NASM(1)NASM(1)NAMEnasm - the Netwide Assembler, a portable 80x86 assembler
SYNOPSISnasm [ -@ response file ] [ -f format ] [ -o outfile ] [ -l listfile ]
[ options... ] filename
nasm-h
nasm-v
DESCRIPTION
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, usu‐
ally 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'.
OPTIONS
-@ filename
Causes nasm to process options from filename as if they were
included on the command line.
-a Causes nasm to assemble the given input file without first
applying the macro preprocessor.
-D macro[=value]
Pre-defines a single-line macro.
-d macro[=value]
Same as the -D option.
-e Causes nasm to preprocess the given input file, and write the
output to stdout (or the specified output file name), and not
actually assemble anything.
-f format
Specifies the output file format. To see a list of valid output
formats, use the -hf option.
-g Causes nasm to generate debug information in selected format
-h Causes nasm to exit immediately, after giving a summary of its
invocation options.
-hf Same as -h , but also lists all valid output formats.
-I directory
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.
-i directory
Same as the -I option.
-l listfile
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.
-M Causes nasm to output Makefile-style dependencies to stdout;
normal output is suppressed.
-O number
optimize branch offsets (-O0 disables, default).
-o outfile
Specifies a precise name for the output file, overriding nasm's
default means of determining it.
-P file
Specifies a file to be pre-included, before the main source file
starts to be processed.
-p file
Same as the -P option.
-r Causes nasm to exit immediately, after displaying its version
number. (obsolete)
-s Causes nasm to send its error messages and/or help text to std‐
out instead of stderr.
-t Causes nasm to assemble in SciTech TASM compatible mode
-U macro
Undefines a single-line macro.
-u macro
Same as the -U option.
-v Causes nasm to exit immediately, after displaying its version
number.
-w[+-]foo
Causes nasm to enable or disable certain classes of warning mes‐
sages, for example -w+orphan-labels or -w-macro-params
-X format
specifies error reporting format (gnu or vc).
-Z filename
Causes nasm to redirect error messages to filename. This option
exists to support operating systems on which stderr is not eas‐
ily redirected.
SYNTAX
This man page does not fully describe the syntax of nasm's assembly
language, but does give a summary of the differences from other assem‐
blers.
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
fadd st0,st1
fadd st1,st0
or one could use the alternative single-operand forms
fadd st1
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-$+message db 0
which defines the string `abcabcabc', followed by the right number of
zero bytes to make the total length up to 64 bytes.
Symbol references are always understood to be immediate (i.e. the
address of the symbol), unless square brackets are used, in which case
the contents of the memory location are used. Thus:
mov ax,wordvar
loads AX with the address of the variable `wordvar', whereas
mov ax,[wordvar]
mov ax,[wordvar+1]
mov ax,[es:wordvar+bx]
all refer to the contents of memory locations. The syntaxes
mov ax,es:wordvar[bx]
es mov ax,wordvar[1]
are not legal at all, although the use of a segment register name as an
instruction prefix is valid, and can be used with instructions such as
LODSB which can't be overridden any other way.
Constants may be expressed numerically in most formats: a trailing H, Q
or B denotes hex, octal or binary respectively, and a leading `0x' or
`$' 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'.
DIRECTIVES
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 struc‐
tures. When you have finished doing absolute assembly, you must issue
another SECTION directive to return to normal assembly.
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 assemblers.
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.
FORMAT-SPECIFIC DIRECTIVES
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) out‐
put format, and defines segment groups. This format also uses UPPER‐
CASE, 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 sensitive.
LIBRARY libname is used by the rdf output format, and causes a depen‐
dency record to be written to the output file which indicates that the
program requires a certain library in order to run.
MACRO PREPROCESSOR
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 tak‐
ing 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>
%endmacro
Again, these macros may be overloaded. The trailing plus sign indicates
that any parameters after the last one get subsumed, with their sepa‐
rating 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 %endmacro.
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 con‐
tain 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 sup‐
ported.
BUGS
Please report bugs through the bug tracker function at
http://nasm.sourceforge.org.
SEE ALSOas(1), ld(1).
The Netwide Assembler Project NASM(1)
