:man source: NASM
:man manual: The Netwide Assembler Project
nasm - the Netwide Assembler, a portable 80x86 assembler
*nasm* [*-@* response file] [*-f* format] [*-o* outfile] [*-l* listfile] ['options'...] filename
The *nasm* command assembles the file 'filename' and directs output to the file
'outfile' if specified. If 'outfile' is not specified, *nasm* will derive a default
output file name from the name of its input file, usually by appending `.o' or
`.obj', or by removing all extensions for a raw binary file. Failing that, the
output file name will be `nasm.out'.
Causes *nasm* to process options from filename as if they were included on
the command line.
Causes *nasm* to assemble the given input file without first applying the
Pre-defines a single-line macro.
Causes *nasm* to preprocess the given input file, and write the output to
'stdout' (or the specified output file name), and not actually assemble
Specifies the output file format. To see a list of valid output formats,
use the *-hf* option.
Specifies the debug information format. To see a list of valid output
formats, use the *-y* option (for example *-felf -y*).
Causes *nasm* to generate debug information.
Equivalent to **-g -F**__ format__.
Causes *nasm* to exit immediately, after giving a summary of its
Same as *-h* , but also lists all valid output formats.
Adds a directory to the search path for include files. The directory
specification must include the trailing slash, as it will be directly
prepended to the name of the include file.
Causes an assembly listing to be directed to the given file, in which
the original source is displayed on the right hand side (plus the source
for included files and the expansions of multi-line macros) and the
generated code is shown in hex on the left.
Causes *nasm* to output Makefile-style dependencies to stdout; normal
output is suppressed.
Same as *-M* but assumes that missing Makefile dependecies are generated
and added to dependency list without a prefix.
Output Makefile-style dependencies to the specified file.
Same as a combination of *-M* and *-MF* options.
Override the default name of the dependency target dependency target name.
This is normally the same as the output filename, specified by
the *-o* option.
The same as *-MT* except it tries to quote characters that have special
meaning in Makefile syntax. This is not foolproof, as not all characters
with special meaning are quotable in Make.
Emit phony target.
Optimize branch offsets.
* *-O0*: No optimization
* *-O1*: Minimal optimization
* *-Ox*: Multipass optimization (default)
Specifies a precise name for the output file, overriding *nasm*'s default
means of determining it.
Specifies a file to be pre-included, before the main source file
starts to be processed.
Causes *nasm* to send its error messages and/or help text to stdout
instead of stderr.
Causes *nasm* to assemble in SciTech TASM compatible mode.
Undefines a single-line macro.
Causes *nasm* to exit immediately, after displaying its version number.
Causes *nasm* to enable or disable certain classes of warning messages,
in gcc-like style, for example *-Wlabel-orphan* or *-Wno-orphan-labels*.
Causes *nasm* to enable or disable certain classes of warning messages,
for example *-w+label-orphan* or *-w-macro-params*.
Specifies error reporting format (gnu or vc).
Causes *nasm* to list supported debug formats.
Causes *nasm* to redirect error messages to 'filename'. This option exists
to support operating systems on which stderr is not easily redirected.
Prepend or append (respectively) the given argument to all global or
This man page does not fully describe the syntax of *nasm*'s assembly language,
but does give a summary of the differences from other assemblers.
'Registers' have no leading `%' sign, unlike *gas*, and floating-point stack
registers are referred to as 'st0', 'st1', and so on.
'Floating-point instructions' may use either the single-operand form or the
double. A 'TO' keyword is provided; thus, one could either write
or one could use the alternative single-operand forms
fadd to st1
'Uninitialised storage' is reserved using the 'RESB', 'RESW', 'RESD', 'RESQ',
'REST' and 'RESO' pseudo-opcodes, each taking one parameter which gives the
number of bytes, words, doublewords, quadwords or ten-byte words to reserve.
'Repetition' of data items is not done by the 'DUP' keyword as seen in DOS
assemblers, but by the use of the 'TIMES' prefix, like this:
message: times 3 db 'abc'
times 64-$+message db 0
which defines the string `abcabcabc`, followed by the right number of zero
bytes to make the total length up to 64 bytes.
'Symbol references' are always understood to be immediate (i.e. the address
of the symbol), unless square brackets are used, in which case the contents
of the memory location are used. Thus:
loads AX with the address of the variable `wordvar`, whereas
all refer to the 'contents' of memory locations. The syntaxes
es mov ax,wordvar
are not legal at all, although the use of a segment register name as an instruction
prefix is valid, and can be used with instructions such as 'LODSB' which can't
be overridden any other way.
