NASM - The Netwide Assembler

version 2.16.02

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. As a result, some care needs to be taken when using the % arithmetic operator to avoid it being confused with a preprocessor directive; it is recommended that it always be surrounded by whitespace.

The NASM preprocessor borrows concepts from both the C preprocessor and the macro facilities of many other assemblers.

4.1 Preprocessor Expansions

The input to the preprocessor is expanded in the following ways in the order specified here.

4.1.1 Continuation Line Collapsing

The preprocessor first collapses all lines which end with a backslash (\) character into a single line. Thus:


will work like a single-line macro without the backslash-newline sequence.

4.1.2 Comment Removal

After concatenation, comments are removed. Comments begin with the character ; unless contained inside a quoted string or a handful of other special contexts.

Note that this is applied after continuation lines are collapsed. This means that

      add al,'\'     ; Add the ASCII code for \ 
      mov [ecx],al   ; Save the character

will probably not do what you expect, as the second line will be considered part of the preceeding comment. Although this behavior is sometimes confusing, it is both the behavior of NASM since the very first version as well as the behavior of the C preprocessor.

4.1.3 %line directives

In this step, %line directives are processed. See section 4.13.1.

4.1.4 Conditionals, Loops and Multi-Line Macro Definitions

In this step, the following preprocessor directives are processed:

These constructs are required to be balanced, so that the ending of a block can be detected, but no further processing is done at this time; stored blocks will be inserted at this step when they are expanded (see below.)

It is specific to each directive to what extent inline expansions and detokenization are performed for the arguments of the directives.

4.1.5 Directives processing

Remaining preprocessor directives are processed. It is specific to each directive to what extend the above expansions or the ones specified in section 4.1.8 are performed on their arguments.

It is specific to each directive to what extent inline expansions and detokenization are performed for the arguments of the directives.

4.1.6 Inline expansions and other directives

In this step, the following expansions are performed on each line:

4.1.7 Multi-Line Macro Expansion

In this step, multi-line macros are expanded into new lines of source, like the typical macro feature of many other assemblers. See section 4.5.

After expansion, the newly injected lines of source are processed starting with the step defined in section 4.1.4.

4.1.8 Detokenization

In this step, the final line of source code is produced. It performs the following operations:

The resulting line of text either is sent to the assembler, or, if running in preprocessor-only mode, to the output file (see section 2.1.22); if necessary prefixed by a newly inserted %line directive.

4.2 Single-Line Macros

Single-line macros are expanded inline, much like macros in the C preprocessor.

4.2.1 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.

Note that single-line macro argument list cannot be preceded by whitespace. Otherwise it will be treated as an expansion. For example:

   %define foo (a,b)               ; no arguments, (a,b) is the expansion 
   %define bar(a,b)                ; two arguments, empty expansion

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 10.1 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 4.2.8).

The following additional features were added in NASM 2.15:

It is possible to define an empty string instead of an argument name if the argument is never used. For example:

   %define ereg(foo,) e %+ foo 
     mov eax,ereg(dx,cx)

A single pair of parentheses is a subcase of a single, unused argument:

   %define myreg() eax 
     mov edx,myreg()

This is similar to the behavior of the C preprocessor.

For example:

    %define xyzzy(=expr,&val,=hex/x) expr, str, hex 
    %define plugh(x) xyzzy(x,x,x) 
    db plugh(13+5), `\0` ; Expands to: db 18, "13+5", 0x12, `\0`

You can pre-define single-line macros using the `-d' option on the NASM command line: see section 2.1.20.

4.2.2 Resolving %define: %xdefine

To have a reference to an embedded single-line macro resolved at the time that the embedding macro is defined, as opposed to when the embedding macro is expanded, you need a different mechanism to the one offered by %define. The solution is to use %xdefine, or it's case-insensitive counterpart %ixdefine.

Suppose you have the following code:

%define  isTrue  1 
%define  isFalse isTrue 
%define  isTrue  0 

val1:    db      isFalse 

%define  isTrue  1 

val2:    db      isFalse

In this case, val1 is equal to 0, and val2 is equal to 1. This is because, when a single-line macro is defined using %define, it is expanded only when it is called. As isFalse expands to isTrue, the expansion will be the current value of isTrue. The first time it is called that is 0, and the second time it is 1.

If you wanted isFalse to expand to the value assigned to the embedded macro isTrue at the time that isFalse was defined, you need to change the above code to use %xdefine.

%xdefine isTrue  1 
%xdefine isFalse isTrue 
%xdefine isTrue  0 

val1:    db      isFalse 

%xdefine isTrue  1 

val2:    db      isFalse

Now, each time that isFalse is called, it expands to 1, as that is what the embedded macro isTrue expanded to at the time that isFalse was defined.

%xdefine and %ixdefine supports argument expansion exactly the same way that %define and %idefine does.

