NASM - The Netwide Assembler ============================ Chapter 1: Introduction ----------------------- 1.1 What Is NASM? The Netwide Assembler, NASM, is an 80x86 and x86-64 assembler designed for portability and modularity. It supports a range of object file formats, including Linux and `*BSD' `a.out', `ELF', `COFF', `Mach-O', 16-bit and 32-bit `OBJ' (OMF) format, `Win32' and `Win64'. It will also output plain binary files, Intel hex and Motorola S-Record formats. Its syntax is designed to be simple and easy to understand, similar to the syntax in the Intel Software Developer Manual with minimal complexity. It supports all currently known x86 architectural extensions, and has strong support for macros. NASM also comes with a set of utilities for handling the `RDOFF' custom object-file format. 1.1.1 License Conditions Please see the file `LICENSE', supplied as part of any NASM distribution archive, for the license conditions under which you may use NASM. NASM is now under the so-called 2-clause BSD license, also known as the simplified BSD license. Copyright 1996-2017 the NASM Authors - All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: (*) Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. (*) Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. Chapter 2: Running NASM ----------------------- 2.1 NASM Command-Line Syntax To assemble a file, you issue a command of the form nasm -f [-o ] For example, nasm -f elf myfile.asm will assemble `myfile.asm' into an `ELF' object file `myfile.o'. And nasm -f bin myfile.asm -o myfile.com will assemble `myfile.asm' into a raw binary file `myfile.com'. To produce a listing file, with the hex codes output from NASM displayed on the left of the original sources, use the `-l' option to give a listing file name, for example: nasm -f coff myfile.asm -l myfile.lst To get further usage instructions from NASM, try typing nasm -h The option `--help' is an alias for the `-h' option. The option `-hf' will also list the available output file formats, and what they are. If you use Linux but aren't sure whether your system is `a.out' or `ELF', type file nasm (in the directory in which you put the NASM binary when you installed it). If it says something like nasm: ELF 32-bit LSB executable i386 (386 and up) Version 1 then your system is `ELF', and you should use the option `-f elf' when you want NASM to produce Linux object files. If it says nasm: Linux/i386 demand-paged executable (QMAGIC) or something similar, your system is `a.out', and you should use `-f aout' instead (Linux `a.out' systems have long been obsolete, and are rare these days.) Like Unix compilers and assemblers, NASM is silent unless it goes wrong: you won't see any output at all, unless it gives error messages. 2.1.1 The `-o' Option: Specifying the Output File Name NASM will normally choose the name of your output file for you; precisely how it does this is dependent on the object file format. For Microsoft object file formats (`obj', `win32' and `win64'), it will remove the `.asm' extension (or whatever extension you like to use - NASM doesn't care) from your source file name and substitute `.obj'. For Unix object file formats (`aout', `as86', `coff', `elf32', `elf64', `elfx32', `ieee', `macho32' and `macho64') it will substitute `.o'. For `dbg', `rdf', `ith' and `srec', it will use `.dbg', `.rdf', `.ith' and `.srec', respectively, and for the `bin' format it will simply remove the extension, so that `myfile.asm' produces the output file `myfile'. If the output file already exists, NASM will overwrite it, unless it has the same name as the input file, in which case it will give a warning and use `nasm.out' as the output file name instead. For situations in which this behaviour is unacceptable, NASM provides the `-o' command-line option, which allows you to specify your desired output file name. You invoke `-o' by following it with the name you wish for the output file, either with or without an intervening space. For example: nasm -f bin program.asm -o program.com nasm -f bin driver.asm -odriver.sys Note that this is a small o, and is different from a capital O , which is used to specify the number of optimisation passes required. See section 2.1.23. 2.1.2 The `-f' Option: Specifying the Output File Format If you do not supply the `-f' option to NASM, it will choose an output file format for you itself. In the distribution versions of NASM, the default is always `bin'; if you've compiled your own copy of NASM, you can redefine `OF_DEFAULT' at compile time and choose what you want the default to be. Like `-o', the intervening space between `-f' and the output file format is optional; so `-f elf' and `-felf' are both valid. A complete list of the available output file formats can be given by issuing the command `nasm -hf'. 