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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
       st_, 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)