4.2.3 Macro Indirection: %[...]

The %[...] construct can be used to expand macros in contexts where macro expansion would otherwise not occur, including in the names other macros. For example, if you have a set of macros named Foo16, Foo32 and Foo64, you could write:

     mov ax,Foo%[__?BITS?__] ; The Foo value

to use the builtin macro __?BITS?__ (see section 5.3) to automatically select between them. Similarly, the two statements:

%xdefine Bar         Quux    ; Expands due to %xdefine 
%define  Bar         %[Quux] ; Expands due to %[...]

have, in fact, exactly the same effect.

%[...] concatenates to adjacent tokens in the same way that multi-line macro parameters do, see section 4.5.9 for details.

4.2.4 Concatenating Single Line Macro Tokens: %+

Individual tokens in single line macros can be concatenated, to produce longer tokens for later processing. This can be useful if there are several similar macros that perform similar functions.

Please note that a space is required after %+, in order to disambiguate it from the syntax %+1 used in multiline macros.

As an example, consider the following:

%define BDASTART 400h                ; Start of BIOS data area

struc   tBIOSDA                      ; its structure 
        .COM1addr       RESW    1 
        .COM2addr       RESW    1 
        ; ..and so on 

Now, if we need to access the elements of tBIOSDA in different places, we can end up with:

        mov     ax,BDASTART + tBIOSDA.COM1addr 
        mov     bx,BDASTART + tBIOSDA.COM2addr

This will become pretty ugly (and tedious) if used in many places, and can be reduced in size significantly by using the following macro:

; Macro to access BIOS variables by their names (from tBDA):

%define BDA(x)  BDASTART + tBIOSDA. %+ x

Now the above code can be written as:

        mov     ax,BDA(COM1addr) 
        mov     bx,BDA(COM2addr)

Using this feature, we can simplify references to a lot of macros (and, in turn, reduce typing errors).

4.2.5 The Macro Name Itself: %? and %??

The special symbols %? and %?? can be used to reference the macro name itself inside a macro expansion, this is supported for both single-and multi-line macros. %? refers to the macro name as invoked, whereas %?? refers to the macro name as declared. The two are always the same for case-sensitive macros, but for case-insensitive macros, they can differ.

For example:

%imacro Foo 0 
        mov %?,%?? 


will expand to:

        mov foo,Foo 
        mov FOO,Foo

These tokens can be used for single-line macros if defined outside any multi-line macros. See below.

4.2.6 The Single-Line Macro Name: %*? and %*??

If the tokens %? and %?? are used inside a multi-line macro, they are expanded before any directives are processed. As a result,

%imacro Foo 0 
      %idefine Bar _%? 
      mov BAR,bAr 

      mov eax,bar

will expand to:

      mov _foo,_foo 
      mov eax,_foo

which may or may not be what you expected. The tokens %*? and %*?? behave like %? and %?? but are only expanded inside single-line macros. Thus:

%imacro Foo 0 
      %idefine Bar _%*? 
      mov BAR,bAr 

      mov eax,bar

will expand to:

      mov _BAR,_bAr 
      mov eax,_bar

The %*? can be used to make a keyword "disappear", for example in case a new instruction has been used as a label in older code. For example:

%idefine pause $%*?                 ; Hide the PAUSE instruction

%*? and %*?? were introduced in NASM 2.15.04.

4.2.7 Undefining Single-Line Macros: %undef

Single-line macros can be removed with the %undef directive. 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 2.1.21.

4.2.8 Preprocessor Variables: %assign

An alternative way to define single-line macros is by means of the %assign command (and its case-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 4.7 for an example of this. Another use for %assign is given in section 9.4 and section 10.1.

The expression passed to %assign is a critical expression (see section 3.8), 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).

See also the %eval() preprocessor function, section 4.4.4.

4.2.9 Defining Strings: %defstr

%defstr, and its case-insensitive counterpart %idefstr, define or redefine a single-line macro without parameters but converts the entire right-hand side, after macro expansion, to a quoted string before definition.

For example:

%defstr test TEST

is equivalent to

%define test 'TEST'

This can be used, for example, with the %! construct (see section 4.13.2):

%defstr PATH %!PATH          ; The operating system PATH variable

See also the %str() preprocessor function, section 4.4.10.

4.2.10 Defining Tokens: %deftok

%deftok, and its case-insensitive counterpart %ideftok, define or redefine a single-line macro without parameters but converts the second parameter, after string conversion, to a sequence of tokens.

For example:

%deftok test 'TEST'

is equivalent to

%define test TEST

See also the %tok() preprocessor function, section 4.4.14.

4.2.11 Defining Aliases: %defalias

%defalias, and its case-insensitive counterpart %idefalias, define an alias to a macro, i.e. equivalent of a symbolic link.

When used with various macro defining and undefining directives, it affects the aliased macro. This functionality is intended for being able to rename macros while retaining the legacy names.

When an alias is defined, but the aliased macro is then undefined, the aliases can legitimately point to nonexistent macros.

The alias can be undefined using the %undefalias directive. All aliases can be undefined using the %clear defalias directive. This includes backwards compatibility aliases defined by NASM itself.