2.1.3 The `-l' Option: Generating a Listing File If you supply the `-l' option to NASM, followed (with the usual optional space) by a file name, NASM will generate a source-listing file for you, in which addresses and generated code are listed on the left, and the actual source code, with expansions of multi-line macros (except those which specifically request no expansion in source listings: see section 4.3.11) on the right. For example: nasm -f elf myfile.asm -l myfile.lst If a list file is selected, you may turn off listing for a section of your source with `[list -]', and turn it back on with `[list +]', (the default, obviously). There is no "user form" (without the brackets). This can be used to list only sections of interest, avoiding excessively long listings. 2.1.4 The `-M' Option: Generate Makefile Dependencies This option can be used to generate makefile dependencies on stdout. This can be redirected to a file for further processing. For example: nasm -M myfile.asm > myfile.dep 2.1.5 The `-MG' Option: Generate Makefile Dependencies This option can be used to generate makefile dependencies on stdout. This differs from the `-M' option in that if a nonexisting file is encountered, it is assumed to be a generated file and is added to the dependency list without a prefix. 2.1.6 The `-MF' Option: Set Makefile Dependency File This option can be used with the `-M' or `-MG' options to send the output to a file, rather than to stdout. For example: nasm -M -MF myfile.dep myfile.asm 2.1.7 The `-MD' Option: Assemble and Generate Dependencies The `-MD' option acts as the combination of the `-M' and `-MF' options (i.e. a filename has to be specified.) However, unlike the `-M' or `-MG' options, `-MD' does _not_ inhibit the normal operation of the assembler. Use this to automatically generate updated dependencies with every assembly session. For example: nasm -f elf -o myfile.o -MD myfile.dep myfile.asm If the argument after `-MD' is an option rather than a filename, then the output filename is the first applicable one of: (*) the filename set in the `-MF' option; (*) the output filename from the `-o' option with `.d' appended; (*) the input filename with the extension set to `.d'. 2.1.8 The `-MT' Option: Dependency Target Name The `-MT' option can be used to override the default name of the dependency target. This is normally the same as the output filename, specified by the `-o' option. 2.1.9 The `-MQ' Option: Dependency Target Name (Quoted) The `-MQ' option acts as the `-MT' option, 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. The default output (if no `-MT' or `-MQ' option is specified) is automatically quoted. 2.1.10 The `-MP' Option: Emit phony targets When used with any of the dependency generation options, the `-MP' option causes NASM to emit a phony target without dependencies for each header file. This prevents Make from complaining if a header file has been removed. 2.1.11 The `-MW' Option: Watcom Make quoting style This option causes NASM to attempt to quote dependencies according to Watcom Make conventions rather than POSIX Make conventions (also used by most other Make variants.) This quotes `#' as `$#' rather than `\#', uses `&' rather than `\' for continuation lines, and encloses filenames containing whitespace in double quotes. 2.1.12 The `-F' Option: Selecting a Debug Information Format This option is used to select the format of the debug information emitted into the output file, to be used by a debugger (or _will_ be). Prior to version 2.03.01, the use of this switch did _not_ enable output of the selected debug info format. Use `-g', see section 2.1.13, to enable output. Versions 2.03.01 and later automatically enable `-g' if `-F' is specified. A complete list of the available debug file formats for an output format can be seen by issuing the command `nasm -f -y'. Not all output formats currently support debugging output. See section 2.1.27. This should not be confused with the `-f dbg' output format option, see section 7.14. 2.1.13 The `-g' Option: Enabling Debug Information. This option can be used to generate debugging information in the specified format. See section 2.1.12. Using `-g' without `-F' results in emitting debug info in the default format, if any, for the selected output format. If no debug information is currently implemented in the selected output format, `-g' is _silently ignored_. 