# NASM - The Netwide Assembler

version 2.16.03

## Chapter 5: 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 but context-local preprocessor variables and single-line macros, see section 4.13.3.

Most user-level directives (see chapter 7) are implemented as macros which invoke primitive directives; these are described in chapter 7. The rest of the standard macro set is described here.

For compatibility with NASM versions before NASM 2.15, most standard macros of the form `__?foo?__` have aliases of form `__foo__` (see section 4.2.11). These can be removed with the directive `%clear defalias`.

### 5.1 NASM Version Macros

The single-line macros `__?NASM_MAJOR?__`, `__?NASM_MINOR?__`, `__?NASM_SUBMINOR?__` and `__?NASM_PATCHLEVEL?__` expand to the major, minor, subminor and patch level parts of the version number of NASM being used. So, under NASM 0.98.32p1 for example, `__?NASM_MAJOR?__` would be defined to be 0, `__?NASM_MINOR?__` would be defined as 98, `__?NASM_SUBMINOR?__` would be defined to 32, and `__?NASM_PATCHLEVEL?__` would be defined as 1.

Additionally, the macro `__?NASM_SNAPSHOT?__` is defined for automatically generated snapshot releases only.

#### 5.1.1 `__?NASM_VERSION_ID?__`: NASM Version ID

The single-line macro `__?NASM_VERSION_ID?__` expands to a dword integer representing the full version number of the version of nasm being used. The value is the equivalent to `__?NASM_MAJOR?__`, `__?NASM_MINOR?__`, `__?NASM_SUBMINOR?__` and `__?NASM_PATCHLEVEL?__` concatenated to produce a single doubleword. Hence, for 0.98.32p1, the returned number would be equivalent to:

            dd      0x00622001

or

            db      1,32,98,0

Note that the above lines are generate exactly the same code, the second line is used just to give an indication of the order that the separate values will be present in memory.

#### 5.1.2 `__?NASM_VER?__`: NASM Version String

The single-line macro `__?NASM_VER?__` expands to a string which defines the version number of nasm being used. So, under NASM 0.98.32 for example,

            db      __?NASM_VER?__

would expand to

            db      "0.98.32"

### 5.2 `__?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.

### 5.3 `__?BITS?__`: Current Code Generation Mode

The `__?BITS?__` standard macro is updated every time that the BITS mode is set using the `BITS XX` or `[BITS XX]` directive, where XX is a valid mode number of 16, 32 or 64. `__?BITS?__` receives the specified mode number and makes it globally available. This can be very useful for those who utilize mode-dependent macros.

### 5.4 `__?OUTPUT_FORMAT?__`: Current Output Format

The `__?OUTPUT_FORMAT?__` standard macro holds the current output format name, as given by the `-f` option or NASM's default. Type `nasm -h` for a list.

    %ifidn __?OUTPUT_FORMAT?__, win32
     %define NEWLINE 13, 10
    %elifidn __?OUTPUT_FORMAT?__, elf32
     %define NEWLINE 10
    %endif

### 5.5 `__?DEBUG_FORMAT?__`: Current Debug Format

If debugging information generation is enabled, The `__?DEBUG_FORMAT?__` standard macro holds the current debug format name as specified by the `-F` or `-g` option or the output format default. Type `nasm -f` output `y` for a list.

`__?DEBUG_FORMAT?__` is not defined if debugging is not enabled, or if the debug format specified is `null`.

### 5.6 Assembly Date and Time Macros

NASM provides a variety of macros that represent the timestamp of the assembly session.

- The `__?DATE?__` and `__?TIME?__` macros give the assembly date and time as strings, in ISO 8601 format (`"YYYY-MM-DD"` and `"HH:MM:SS"`, respectively.)

- The `__?DATE_NUM?__` and `__?TIME_NUM?__` macros give the assembly date and time in numeric form; in the format `YYYYMMDD` and `HHMMSS` respectively.

- The `__?UTC_DATE?__` and `__?UTC_TIME?__` macros give the assembly date and time in universal time (UTC) as strings, in ISO 8601 format (`"YYYY-MM-DD"` and `"HH:MM:SS"`, respectively.) If the host platform doesn't provide UTC time, these macros are undefined.