To disable aliases without undefining them, use the %aliases off directive.

To check whether an alias is defined, regardless of the existence of the aliased macro, use %ifdefalias.

For example:

%defalias OLD NEW 
   ; OLD and NEW both undefined 
%define NEW 123 
   ; OLD and NEW both 123 
%undef OLD 
   ; OLD and NEW both undefined 
%define OLD 456 
   ; OLD and NEW both 456 
%undefalias OLD 
   ; OLD undefined, NEW defined to 456

4.2.12 Conditional Comma Operator: %,

As of version 2.15, NASM has a conditional comma operator %, that expands to a comma unless followed by a null expansion, which allows suppressing the comma before an empty argument. This is especially useful with greedy single-line macros.

For example, all the expressions below are valid:

%define greedy(a,b,c+) a + 66 %, b * 3 %, c 

       db greedy(1,2)          ; db 1 + 66, 2 * 3 
       db greedy(1,2,3)        ; db 1 + 66, 2 * 3, 3 
       db greedy(1,2,3,4)      ; db 1 + 66, 2 * 3, 3, 4 
       db greedy(1,2,3,4,5)    ; db 1 + 66, 2 * 3, 3, 4, 5

4.3 String Manipulation in Macros

It's often useful to be able to handle strings in macros. NASM supports a few simple string handling macro operators from which more complex operations can be constructed.

All the string operators define or redefine a value (either a string or a numeric value) to a single-line macro. When producing a string value, it may change the style of quoting of the input string or strings, and possibly use \–escapes inside `–quoted strings.

These directives are also available as preprocessor functions, see section 4.4.

4.3.1 Concatenating Strings: %strcat

The %strcat operator concatenates quoted strings and assign them to a single-line macro.

For example:

%strcat alpha "Alpha: ", '12" screen'

... would assign the value 'Alpha: 12" screen' to alpha. Similarly:

%strcat beta '"foo"\', "'bar'"

... would assign the value `"foo"\\'bar'` to beta.

The use of commas to separate strings is permitted but optional.

The corresponding preprocessor function is %strcat(), see section 4.4.11.

4.3.2 String Length: %strlen

The %strlen operator assigns the length of a string to a macro. For example:

%strlen charcnt 'my string'

In this example, charcnt would receive the value 9, just as if an %assign had been used. In this example, 'my string' was a literal string but it could also have been a single-line macro that expands to a string, as in the following example:

%define sometext 'my string' 
%strlen charcnt sometext

As in the first case, this would result in charcnt being assigned the value of 9.

The corresponding preprocessor function is %strlen(), see section 4.4.12.

4.3.3 Extracting Substrings: %substr

Individual letters or substrings in strings can be extracted using the %substr operator. An example of its use is probably more useful than the description:

%substr mychar 'xyzw' 1       ; equivalent to %define mychar 'x' 
%substr mychar 'xyzw' 2       ; equivalent to %define mychar 'y' 
%substr mychar 'xyzw' 3       ; equivalent to %define mychar 'z' 
%substr mychar 'xyzw' 2,2     ; equivalent to %define mychar 'yz' 
%substr mychar 'xyzw' 2,-1    ; equivalent to %define mychar 'yzw' 
%substr mychar 'xyzw' 2,-2    ; equivalent to %define mychar 'yz'

As with %strlen (see section 4.3.2), the first parameter is the single-line macro to be created and the second is the string. The third parameter specifies the first character to be selected, and the optional fourth parameter preceded by comma) is the length. Note that the first index is 1, not 0 and the last index is equal to the value that %strlen would assign given the same string. Index values out of range result in an empty string. A negative length means "until N-1 characters before the end of string", i.e. -1 means until end of string, -2 until one character before, etc.

The corresponding preprocessor function is %substr(), see section 4.4.13, however please note that the default value for the length parameter, if omitted, is -1 rather than 1 for %substr().

4.4 Preprocessor Functions

Preprocessor functions are, fundamentally, a kind of built-in single-line macros. They expand to a string depending on its arguments, and can be used in any context where single-line macro expansion would be performed. Preprocessor functions were introduced in NASM 2.16.

4.4.1 %abs() Function

The %abs() function evaluates its first argument as an expression, and then emits the absolute value. This will always be emitted as a single token containing a decimal number; no minus sign will be emitted even if the input value is the maximum negative number.

4.4.2 %cond() Function

The %cond() function evaluates its first argument as an expression, then expands to its second argument if true (nonzero), and the third, if present, if false (zero). This is in effect a specialized version of the %sel() function; %cond(x,y,z) is equivalent to %sel(2-!(x),y,z).

%define a 1 
%xdefine astr %cond(a,"true","false") ; %define astr "true"

The argument not selected is never expanded.

4.4.3 %count() Function

The %count() function expands to the number of argments passed to the macro. Note that just as for single-line macros, %count() treats an empty argument list as a single empty argument.

