NASM(1) NASM(1)
NAME
nasm - the Netwide Assembler, a portable 80x86 assembler
SYNOPSIS
nasm [ -f format ] [ -o outfile ] [ options... ] infile
nasm -h
nasm -r
DESCRIPTION
The nasm command assembles the file infile 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. Fail-ing Failing
ing that, the output file name will be `nasm.out'.
OPTIONS
-h Causes nasm to exit immediately, after giving a summary of its invocation options, and listing
all its supported output file formats.
-a Causes nasm to assemble the given input file without first applying the macro preprocessor.
-e Causes nasm to preprocess the given input file, and write the output to stdout (or the speci-fied specified
fied output file name), and not actually assemble anything.
-M Causes nasm to output Makefile-style dependencies to stdout; normal output is suppressed.
-E filename
Causes nasm to redirect error messages to filename. This option exists to support operating
systems on which stderr is not easily redirected.
-r Causes nasm to exit immediately, after displaying its version number. (obsolete)
-v Causes nasm to exit immediately, after displaying its version number.
-f format
Specifies the output file format. Formats include bin, to produce flat-form binary files, and
aout and elf to produce Linux a.out and ELF object files, respectively.
-o outfile
Specifies a precise name for the output file, overriding nasm's default means of determining
it.
-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.
-s Causes nasm to send its error messages and/or help text to stdout instead of stderr.
-w[+-]foo
Causes nasm to enable or disable certain classes of warning messages, for example -w+orphan-labels -w+orphanlabels
labels or -w-macro-params to, respectively, enable warnings about labels alone on lines or
disable warnings about incorrect numbers of parameters in macro calls.
-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.
-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.
-D macro[=value]
Pre-defines a single-line macro.
-d macro[=value]
Same as the -D option.
-U macro
Undefines a single-line macro.
-u macro
Same as the -U option.
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 and REST pseudo-opcodes, each tak-ing taking
ing 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 actu-ally actually
ally 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 assem-bly. assembly.
bly.
BITS 16 or BITS 32 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 out-put output
put 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 syn-onym synonym
onym 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
There is a reported seg-fault on some (Linux) systems with some large source files. This appears to
be very hard to reproduce. All other known bugs have been fixed...
RESTRICTIONS
There is no support for listing files, symbol maps, or debugging object-file records. The advanced
features of the ELF and Win32 object file formats are not supported, and there is no means for warn-ing warning
ing the programmer against using an instruction beyond the capability of the target processor.
SEE ALSO
as(1), ld(1).
The Netwide Assembler Project NASM(1)
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