macros by means of code like this:

%ifdef SOME_MACRO 
; do some setup 
; do some different setup 
%error Neither SOME_MACRO nor SOME_OTHER_MACRO was defined. 

Then any user who fails to understand the way your code is supposed to be assembled will be quickly warned of their mistake, rather than having to wait until the program crashes on being run and then not knowing what went wrong.

4.4 Preprocessor Loops: %rep

NASM's TIMES prefix, though useful, cannot be used to invoke a multi-line macro multiple times, because it is processed by NASM after macros have already been expanded. Therefore NASM provides another form of loop, this time at the preprocessor level: %rep.

The directives %rep and %endrep (%rep takes a numeric argument, which can be an expression; %endrep takes no arguments) can be used to enclose a chunk of code, which is then replicated as many times as specified by the preprocessor:

%assign i 0 
%rep 64 
          inc word [table+2*i] 
%assign i i+1 

This will generate a sequence of 64 INC instructions, incrementing every word of memory from [table] to [table+126].

For more complex termination conditions, or to break out of a repeat loop part way along, you can use the %exitrep directive to terminate the loop, like this:

%assign i 0 
%assign j 1 
%rep 100 
%if j > 65535 
          dw j 
%assign k j+i 
%assign i j 
%assign j k 
fib_number equ ($-fibonacci)/2

This produces a list of all the Fibonacci numbers that will fit in 16 bits. Note that a maximum repeat count must still be given to %rep. This is to prevent the possibility of NASM getting into an infinite loop in the preprocessor, which (on multitasking or multi-user systems) would typically cause all the system memory to be gradually used up and other applications to start crashing.

4.5 Including Other Files

Using, once again, a very similar syntax to the C preprocessor, NASM's preprocessor lets you include other source files into your code. This is done by the use of the %include directive:

%include "macros.mac"

will include the contents of the file macros.mac into the source file containing the %include directive.

Include files are searched for in the current directory (the directory you're in when you run NASM, as opposed to the location of the NASM executable or the location of the source file), plus any directories specified on the NASM command line using the -i option.

The standard C idiom for preventing a file being included more than once is just as applicable in NASM: if the file macros.mac has the form

%ifndef MACROS_MAC 
%define MACROS_MAC 
; now define some macros 

then including the file more than once will not cause errors, because the second time the file is included nothing will happen because the macro MACROS_MAC will already be defined.

You can force a file to be included even if there is no %include directive that explicitly includes it, by using the -p option on the NASM command line (see section 2.1.7).

4.6 The Context Stack

Having labels that are local to a macro definition is sometimes not quite powerful enough: sometimes you want to be able to share labels between several macro calls. An example might be a REPEAT ... UNTIL loop, in which the expansion of the REPEAT macro would need to be able to refer to a label which the UNTIL macro had defined. However, for such a macro you would also want to be able to nest these loops.

NASM provides this level of power by means of a context stack. The preprocessor maintains a stack of contexts, each of which is characterised by a name. You add a new context to the stack using the %push directive, and remove one using %pop. You can define labels that are local to a particular context on the stack.

4.6.1 %push and %pop: Creating and Removing Contexts

The %push directive is used to create a new context and place it on the top of the context stack. %push requires one argument, which is the name of the context. For example:

%push foobar

This pushes a new context called foobar on the stack. You can have several contexts on the stack with the same name: they can still be distinguished.

The directive %pop, requiring no arguments, removes the top context from the context stack and destroys it, along with any labels associated with it.

4.6.2 Context-Local Labels

Just as the usage %%foo defines a label which is local to the particular macro call in which it is used, the usage %$foo is used to define a label which is local to the context on the top of the context stack. So the REPEAT and UNTIL example given above could be implemented by means of:

%macro repeat 0 
%push repeat 

%macro until 1 
          j%-1 %$begin 

and invoked by means of, for example,

          mov cx,string 
          add cx,3 
          until e

which would scan every fourth byte of a string in search of the byte in AL.

If you need to define, or access, labels local to the context below the top one on the stack, you can use %$$foo, or %$$$foo for the context below that, and so on.

