NASM(1) The Netwide Assembler Project NASM(1)
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'.
-@ 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|-d macro[=value]
Pre-defines a single-line macro.
-E|-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.
-F format
Specifies the debug information format. To see a list of valid
output formats, use the -y option (for example -felf -y).
-g
Causes nasm to generate debug information.
-gformat
Equivalent to -g -F 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|-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.
-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.
-MG file
Same as -M but assumes that missing Makefile dependencies are
generated and added to dependency list without a prefix.
-MF file
Output Makefile-style dependencies to the specified file.
-MD file
Same as a combination of -M and -MF options.
-MT file
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.
-MQ file
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.
-MP
Emit phony target.
-O number
Optimize branch offsets.
+o -O0: No optimization
+o -O1: Minimal optimization
+o -Ox: Multipass optimization (default)
-o outfile
Specifies a precise name for the output file, overriding nasm's
default means of determining it.
-P|-p file
Specifies a file to be pre-included, before the main source file
starts to be processed.
-s
Causes nasm to send its error messages and/or help text to stdout
instead of stderr.
-t
Causes nasm to assemble in SciTech TASM compatible mode.
-U|-u macro
Undefines a single-line macro.
-v
Causes nasm to exit immediately, after displaying its version
number.
*-W[no-]foo'
Causes nasm to enable or disable certain classes of warning
messages, in gcc-like style, for example -Wlabel-orphan or
-Wno-orphan-labels.
-w[+-]foo
Causes nasm to enable or disable certain classes of warning
messages, for example -w+label-orphan or -w-macro-params.
-X format
Specifies error reporting format (gnu or vc).
-y
Causes nasm to list supported debug formats.
-Z filename
Causes nasm to redirect error messages to filename. This option
exists to support operating systems on which stderr is not easily
redirected.
--prefix, --postfix
Prepend or append (respectively) the given argument to all global
or extern variables.
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
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'.
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 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)
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 sensitive.
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>
%endmacro
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 %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
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.
as(1), ld(1).
NASM 04/17/2024 NASM(1)
NAME
nasm - the Netwide Assembler, a portable 80x86 assembler
SYNOPSIS
nasm [-@ response file] [-f format] [-o outfile] [-l listfile]
[options...] filename
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,
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'.
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|-d macro[=value]
Pre-defines a single-line macro.
-E|-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.
-F format
Specifies the debug information format. To see a list of valid
output formats, use the -y option (for example -felf -y).
-g
Causes nasm to generate debug information.
-gformat
Equivalent to -g -F 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|-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.
-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.
-MG file
Same as -M but assumes that missing Makefile dependencies are
generated and added to dependency list without a prefix.
-MF file
Output Makefile-style dependencies to the specified file.
-MD file
Same as a combination of -M and -MF options.
-MT file
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.
-MQ file
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.
-MP
Emit phony target.
-O number
Optimize branch offsets.
+o -O0: No optimization
+o -O1: Minimal optimization
+o -Ox: Multipass optimization (default)
-o outfile
Specifies a precise name for the output file, overriding nasm's
default means of determining it.
-P|-p file
Specifies a file to be pre-included, before the main source file
starts to be processed.
-s
Causes nasm to send its error messages and/or help text to stdout
instead of stderr.
-t
Causes nasm to assemble in SciTech TASM compatible mode.
-U|-u macro
Undefines a single-line macro.
-v
Causes nasm to exit immediately, after displaying its version
number.
*-W[no-]foo'
Causes nasm to enable or disable certain classes of warning
messages, in gcc-like style, for example -Wlabel-orphan or
-Wno-orphan-labels.
-w[+-]foo
Causes nasm to enable or disable certain classes of warning
messages, for example -w+label-orphan or -w-macro-params.
-X format
Specifies error reporting format (gnu or vc).
-y
Causes nasm to list supported debug formats.
-Z filename
Causes nasm to redirect error messages to filename. This option
exists to support operating systems on which stderr is not easily
redirected.
--prefix, --postfix
Prepend or append (respectively) the given argument to all global
or extern variables.
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
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
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 structures. 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)
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 sensitive.
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.
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 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>
%endmacro
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 %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
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.
BUGS
Please report bugs through the bug tracker function at
http://nasm.us.
SEE ALSO
as(1), ld(1).
NASM 04/17/2024 NASM(1)