1NASM(1) General Commands Manual NASM(1)
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6 nasm - the Netwide Assembler, a portable 80x86 assembler
7
9 nasm [ -f format ] [ -o outfile ] [ options... ] infile
10 nasm -h
11 nasm -r
12
14 The nasm command assembles the file infile and directs output to the
15 file outfile if specified. If outfile is not specified, nasm will
16 derive a default output file name from the name of its input file, usu‐
17 ally by appending `.o' or `.obj', or by removing all extensions for a
18 raw binary file. Failing that, the output file name will be `nasm.out'.
19 OPTIONS
21 -h Causes nasm to exit immediately, after giving a summary of its
22 invocation options, and listing all its supported output file
23 formats.
24 -a Causes nasm to assemble the given input file without first
26 applying the macro preprocessor.
27 -e Causes nasm to preprocess the given input file, and write the
29 output to stdout (or the specified output file name), and not
30 actually assemble anything.
31 -M Causes nasm to output Makefile-style dependencies to stdout;
33 normal output is suppressed.
34 -E filename
36 Causes nasm to redirect error messages to filename. This option
37 exists to support operating systems on which stderr is not eas‐
38 ily redirected.
39 -r Causes nasm to exit immediately, after displaying its version
41 number. (obsolete)
42 -v Causes nasm to exit immediately, after displaying its version
44 number.
45 -f format
47 Specifies the output file format. Formats include bin, to pro‐
48 duce flat-form binary files, and aout and elf to produce Linux
49 a.out and ELF object files, respectively.
50 -o outfile
52 Specifies a precise name for the output file, overriding nasm's
53 default means of determining it.
54 -l listfile
56 Causes an assembly listing to be directed to the given file, in
57 which the original source is displayed on the right hand side
58 (plus the source for included files and the expansions of multi-
59 line macros) and the generated code is shown in hex on the left.
60 -s Causes nasm to send its error messages and/or help text to std‐
62 out instead of stderr.
63 -w[+-]foo
65 Causes nasm to enable or disable certain classes of warning mes‐
66 sages, for example -w+orphan-labels or -w-macro-params to,
67 respectively, enable warnings about labels alone on lines or
68 disable warnings about incorrect numbers of parameters in macro
69 calls.
70 -I directory
72 Adds a directory to the search path for include files. The
73 directory specification must include the trailing slash, as it
74 will be directly prepended to the name of the include file.
75 -i directory
77 Same as the -I option.
78 -P file
80 Specifies a file to be pre-included, before the main source file
81 starts to be processed.
82 -p file
84 Same as the -P option.
85 -D macro[=value]
87 Pre-defines a single-line macro.
88 -d macro[=value]
90 Same as the -D option.
91 -U macro
93 Undefines a single-line macro.
94 -u macro
96 Same as the -U option.
97 SYNTAX
99 This man page does not fully describe the syntax of nasm's assembly
100 language, but does give a summary of the differences from other assem‐
101 blers.
102 Registers have no leading `%' sign, unlike gas, and floating-point
104 stack registers are referred to as st0, st1, and so on.
105 Floating-point instructions may use either the single-operand form or
107 the double. A TO keyword is provided; thus, one could either write
108 fadd st0,st1
110 fadd st1,st0
111 or one could use the alternative single-operand forms
113 fadd st1
115 fadd to st1
116 Uninitialised storage is reserved using the RESB, RESW, RESD, RESQ and
118 REST pseudo-opcodes, each taking one parameter which gives the number
119 of bytes, words, doublewords, quadwords or ten-byte words to reserve.
120 Repetition of data items is not done by the DUP keyword as seen in DOS
122 assemblers, but by the use of the TIMES prefix, like this:
123 message: times 3 db 'abc'
125 times 64-$+message db 0
126 which defines the string `abcabcabc', followed by the right number of
128 zero bytes to make the total length up to 64 bytes.
129 Symbol references are always understood to be immediate (i.e. the
131 address of the symbol), unless square brackets are used, in which case
132 the contents of the memory location are used. Thus:
133 mov ax,wordvar
135 loads AX with the address of the variable `wordvar', whereas
137 mov ax,[wordvar]
139 mov ax,[wordvar+1]
140 mov ax,[es:wordvar+bx]
141 all refer to the contents of memory locations. The syntaxes
143 mov ax,es:wordvar[bx]
145 es mov ax,wordvar[1]
146 are not legal at all, although the use of a segment register name as an
148 instruction prefix is valid, and can be used with instructions such as
149 LODSB which can't be overridden any other way.
150 Constants may be expressed numerically in most formats: a trailing H, Q
152 or B denotes hex, octal or binary respectively, and a leading `0x' or
153 `$' denotes hex as well. Leading zeros are not treated specially at
154 all. Character constants may be enclosed in single or double quotes;
155 there is no escape character. The ordering is little-endian (reversed),
156 so that the character constant 'abcd' denotes 0x64636261 and not
157 0x61626364.