'Constants' may be expressed numerically in most formats: a trailing H, Q or
B denotes hex, octal or binary respectively, and a leading `0x' or `$' denotes
hex as well. Leading zeros are not treated specially at all. Character constants
may be enclosed in single or double quotes; there is no escape character. The
ordering is little-endian (reversed), so that the character constant ''abcd''
denotes 0x64636261 and not 0x61626364.
Local labels begin with a period, and their `locality' is granted by the assembler
prepending the name of the previous non-local symbol. Thus declaring a label
`.loop' after a label `label' has actually defined a symbol called `label.loop'.
'SECTION' 'name' or 'SEGMENT' 'name' causes *nasm* to direct all following code
to the named section. Section names vary with output file format, although most
formats support the names '.text', '.data' and '.bss'. (The exception is the
'obj' format, in which all segments are user-definable.)
'ABSOLUTE' 'address' causes *nasm* to position its notional assembly point at
an absolute address: so no code or data may be generated, but you can use 'RESB',
'RESW' and 'RESD' to move the assembly point further on, and you can define labels.
So this directive may be used to define data structures. When you have finished
doing absolute assembly, you must issue another 'SECTION' directive to return to
'BITS' '16', 'BITS' '32' or 'BITS' '64' switches the default processor mode for
which *nasm* is generating code: it is equivalent to 'USE16' or 'USE32' in DOS
'EXTERN' 'symbol' and 'GLOBAL' 'symbol' import and export symbol definitions,
respectively, from and to other modules. Note that the 'GLOBAL' directive must
appear before the definition of the symbol it refers to.
'STRUC' 'strucname' and 'ENDSTRUC', when used to bracket a number of 'RESB',
'RESW' or similar instructions, define a data structure. In addition to
defining the offsets of the structure members, the construct also defines a symbol
for the size of the structure, which is simply the structure name with 'size'
tacked on to the end.
'ORG' 'address' is used by the 'bin' flat-form binary output format, and
specifies the address at which the output code will eventually be loaded.
'GROUP' 'grpname' 'seg1' 'seg2'... is used by the obj (Microsoft 16-bit)
output format, and defines segment groups. This format also uses 'UPPERCASE',
which directs that all segment, group and symbol names output to the object
file should be in uppercase. Note that the actual assembly is still case
'LIBRARY' 'libname' is used by the 'rdf' output format, and causes a
dependency record to be written to the output file which indicates that
the program requires a certain library in order to run.
Single-line macros are defined using the '%define' or '%idefine' commands, in
a similar fashion to the C preprocessor. They can be overloaded with respect
to number of parameters, although defining a macro with no parameters prevents
the definition of any macro with the same name taking parameters, and vice versa.
'%define' defines macros whose names match case-sensitively, whereas '%idefine'
defines case-insensitive macros.
Multi-line macros are defined using '%macro' and '%imacro' (the distinction is the
same as that between '%define' and '%idefine'), whose syntax is as follows
%macro name minprm[-maxprm][+][.nolist] [defaults]
<some lines of macro expansion text>
Again, these macros may be overloaded. The trailing plus sign indicates that
any parameters after the last one get subsumed, with their separating commas,
into the last parameter. The 'defaults' part can be used to specify defaults for
unspecified macro parameters after 'minparam'. '%endm' is a valid synonym for
To refer to the macro parameters within a macro expansion, you use '%1', '%2' and
so on. You can also enforce that a macro parameter should contain a condition
code by using '%+1', and you can invert the condition code by using '%-1'. You can also
define a label specific to a macro invocation by prefixing it with a double `%' sign.
Files can be included using the '%include' directive, which works like C.
The preprocessor has a `context stack', which may be used by one macro to store
information that a later one will retrieve. You can push a context on the stack
using '%push', remove one using '%pop', and change the name of the top context (without
disturbing any associated definitions) using '%repl'. Labels and '%define' macros
specific to the top context may be defined by prefixing their names with %$,
and things specific to the next context down with %$$, and so on.
Conditional assembly is done by means of '%ifdef', '%ifndef', '%else' and '%endif'
as in C. (Except that '%ifdef' can accept several putative macro names, and
will evaluate TRUE if any of them is defined.) In addition, the directives
'%ifctx' and '%ifnctx' can be used to condition on the name of the top context
on the context stack. The obvious set of `else-if' directives, '%elifdef',
'%elifndef', '%elifctx' and '%elifnctx' are also supported.
Please report bugs through the bug tracker function at http://nasm.us.