%xdefine empty %count()        ; %define empty 1 
%xdefine one   %count(1)       ; %define one 1 
%xdefine two   %count(5,q)     ; %define two 2 
%define  list  a,b,46 
%xdefine lc1   %count(list)    ; %define lc 1 (just one argument) 
%xdefine lc2   %count(%[list]) ; %define lc 3 (indirection expands)

4.4.4 %eval() Function

The %eval() function evaluates its argument as a numeric expression and expands to the result as an integer constant in much the same way the %assign directive would, see section 4.2.8. Unlike %assign, %eval() supports more than one argument; if more than one argument is specified, it is expanded to a comma-separated list of values.

%assign a    2 
%assign b    3 
%defstr what %eval(a+b,a*b)  ; equivalent to %define what "5,6"

The expressions passed to %eval() are critical expressions, see section 3.8.

4.4.5 %hex() Function

Equivalent to %eval(), except that the results generated are given as unsigned hexadecimal, with a 0x prefix.

4.4.6 %is() Family Functions

Each %if family directive (see section 4.6) has an equivalent %is() family function, that expands to 1 if the equivalent %if directive would process as true, and 0 if the equivalent %if directive would process as false.

; Instead of !%isidn() could have used %isnidn() 
%if %isdef(foo) && !%isidn(foo,bar) 
      db "foo is defined, but not as 'bar'" 

Note that, being functions, the arguments (before expansion) will always need to have balanced parentheses so that the end of the argument list can be defined. This means that the syntax of e.g. %istoken() and %isidn() is somewhat stricter than their corresponding %if directives; it may be necessary to escape the argument to the conditional using {}:

; Instead of !%isidn() could have used %isnidn() 
%if %isdef(foo) && !%isidn({foo,)}) 
      db "foo is defined, but not as ')'" 

4.4.7 %map() Function

The %map() function takes as its first parameter the name of a single-line macro, followed by up to two optional colon-separated subparameters:

Further parameters, if any, are then passed as additional parameters to the given macro for expansion, in sets given by the specified group size, and the results turned into a comma-separated list. If no additional parameters are given, %map() expands to nothing.

For example:

%define alpha(&x)     x 
%define alpha(&x,y)   y dup (x) 
%define alpha(s,&x,y) y dup (x,s) 
; 0 fixed + 1 grouped parameters per call, calls alpha(&x) 
      db %map(alpha,foo,bar,baz,quux) 
; 0 fixed + 2 grouped parameters per call, calls alpha(&x,y) 
      db %map(alpha::2,foo,bar,baz,quux) 
; 1 fixed + 2 grouped parameters per call, calls alpha(s,&x,y) 
      db %map(alpha:("!"):2,foo,bar,baz,quux)

... expands to:

      db 'foo','bar','baz','quux' 
      db bar dup ('foo'),quux dup ('baz') 
      db bar dup ('foo',"!"),quux dup ('baz',"!")

As a more complex example, a macro that joins quoted strings together with a user-specified delimiter string:

%define join(sep)        ''      ; handle the case of zero strings 
%define _join(sep,str)   sep,str ; helper macro 
%define join(sep,s1,sn+) %strcat(s1, %map(_join:(sep) %, sn)) 

      db join(':') 
      db join(':','a') 
      db join(':','a','b') 
      db join(':','a','b','c') 
      db join(':','a','b','c','d')

... expands to:

      db '' 
      db 'a' 
      db 'a:b' 
      db 'a:b:c' 
      db 'a:b:c:d'

4.4.8 %num() Function

The %num() function evaluates its arguments as expressions, and then produces a quoted string encoding the first argument as an unsigned 64-bit integer.

The second argument is the desired number of digits (max 255, default –1).

The third argument is the encoding base (from 2 to 64, default 10); if the base is given as –2, –8, –10, or –16, then 0b, 0q, 0d or 0x is prepended, respectively; all other negative values are disallowed.

Only the first argument is required.

If the number of digits is negative, NASM will add additional digits if needed; if positive the string is truncated to the number of digits specified. 0 is treated as –1, except that the input number 0 always generates an empty string (thus, the first digit will never be zero), even if the base given is negative.

The full 64-symbol set used is, in order:


If a signed number needs to be converted to a string, use %abs(), %cond(), and %strcat() to format the signed number string to your specific output requirements.

4.4.9 %sel() Function

The %sel() function evaluates its first argument as an expression, then expands to its second argument if 1, the third argument if 2, and so on. If the value is less than 1 or larger than the number of arguments minus one, then the %sel() function expands to nothing.

%define b 2 
%xdefine bstr %sel(b,"one","two","three") ; %define bstr "two"

The arguments not selected are never expanded.

4.4.10 %str() Function

The %str() function converts its argument, including any commas, to a quoted string, similar to the way the %defstr directive would, see section 4.2.9.

Being a function, the argument will need to have balanced parentheses or be escaped using {}.

; The following lines are all equivalent 
%define  test 'TEST' 
%defstr  test TEST 
%xdefine test %str(TEST)

4.4.11 %strcat() Function

The %strcat() function concatenates a list of quoted strings, in the same way the %strcat directive would, see section 4.3.1.