- The `__?UTC_DATE_NUM?__` and `__?UTC_TIME_NUM?__` macros give the assembly date and time universal time (UTC) in numeric form; in the format `YYYYMMDD` and `HHMMSS` respectively. If the host platform doesn't provide UTC time, these macros are undefined.

- The `__?POSIX_TIME?__` macro is defined as a number containing the number of seconds since the POSIX epoch, 1 January 1970 00:00:00 UTC; excluding any leap seconds. This is computed using UTC time if available on the host platform, otherwise it is computed using the local time as if it was UTC.

All instances of time and date macros in the same assembly session produce consistent output. For example, in an assembly session started at 42 seconds after midnight on January 1, 2010 in Moscow (timezone UTC+3) these macros would have the following values, assuming, of course, a properly configured environment with a correct clock:

          __?DATE?__             "2010-01-01"
          __?TIME?__             "00:00:42"
          __?DATE_NUM?__         20100101
          __?TIME_NUM?__         000042
          __?UTC_DATE?__         "2009-12-31"
          __?UTC_TIME?__         "21:00:42"
          __?UTC_DATE_NUM?__     20091231
          __?UTC_TIME_NUM?__     210042
          __?POSIX_TIME?__       1262293242

### 5.7 `__?USE_`package`?__`: Package Include Test

When a standard macro package (see chapter 6) is included with the `%use` directive (see section 4.8.4), a single-line macro of the form `__?USE_`package`?__` is automatically defined. This allows testing if a particular package is invoked or not.

For example, if the `altreg` package is included (see section 6.1), then the macro `__?USE_ALTREG?__` is defined.

### 5.8 `__?PASS?__`: Assembly Pass

The macro `__?PASS?__` is defined to be `1` on preparatory passes, and `2` on the final pass. In preprocess-only mode, it is set to `3`, and when running only to generate dependencies (due to the `-M` or `-MG` option, see section 2.1.5) it is set to `0`.

Avoid using this macro if at all possible. It is tremendously easy to generate very strange errors by misusing it, and the semantics may change in future versions of NASM.

### 5.9 Structure Data Types

#### 5.9.1 `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 or two parameters. The first parameter is the name of the data type. The second, optional parameter is the base offset of the structure. The name of the data type is defined as a symbol with the value of the base offset, and the name of the data type with the suffix `_size` appended to it is 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 by default 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]`.

Sometimes you only have the address of the structure displaced by an offset. For example, consider this standard stack frame setup:

    push ebp
    mov ebp, esp
    sub esp, 40

In this case, you could access an element by subtracting the offset:

    mov [ebp - 40 + mytype.word], ax

However, if you do not want to repeat this offset, you can use –40 as a base offset:

    struc mytype, -40

And access an element this way:

    mov [ebp + mytype.word], ax

#### 5.9.2 `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

### 5.10 Alignment Control

#### 5.10.1 `ALIGN` and `ALIGNB`: Code and 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`.

Both `ALIGN` and `ALIGNB` do call `SECTALIGN` macro implicitly. See section 5.10.2 for details.

See also the `smartalign` standard macro package, section 6.2.

#### 5.10.2 `SECTALIGN`: Section Alignment

The `SECTALIGN` macros provides a way to modify alignment attribute of output file section. Unlike the `align=` attribute (which is allowed at section definition only) the `SECTALIGN` macro may be used at any time.

For example the directive

    SECTALIGN 16

sets the section alignment requirements to 16 bytes. Once increased it can not be decreased, the magnitude may grow only.

Note that `ALIGN` (see section 5.10.1) calls the `SECTALIGN` macro implicitly so the active section alignment requirements may be updated. This is by default behaviour, if for some reason you want the `ALIGN` do not call `SECTALIGN` at all use the directive

    SECTALIGN OFF

It is still possible to turn in on again by

    SECTALIGN ON

Note that `SECTALIGN <ON|OFF>` affects only the `ALIGN`/`ALIGNB` directives, not an explicit `SECTALIGN` directive.