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 dependecies 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.

           •   -O0: No optimization

           •   -O1: Minimal optimization

           •   -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
           -Worphan-labels or -Wno-orphan-labels.

       -w[+-]foo
           Causes nasm to enable or disable certain classes of warning messages, for example -w+orphan-labels 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).