; The following lines are all equivalent 
%define  alpha 'Alpha: 12" screen' 
%strcat  alpha "Alpha: ", '12" screen' 
%xdefine alpha %strcat("Alpha: ", '12" screen')

4.4.12 %strlen() Function

The %strlen() function expands to the length of a quoted string, in the same way the %strlen directive would, see section 4.3.2.

; The following lines are all equivalent 
%define  charcnt 9 
%strlen  charcnt 'my string' 
%xdefine charcnt %strlen('my string')

4.4.13 %substr() Function

The %substr() function extracts a substring of a quoted string, in the same way the %substr directive would, see section 4.3.3. Note that unlike the %substr directive, commas are required between all parameters, is required after the string argument, and that the default for the length argument, if omitted, is -1 (i.e. the remainder of the string) rather than 1.

; The following lines are all equivalent 
%define  mychar 'yzw' 
%substr  mychar 'xyzw' 2,-1 
%xdefine mychar %substr('xyzw',2,3) 
%xdefine mychar %substr('xyzw',2,-1) 
%xdefine mychar %substr('xyzw',2)

4.4.14 %tok() function

The %tok() function converts a quoted string into a sequence of tokens, in the same way the %deftok directive would, see section 4.2.10.

; The following lines are all equivalent 
%define test TEST 
%deftok test 'TEST' 
%define test %tok('TEST')

4.5 Multi-Line Macros: %macro

Multi-line macros much like the type of macro seen in MASM and TASM, and expand to a new set of lines of source code. 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

The behavior with regards to empty arguments at the end of multi-line macros before NASM 2.15 was often very strange. For backwards compatibility, NASM attempts to recognize cases where the legacy behavior would give unexpected results, and issues a warning, but largely tries to match the legacy behavior. This can be disabled with the %pragma (see section 4.12.1):

%pragma preproc sane_empty_expansion

4.5.1 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 2.1.26).

4.5.2 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 3.9. 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.

These labels are really macro-local tokens, and can be used for other purposes where a token unique to each macro invocation is desired, e.g. to name single-line macros without using the context feature (section 4.9.2).

4.5.3 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 
        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 7.3.1 for a better way to write the above macro.

4.5.4 Macro Parameters Range

NASM allows you to expand parameters via special construction %{x:y} where x is the first parameter index and y is the last. Any index can be either negative or positive but must never be zero.

For example

%macro mpar 1-* 
     db %{3:5} 

mpar 1,2,3,4,5,6

expands to 3,4,5 range.

Even more, the parameters can be reversed so that

%macro mpar 1-* 
     db %{5:3} 

mpar 1,2,3,4,5,6

expands to 5,4,3 range.

But even this is not the last. The parameters can be addressed via negative indices so NASM will count them reversed. The ones who know Python may see the analogue here.

%macro mpar 1-* 
     db %{-1:-3} 

mpar 1,2,3,4,5,6

expands to 6,5,4 range.

Note that NASM uses comma to separate parameters being expanded.

By the way, here is a trick – you might use the index %{-1:-1} which gives you the last argument passed to a macro.

4.5.5 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 4.5.3) 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 can provide extra information to a macro by providing too many default parameters:

%macro quux 1 something

This will trigger a warning by default; see section 2.1.26 for more information. When quux is invoked, it receives not one but two parameters. something can be referred to as %2. The difference between passing something this way and writing something in the macro body is that with this way something is evaluated when the macro is defined, not when it is expanded.

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 4.5.6) 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 4.5.8.

4.5.6 %0: Macro Parameter Counter

The parameter reference %0 will return a numeric constant giving the number of parameters received, that is, if %0 is n then %n is the last parameter. %0 is mostly useful for macros that can take a variable number of parameters. It can be used as an argument to %rep (see section 4.7) in order to iterate through all the parameters of a macro. Examples are given in section 4.5.8.

4.5.7 %00: Label Preceding Macro

%00 will return the label preceding the macro invocation, if any. The label must be on the same line as the macro invocation, may be a local label (see section 3.9), and need not end in a colon.

If %00 is present anywhere in the macro body, the label itself will not be emitted by NASM. You can, of course, put %00: explicitly at the beginning of your macro.

4.5.8 %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.

4.5.9 Concatenating Macro Parameters

NASM can concatenate macro parameters and macro indirection constructs 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 4.5.2) and context-local labels (section 4.9.2). 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.)

The single-line macro indirection construct, %[...] (section 4.2.3), behaves the same way as macro parameters for the purpose of concatenation.

See also the %+ operator, section 4.2.4.

4.5.10 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 4.5.2 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.

4.5.11 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

4.5.12 Undefining Multi-Line Macros: %unmacro

Multi-line macros can be removed with the %unmacro directive. Unlike the %undef directive, however, %unmacro takes an argument specification, and will only remove exact matches with that argument specification.

For example:

%macro foo 1-3 
        ; Do something 
%unmacro foo 1-3

removes the previously defined macro foo, but

%macro bar 1-3 
        ; Do something 
%unmacro bar 1

does not remove the macro bar, since the argument specification does not match exactly.