4.6.3 Context-Local Single-Line Macros

NASM also allows you to define single-line macros which are local to a particular context, in just the same way:

%define %$localmac 3

will define the single-line macro %$localmac to be local to the top context on the stack. Of course, after a subsequent %push, it can then still be accessed by the name %$$localmac.

4.6.4 %repl: Renaming a Context

If you need to change the name of the top context on the stack (in order, for example, to have it respond differently to %ifctx), you can execute a %pop followed by a %push; but this will have the side effect of destroying all context-local labels and macros associated with the context that was just popped.

NASM provides the directive %repl, which replaces a context with a different name, without touching the associated macros and labels. So you could replace the destructive code

%push newname

with the non-destructive version %repl newname.

4.6.5 Example Use of the Context Stack: Block IFs

This example makes use of almost all the context-stack features, including the conditional-assembly construct %ifctx, to implement a block IF statement as a set of macros.

%macro if 1 
    %push if 
    j%-1 %$ifnot 

%macro else 0 
    %ifctx if 
        %repl else 
        jmp %$ifend 
        %error "expected `if' before `else'" 

%macro endif 0 
    %ifctx if 
    %elifctx else 
        %error "expected `if' or `else' before `endif'" 

This code is more robust than the REPEAT and UNTIL macros given in section 4.6.2, because it uses conditional assembly to check that the macros are issued in the right order (for example, not calling endif before if) and issues a %error if they're not.

In addition, the endif macro has to be able to cope with the two distinct cases of either directly following an if, or following an else. It achieves this, again, by using conditional assembly to do different things depending on whether the context on top of the stack is if or else.

The else macro has to preserve the context on the stack, in order to have the %$ifnot referred to by the if macro be the same as the one defined by the endif macro, but has to change the context's name so that endif will know there was an intervening else. It does this by the use of %repl.

A sample usage of these macros might look like:

          cmp ax,bx 
          if ae 
            cmp bx,cx 
            if ae 
              mov ax,cx 
              mov ax,bx 
            cmp ax,cx 
            if ae 
              mov ax,cx 

The block-IF macros handle nesting quite happily, by means of pushing another context, describing the inner if, on top of the one describing the outer if; thus else and endif always refer to the last unmatched if or else.

4.7 Standard Macros

NASM defines a set of standard macros, which are already defined when it starts to process any source file. If you really need a program to be assembled with no pre-defined macros, you can use the %clear directive to empty the preprocessor of everything.

Most user-level assembler directives (see chapter 5) are implemented as macros which invoke primitive directives; these are described in chapter 5. The rest of the standard macro set is described here.

4.7.1 __NASM_MAJOR__ and __NASM_MINOR__: NASM Version

The single-line macros __NASM_MAJOR__ and __NASM_MINOR__ expand to the major and minor parts of the version number of NASM being used. So, under NASM 0.96 for example, __NASM_MAJOR__ would be defined to be 0 and __NASM_MINOR__ would be defined as 96.

4.7.2 __FILE__ and __LINE__: File Name and Line Number

Like the C preprocessor, NASM allows the user to find out the file name and line number containing the current instruction. The macro __FILE__ expands to a string constant giving the name of the current input file (which may change through the course of assembly if %include directives are used), and __LINE__ expands to a numeric constant giving the current line number in the input file.

These macros could be used, for example, to communicate debugging information to a macro, since invoking __LINE__ inside a macro definition (either single-line or multi-line) will return the line number of the macro call, rather than definition. So to determine where in a piece of code a crash is occurring, for example, one could write a routine stillhere, which is passed a line number in EAX and outputs something like `line 155: still here'. You could then write a macro

%macro notdeadyet 0 
          push eax 
          mov eax,__LINE__ 
          call stillhere 
          pop eax 

and then pepper your code with calls to notdeadyet until you find the crash point.

4.7.3 STRUC and ENDSTRUC: Declaring Structure Data Types

The core of NASM contains no intrinsic means of defining data structures; instead, the preprocessor is sufficiently powerful that data structures can be implemented as a set of macros. The macros STRUC and ENDSTRUC are used to define a structure data type.

STRUC takes one parameter, which is the name of the data type. This name is defined as a symbol with the value zero, and also has the suffix _size appended to it and is then defined as an EQU giving the size of the structure. Once STRUC has been issued, you are defining the structure, and should define fields using the RESB family of pseudo-instructions, and then invoke ENDSTRUC to finish the definition.

