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.
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.
__?NASM_VERSION_ID?__: NASM Version IDThe 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.
__?NASM_VER?__: NASM Version StringThe 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"
__?FILE?__ and __?LINE?__: File Name and Line NumberLike 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.
__?BITS?__: Current Code Generation ModeThe __?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.
__?OUTPUT_FORMAT?__: Current Output FormatThe __?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
__?DEBUG_FORMAT?__: Current Debug FormatIf 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.
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
__?USE_package?__: Package Include TestWhen 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.
__?PASS?__: Assembly PassThe 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.
STRUC and ENDSTRUC: Declaring Structure Data TypesThe 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
ISTRUC, AT and IEND: Declaring Instances of StructuresHaving 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
ALIGN and ALIGNB: Code and Data AlignmentThe 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.
SECTALIGN: Section AlignmentThe 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.