A case-insensitive macro needs to be removed with the %unimacro directive.

4.6 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 inverse forms %ifn and %elifn are also supported.

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

There are a number of variants of the %if directive. Each has its corresponding %elif, %ifn, and %elifn directives; for example, the equivalents to the %ifdef directive are %elifdef, %ifndef, and %elifndef.

4.6.1 %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.

4.6.2 %ifmacro: Testing Multi-Line Macro Existence

The %ifmacro directive operates in the same way as the %ifdef directive, except that it checks for the existence of a multi-line macro.

For example, you may be working with a large project and not have control over the macros in a library. You may want to create a macro with one name if it doesn't already exist, and another name if one with that name does exist.

The %ifmacro is considered true if defining a macro with the given name and number of arguments would cause a definitions conflict. For example:

%ifmacro MyMacro 1-3 

     %error "MyMacro 1-3" causes a conflict with an existing macro. 


     %macro MyMacro 1-3 

             ; insert code to define the macro 



This will create the macro "MyMacro 1-3" if no macro already exists which would conflict with it, and emits a warning if there would be a definition conflict.

You can test for the macro not existing by using the %ifnmacro instead of %ifmacro. Additional tests can be performed in %elif blocks by using %elifmacro and %elifnmacro.

4.6.3 %ifctx: Testing the Context Stack

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

For more details of the context stack, see section 4.9. For a sample use of %ifctx, see section 4.9.6.

4.6.4 %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 4.7 for a detailed example.

The expression given to %if, and its counterpart %elif, is a critical expression (see section 3.8).

Like other %if constructs, %if has a counterpart %elif, and negative forms %ifn and %elifn.

4.6.5 %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 other %if constructs, %ifidn has a counterpart %elifidn, and negative forms %ifnidn and %elifnidn. Similarly, %ifidni has counterparts %elifidni, %ifnidni and %elifnidni.

4.6.6 %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. $ and $$ are not considered identifiers by %ifid.

%ifnum works similarly, but tests for the token being an integer numeric constant (not an expression!) possibly preceded by + or -; %ifstr tests for it being a quoted string.

For example, the writefile macro defined in section 4.5.3 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 %elif..., %ifn..., and %elifn... versions exist for each of %ifid, %ifnum and %ifstr.

4.6.7 %iftoken: Test for a Single Token

Some macros will want to do different things depending on if it is passed a single token (e.g. paste it to something else using %+) versus a multi-token sequence.

The conditional assembly construct %iftoken assembles the subsequent code if and only if the expanded parameters consist of exactly one token, possibly surrounded by whitespace.

For example:

%iftoken 1

will assemble the subsequent code, but

%iftoken -1

will not, since -1 contains two tokens: the unary minus operator -, and the number 1.

The usual %eliftoken, %ifntoken, and %elifntoken variants are also provided.

4.6.8 %ifempty: Test for Empty Expansion

The conditional assembly construct %ifempty assembles the subsequent code if and only if the expanded parameters do not contain any tokens at all, whitespace excepted.

The usual %elifempty, %ifnempty, and %elifnempty variants are also provided.

4.6.9 %ifenv: Test If Environment Variable Exists

The conditional assembly construct %ifenv assembles the subsequent code if and only if the environment variable referenced by the %!variable directive exists.

The usual %elifenv, %ifnenv, and %elifnenv variants are also provided.

Just as for %!variable the argument should be written as a string if it contains characters that would not be legal in an identifier. See section 4.13.2.

4.7 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.

Note the maximum repeat count is limited to the value specified by the --limit-rep option or %pragma limit rep, see section 2.1.31.

4.8 Source Files and Dependencies

These commands allow you to split your sources into multiple files.

4.8.1 %include: 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 2.1.19).

4.8.2 %pathsearch: Search the Include Path

The %pathsearch directive takes a single-line macro name and a filename, and declare or redefines the specified single-line macro to be the include-path-resolved version of the filename, if the file exists (otherwise, it is passed unchanged.)

For example,

%pathsearch MyFoo "foo.bin"

... with -Ibins/ in the include path may end up defining the macro MyFoo to be "bins/foo.bin".

4.8.3 %depend: Add Dependent Files

The %depend directive takes a filename and adds it to the list of files to be emitted as dependency generation when the -M options and its relatives (see section 2.1.5) are used. It produces no output.

This is generally used in conjunction with %pathsearch. For example, a simplified version of the standard macro wrapper for the INCBIN directive looks like:

%imacro incbin 1-2+ 0 
%pathsearch dep %1 
%depend dep 
        incbin dep,%2 

This first resolves the location of the file into the macro dep, then adds it to the dependency lists, and finally issues the assembler-level INCBIN directive.

4.8.4 %use: Include Standard Macro Package

The %use directive is similar to %include, but rather than including the contents of a file, it includes a named standard macro package. The standard macro packages are part of NASM, and are described in chapter 6.