For example, to define a structure called mytype containing a longword, a word, a byte and a string of bytes, you might code

          struc mytype 
mt_long:  resd 1 
mt_word:  resw 1 
mt_byte:  resb 1 
mt_str:   resb 32 

The above code defines six symbols: mt_long as 0 (the offset from the beginning of a mytype structure to the longword field), mt_word as 4, mt_byte as 6, mt_str as 7, mytype_size as 39, and mytype itself as zero.

The reason why the structure type name is defined at zero is a side effect of allowing structures to work with the local label mechanism: if your structure members tend to have the same names in more than one structure, you can define the above structure like this:

          struc mytype 
.long:    resd 1 
.word:    resw 1 
.byte:    resb 1 
.str:     resb 32 

This defines the offsets to the structure fields as mytype.long, mytype.word, mytype.byte and mytype.str.

NASM, since it has no intrinsic structure support, does not support any form of period notation to refer to the elements of a structure once you have one (except the above local-label notation), so code such as mov ax,[mystruc.mt_word] is not valid. mt_word is a constant just like any other constant, so the correct syntax is mov ax,[mystruc+mt_word] or mov ax,[mystruc+mytype.word].

4.7.4 ISTRUC, AT and IEND: Declaring Instances of Structures

Having defined a structure type, the next thing you typically want to do is to declare instances of that structure in your data segment. NASM provides an easy way to do this in the ISTRUC mechanism. To declare a structure of type mytype in a program, you code something like this:

mystruc:  istruc mytype 
          at mt_long, dd 123456 
          at mt_word, dw 1024 
          at mt_byte, db 'x' 
          at mt_str, db 'hello, world', 13, 10, 0 

The function of the AT macro is to make use of the TIMES prefix to advance the assembly position to the correct point for the specified structure field, and then to declare the specified data. Therefore the structure fields must be declared in the same order as they were specified in the structure definition.

If the data to go in a structure field requires more than one source line to specify, the remaining source lines can easily come after the AT line. For example:

          at mt_str, db 123,134,145,156,167,178,189 
          db 190,100,0

Depending on personal taste, you can also omit the code part of the AT line completely, and start the structure field on the next line:

          at mt_str 
          db 'hello, world' 
          db 13,10,0

4.7.5 ALIGN and ALIGNB: Data Alignment

The ALIGN and ALIGNB macros provides a convenient way to align code or data on a word, longword, paragraph or other boundary. (Some assemblers call this directive EVEN.) The syntax of the ALIGN and ALIGNB macros is

          align 4                ; align on 4-byte boundary 
          align 16               ; align on 16-byte boundary 
          align 8,db 0           ; pad with 0s rather than NOPs 
          align 4,resb 1         ; align to 4 in the BSS 
          alignb 4               ; equivalent to previous line

Both macros require their first argument to be a power of two; they both compute the number of additional bytes required to bring the length of the current section up to a multiple of that power of two, and then apply the TIMES prefix to their second argument to perform the alignment.

If the second argument is not specified, the default for ALIGN is NOP, and the default for ALIGNB is RESB 1. So if the second argument is specified, the two macros are equivalent. Normally, you can just use ALIGN in code and data sections and ALIGNB in BSS sections, and never need the second argument except for special purposes.

ALIGN and ALIGNB, being simple macros, perform no error checking: they cannot warn you if their first argument fails to be a power of two, or if their second argument generates more than one byte of code. In each of these cases they will silently do the wrong thing.

ALIGNB (or ALIGN with a second argument of RESB 1) can be used within structure definitions:

          struc mytype2 
mt_byte:  resb 1 
          alignb 2 
mt_word:  resw 1 
          alignb 4 
mt_long:  resd 1 
mt_str:   resb 32 

This will ensure that the structure members are sensibly aligned relative to the base of the structure.

A final caveat: ALIGN and ALIGNB work relative to the beginning of the section, not the beginning of the address space in the final executable. Aligning to a 16-byte boundary when the section you're in is only guaranteed to be aligned to a 4-byte boundary, for example, is a waste of effort. Again, NASM does not check that the section's alignment characteristics are sensible for the use of ALIGN or ALIGNB.

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