NASM(1)                                                                NASM(1)


       nasm - the Netwide Assembler, a portable 80x86 assembler


       nasm [ -f format ] [ -o outfile ] [ options...  ] infile
       nasm -h
       nasm -r


       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, 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’.

       -h     Causes nasm to exit immediately, after giving a summary  of  its
              invocation  options,  and  listing all its supported output file

       -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 specified 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 eas-
              ily redirected.

       -r     Causes nasm to exit immediately, after  displaying  its  version
              number.  (obsolete)

       -v     Causes  nasm  to  exit immediately, after displaying its version

       -f format
              Specifies the output file format. Formats include bin,  to  pro-
              duce  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 std-
              out instead of stderr.

              Causes nasm to enable or disable certain classes of warning mes-
              sages,  for  example  -w+orphan-labels  or  -w-macro-params  to,
              respectively, enable warnings about labels  alone  on  lines  or
              disable  warnings about incorrect numbers of parameters in macro

       -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.

       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-

       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 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

       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 struc-
       tures. When you have finished doing absolute assembly, you  must  issue
       another SECTION directive to return to normal assembly.

       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  assem-

       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.

       ORG  address  is  used  by  the bin flat-form binary output format, and
       specifies the address at which  the  output  code  will  eventually  be

       GROUP  grpname seg1 seg2...  is used by the obj (Microsoft 16-bit) out-
       put  format,  and  defines  segment  groups.  This  format  also   uses
       UPPERCASE,  which directs that all segment, group and symbol names out-
       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 depen-
       dency 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 tak-
       ing  parameters,  and  vice  versa.  %define defines macros whose names
       match  case-sensitively,  whereas  %idefine  defines   case-insensitive

       Multi-line macros are defined using %macro and %imacro (the distinction
       is the same as that between %define and %idefine), whose syntax  is  as

             %macro name minprm[-maxprm][+][.nolist] [defaults]
                      <some lines of macro expansion text>

       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-


       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...


       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  warning  the
       programmer  against  using  an instruction beyond the capability of the
       target processor.


       as(1), ld(1).

                         The Netwide Assembler Project                 NASM(1)

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