# 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.

You can pre-define single-line macros using the `-d' option on the NASM command line: see section 2.1.7.

#### 4.1.2 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.

`%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
    %endmacro

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
    %endmacro
              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

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
    %endmacro

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
    %endmacro

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 2.1.10).

#### 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
              ret
    %%skip:
    %endmacro

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
    %endmacro

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
    %endmacro

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
    %endrep
    %endmacro

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
    %endrep
    %endmacro

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
    %endmacro
    keytab:
              keytab_entry F1,128+1
              keytab_entry F2,128+2
              keytab_entry Return,13

which would expand to

    keytab:
    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
              ret
    %%skip:
    %endmacro

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:

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

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
    %endif
              ; 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
    %%label:
    %else
              push %1
    %endif
    %endmacro

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
    %else
    %%str:	  db %2
    %endif
    %%endstr: mov dx,%%str
              mov cx,%%endstr-%%str
    %else
    	  mov dx,%2
    	  mov cx,%3
    %endif
              mov bx,%1
              mov ah,0x40
              int 0x21
    %endmacro

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
    %elifdef SOME_OTHER_MACRO
    ; do some different setup
    %else
    %error Neither SOME_MACRO nor SOME_OTHER_MACRO was defined.
    %endif

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
    %endrep

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:

    fibonacci:
    %assign i 0
    %assign j 1
    %rep 100
    %if j > 65535
    %exitrep
    %endif
              dw j
    %assign k j+i
    %assign i j
    %assign j k
    %endrep
    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
    %endif

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 2.1.6).

### 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
    %$begin:
    %endmacro

    %macro until 1
              j%-1 %$begin
    %pop
    %endmacro

and invoked by means of, for example,

              mov cx,string
              repeat
              add cx,3
              scasb
              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

    %pop
    %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
    %endmacro

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

    %macro endif 0
        %ifctx if
            %$ifnot:
            %pop
        %elifctx else
            %$ifend:
            %pop
        %else
            %error "expected `if' or `else' before `endif'"
        %endif
    %endmacro

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

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
    %endmacro

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
              endstruc

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
              endstruc

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
              iend

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
              endstruc

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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