Unlike the %include directive, package names for the %use directive do not require quotes, but quotes are permitted. In NASM 2.04 and 2.05 the unquoted form would be macro-expanded; this is no longer true. Thus, the following lines are equivalent:

%use altreg 
%use 'altreg'

Standard macro packages are protected from multiple inclusion. When a standard macro package is used, a testable single-line macro of the form __?USE_package?__ is also defined, see section 5.7.

4.9 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 characterized 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.

4.9.1 %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 takes an optional 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. If no name is given, the context is unnamed (this is normally used when both the %push and the %pop are inside a single macro definition.)

The directive %pop, taking one optional argument, removes the top context from the context stack and destroys it, along with any labels associated with it. If an argument is given, it must match the name of the current context, otherwise it will issue an error.

4.9.2 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.

4.9.3 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.

4.9.4 Context Fall-Through Lookup (deprecated)

Context fall-through lookup (automatic searching of outer contexts) is a feature that was added in NASM version 0.98.03. Unfortunately, this feature is unintuitive and can result in buggy code that would have otherwise been prevented by NASM's error reporting. As a result, this feature has been deprecated. NASM version 2.09 will issue a warning when usage of this deprecated feature is detected. Starting with NASM version 2.10, usage of this deprecated feature will simply result in an expression syntax error.

An example usage of this deprecated feature follows:

%macro ctxthru 0 
%push ctx1 
    %assign %$external 1 
        %push ctx2 
            %assign %$internal 1 
            mov eax, %$external 
            mov eax, %$internal 

As demonstrated, %$external is being defined in the ctx1 context and referenced within the ctx2 context. With context fall-through lookup, referencing an undefined context-local macro like this implicitly searches through all outer contexts until a match is made or isn't found in any context. As a result, %$external referenced within the ctx2 context would implicitly use %$external as defined in ctx1. Most people would expect NASM to issue an error in this situation because %$external was never defined within ctx2 and also isn't qualified with the proper context depth, %$$external.

Here is a revision of the above example with proper context depth:

%macro ctxthru 0 
%push ctx1 
    %assign %$external 1 
        %push ctx2 
            %assign %$internal 1 
            mov eax, %$$external 
            mov eax, %$internal 

As demonstrated, %$external is still being defined in the ctx1 context and referenced within the ctx2 context. However, the reference to %$external within ctx2 has been fully qualified with the proper context depth, %$$external, and thus is no longer ambiguous, unintuitive or erroneous.

4.9.5 %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.

4.9.6 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 4.9.2, 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.

4.10 Stack Relative Preprocessor Directives

The following preprocessor directives provide a way to use labels to refer to local variables allocated on the stack.

4.10.1 %arg Directive

The %arg directive is used to simplify the handling of parameters passed on the stack. Stack based parameter passing is used by many high level languages, including C, C++ and Pascal.

While NASM has macros which attempt to duplicate this functionality (see section 9.4.5), the syntax is not particularly convenient to use and is not TASM compatible. Here is an example which shows the use of %arg without any external macros:


    %push     mycontext        ; save the current context 
    %stacksize large           ; tell NASM to use bp 
    %arg      i:word, j_ptr:word 

        mov     ax,[i] 
        mov     bx,[j_ptr] 
        add     ax,[bx] 

    %pop                       ; restore original context

This is similar to the procedure defined in section 9.4.5 and adds the value in i to the value pointed to by j_ptr and returns the sum in the ax register. See section 4.9.1 for an explanation of push and pop and the use of context stacks.

4.10.2 %stacksize Directive

The %stacksize directive is used in conjunction with the %arg (see section 4.10.1) and the %local (see section 4.10.3) directives. It tells NASM the default size to use for subsequent %arg and %local directives. The %stacksize directive takes one required argument which is one of flat, flat64, large or small.

%stacksize flat

This form causes NASM to use stack-based parameter addressing relative to ebp and it assumes that a near form of call was used to get to this label (i.e. that eip is on the stack).

%stacksize flat64

This form causes NASM to use stack-based parameter addressing relative to rbp and it assumes that a near form of call was used to get to this label (i.e. that rip is on the stack).

%stacksize large

This form uses bp to do stack-based parameter addressing and assumes that a far form of call was used to get to this address (i.e. that ip and cs are on the stack).

%stacksize small

This form also uses bp to address stack parameters, but it is different from large because it also assumes that the old value of bp is pushed onto the stack (i.e. it expects an ENTER instruction). In other words, it expects that bp, ip and cs are on the top of the stack, underneath any local space which may have been allocated by ENTER. This form is probably most useful when used in combination with the %local directive (see section 4.10.3).