158 Local labels begin with a period, and their `locality' is granted by
160 the assembler prepending the name of the previous non-local symbol.
161 Thus declaring a label `.loop' after a label `label' has actually
162 defined a symbol called `label.loop'.
163 DIRECTIVES
165 SECTION name or SEGMENT name causes nasm to direct all following code
166 to the named section. Section names vary with output file format,
167 although most formats support the names .text, .data and .bss. (The
168 exception is the obj format, in which all segments are user-definable.)
169 ABSOLUTE address causes nasm to position its notional assembly point at
171 an absolute address: so no code or data may be generated, but you can
172 use RESB, RESW and RESD to move the assembly point further on, and you
173 can define labels. So this directive may be used to define data struc‐
174 tures. When you have finished doing absolute assembly, you must issue
175 another SECTION directive to return to normal assembly.
176 BITS 16 or BITS 32 switches the default processor mode for which nasm
178 is generating code: it is equivalent to USE16 or USE32 in DOS assem‐
179 blers.
180 EXTERN symbol and GLOBAL symbol import and export symbol definitions,
182 respectively, from and to other modules. Note that the GLOBAL directive
183 must appear before the definition of the symbol it refers to.
184 STRUC strucname and ENDSTRUC, when used to bracket a number of RESB,
186 RESW or similar instructions, define a data structure. In addition to
187 defining the offsets of the structure members, the construct also
188 defines a symbol for the size of the structure, which is simply the
189 structure name with _size tacked on to the end.
190 FORMAT-SPECIFIC DIRECTIVES
192 ORG address is used by the bin flat-form binary output format, and
193 specifies the address at which the output code will eventually be
194 loaded.
195 GROUP grpname seg1 seg2... is used by the obj (Microsoft 16-bit) out‐
197 put format, and defines segment groups. This format also uses UPPER‐
198 CASE, which directs that all segment, group and symbol names output to
199 the object file should be in uppercase. Note that the actual assembly
200 is still case sensitive.
201 LIBRARY libname is used by the rdf output format, and causes a depen‐
203 dency record to be written to the output file which indicates that the
204 program requires a certain library in order to run.
205 MACRO PREPROCESSOR
207 Single-line macros are defined using the %define or %idefine commands,
208 in a similar fashion to the C preprocessor. They can be overloaded with
209 respect to number of parameters, although defining a macro with no
210 parameters prevents the definition of any macro with the same name tak‐
211 ing parameters, and vice versa. %define defines macros whose names
212 match case-sensitively, whereas %idefine defines case-insensitive
213 macros.
214 Multi-line macros are defined using %macro and %imacro (the distinction
216 is the same as that between %define and %idefine), whose syntax is as
217 follows:
218 %macro name minprm[-maxprm][+][.nolist] [defaults]
220 <some lines of macro expansion text>
221 %endmacro
222 Again, these macros may be overloaded. The trailing plus sign indicates
224 that any parameters after the last one get subsumed, with their sepa‐
225 rating commas, into the last parameter. The defaults part can be used
226 to specify defaults for unspecified macro parameters after minparam.
227 %endm is a valid synonym for %endmacro.
228 To refer to the macro parameters within a macro expansion, you use %1,
230 %2 and so on. You can also enforce that a macro parameter should con‐
231 tain a condition code by using %+1, and you can invert the condition
232 code by using %-1. You can also define a label specific to a macro
233 invocation by prefixing it with a double % sign.
234 Files can be included using the %include directive, which works like C.
236 The preprocessor has a `context stack', which may be used by one macro
238 to store information that a later one will retrieve. You can push a
239 context on the stack using %push, remove one using %pop, and change the
240 name of the top context (without disturbing any associated definitions)
241 using %repl. Labels and %define macros specific to the top context may
242 be defined by prefixing their names with %$, and things specific to the
243 next context down with %$$, and so on.
244 Conditional assembly is done by means of %ifdef, %ifndef, %else and
246 %endif as in C. (Except that %ifdef can accept several putative macro
247 names, and will evaluate TRUE if any of them is defined.) In addition,
248 the directives %ifctx and %ifnctx can be used to condition on the name
249 of the top context on the context stack. The obvious set of `else-if'
250 directives, %elifdef, %elifndef, %elifctx and %elifnctx are also sup‐
251 ported.
252
254 There is a reported seg-fault on some (Linux) systems with some large
255 source files. This appears to be very hard to reproduce. All other
256 known bugs have been fixed...
257
259 There is no support for listing files, symbol maps, or debugging
260 object-file records. The advanced features of the ELF and Win32 object
261 file formats are not supported, and there is no means for warning the
262 programmer against using an instruction beyond the capability of the
263 target processor.
264
266 as(1), ld(1).
267 The Netwide Assembler Project NASM(1)