4.10.3 %local Directive

The %local directive is used to simplify the use of local temporary stack variables allocated in a stack frame. Automatic local variables in C are an example of this kind of variable. The %local directive is most useful when used with the %stacksize (see section 4.10.2 and is also compatible with the %arg directive (see section 4.10.1). It allows simplified reference to variables on the stack which have been allocated typically by using the ENTER instruction. An example of its use is the following:


    %push mycontext             ; save the current context 
    %stacksize small            ; tell NASM to use bp 
    %assign %$localsize 0       ; see text for explanation 
    %local old_ax:word, old_dx:word 

        enter   %$localsize,0   ; see text for explanation 
        mov     [old_ax],ax     ; swap ax & bx 
        mov     [old_dx],dx     ; and swap dx & cx 
        mov     ax,bx 
        mov     dx,cx 
        mov     bx,[old_ax] 
        mov     cx,[old_dx] 
        leave                   ; restore old bp 
        ret                     ; 

    %pop                        ; restore original context

The %$localsize variable is used internally by the %local directive and must be defined within the current context before the %local directive may be used. Failure to do so will result in one expression syntax error for each %local variable declared. It then may be used in the construction of an appropriately sized ENTER instruction as shown in the example.

4.11 Reporting User-Defined Errors: %error, %warning, %fatal

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 F1 
    ; do some setup 
%elifdef F2 
    ; do some different setup 
    %error "Neither F1 nor F2 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.

Similarly, %warning issues a warning, but allows assembly to continue:

%ifdef F1 
    ; do some setup 
%elifdef F2 
    ; do some different setup 
    %warning "Neither F1 nor F2 was defined, assuming F1." 
    %define F1 

%error and %warning are issued only on the final assembly pass. This makes them safe to use in conjunction with tests that depend on symbol values.

%fatal terminates assembly immediately, regardless of pass. This is useful when there is no point in continuing the assembly further, and doing so is likely just going to cause a spew of confusing error messages.

It is optional for the message string after %error, %warning or %fatal to be quoted. If it is not, then single-line macros are expanded in it, which can be used to display more information to the user. For example:

%if foo > 64 
    %assign foo_over foo-64 
    %error foo is foo_over bytes too large 

4.12 %pragma: Setting Options

The %pragma directive controls a number of options in NASM. Pragmas are intended to remain backwards compatible, and therefore an unknown %pragma directive is not an error.

The various pragmas are documented with the options they affect.

The general structure of a NASM pragma is:

%pragma namespace directive [arguments...]

Currently defined namespaces are:

In addition, the name of any output or debug format, and sometimes groups thereof, also constitute %pragma namespaces. The namespaces output and debug simply refer to any output or debug format, respectively.

For example, to prepend an underscore to global symbols regardless of the output format (see section 7.10):

%pragma output gprefix _

... whereas to prepend an underscore to global symbols only when the output is either win32 or win64:

%pragma win gprefix _

4.12.1 Preprocessor Pragmas

The only preprocessor %pragma defined in NASM 2.15 is:

4.13 Other Preprocessor Directives

4.13.1 %line Directive

The %line directive is used to notify NASM that the input line corresponds to a specific line number in another file. Typically this other file would be an original source file, with the current NASM input being the output of a pre-processor. The %line directive allows NASM to output messages which indicate the line number of the original source file, instead of the file that is being read by NASM.

This preprocessor directive is not generally used directly by programmers, but may be of interest to preprocessor authors. The usage of the %line preprocessor directive is as follows:

%line nnn[+mmm] [filename]

In this directive, nnn identifies the line of the original source file which this line corresponds to. mmm is an optional parameter which specifies a line increment value; each line of the input file read in is considered to correspond to mmm lines of the original source file. Finally, filename is an optional parameter which specifies the file name of the original source file. It may be a quoted string, in which case any additional argument after the quoted string will be ignored.

After reading a %line preprocessor directive, NASM will report all file name and line numbers relative to the values specified therein.

If the command line option --no-line is given, all %line directives are ignored. This may be useful for debugging preprocessed code. See section 2.1.33.

Starting in NASM 2.15, %line directives are processed before any other processing takes place.

For compatibility with the output from some other preprocessors, including many C preprocessors, a # character followed by whitespace at the very beginning of a line is also treated as a %line directive, except that double quotes surrounding the filename are treated like NASM backquotes, with \–escaped sequences decoded.

4.13.2 %!variable: Read an Environment Variable.

The %!variable directive makes it possible to read the value of an environment variable at assembly time. This could, for example, be used to store the contents of an environment variable into a string, which could be used at some other point in your code.

For example, suppose that you have an environment variable FOO, and you want the contents of FOO to be embedded in your program as a quoted string. You could do that as follows:

%defstr FOO          %!FOO

See section 4.2.9 for notes on the %defstr directive.

If the name of the environment variable contains non-identifier characters, you can use string quotes to surround the name of the variable, for example:

%defstr C_colon      %!'C:'

4.13.3 %clear: Clear All Macro Definitions

The directive %clear clears all definitions of a certain type, including the ones defined by NASM itself. This can be useful when preprocessing non-NASM code, or to drop backwards compatibility aliases.

The syntax is:

   %clear [global|context] type...

... where context indicates that this applies to context-local macros only; the default is global.

type can be one or more of:

In NASM 2.14 and earlier, only the single syntax %clear was supported, which is equivalent to %clear global all.