2.1.14 The `-X' Option: Selecting an Error Reporting Format This option can be used to select an error reporting format for any error messages that might be produced by NASM. Currently, two error reporting formats may be selected. They are the `-Xvc' option and the `-Xgnu' option. The GNU format is the default and looks like this: filename.asm:65: error: specific error message where `filename.asm' is the name of the source file in which the error was detected, `65' is the source file line number on which the error was detected, `error' is the severity of the error (this could be `warning'), and `specific error message' is a more detailed text message which should help pinpoint the exact problem. The other format, specified by `-Xvc' is the style used by Microsoft Visual C++ and some other programs. It looks like this: filename.asm(65) : error: specific error message where the only difference is that the line number is in parentheses instead of being delimited by colons. See also the `Visual C++' output format, section 7.5. 2.1.15 The `-Z' Option: Send Errors to a File Under `MS-DOS' it can be difficult (though there are ways) to redirect the standard-error output of a program to a file. Since NASM usually produces its warning and error messages on `stderr', this can make it hard to capture the errors if (for example) you want to load them into an editor. NASM therefore provides the `-Z' option, taking a filename argument which causes errors to be sent to the specified files rather than standard error. Therefore you can redirect the errors into a file by typing nasm -Z myfile.err -f obj myfile.asm In earlier versions of NASM, this option was called `-E', but it was changed since `-E' is an option conventionally used for preprocessing only, with disastrous results. See section 2.1.21. 2.1.16 The `-s' Option: Send Errors to `stdout' The `-s' option redirects error messages to `stdout' rather than `stderr', so it can be redirected under `MS-DOS'. To assemble the file `myfile.asm' and pipe its output to the `more' program, you can type: nasm -s -f obj myfile.asm | more See also the `-Z' option, section 2.1.15. 2.1.17 The `-i' Option: Include File Search Directories When NASM sees the `%include' or `%pathsearch' directive in a source file (see section 4.6.1, section 4.6.2 or section 3.2.3), it will search for the given file not only in the current directory, but also in any directories specified on the command line by the use of the `-i' option. Therefore you can include files from a macro library, for example, by typing nasm -ic:\macrolib\ -f obj myfile.asm (As usual, a space between `-i' and the path name is allowed, and optional). Prior NASM 2.14 a path provided in the option has been considered as a verbatim copy and providing a path separator been up to a caller. One could implicitly concatenate a search path together with a filename. Still this was rather a trick than something useful. Now the trailing path separator is made to always present, thus `-ifoo' will be considered as the `-ifoo/' directory. If you want to define a _standard_ include search path, similar to `/usr/include' on Unix systems, you should place one or more `-i' directives in the `NASMENV' environment variable (see section 2.1.34). For Makefile compatibility with many C compilers, this option can also be specified as `-I'. 2.1.18 The `-p' Option: Pre-Include a File NASM allows you to specify files to be _pre-included_ into your source file, by the use of the `-p' option. So running nasm myfile.asm -p myinc.inc is equivalent to running `nasm myfile.asm' and placing the directive `%include "myinc.inc"' at the start of the file. `--include' option is also accepted. For consistency with the `-I', `-D' and `-U' options, this option can also be specified as `-P'. 2.1.19 The `-d' Option: Pre-Define a Macro Just as the `-p' option gives an alternative to placing `%include' directives at the start of a source file, the `-d' option gives an alternative to placing a `%define' directive. You could code nasm myfile.asm -dFOO=100 as an alternative to placing the directive %define FOO 100 at the start of the file. You can miss off the macro value, as well: the option `-dFOO' is equivalent to coding `%define FOO'. This form of the directive may be useful for selecting assembly-time options which are then tested using `%ifdef', for example `-dDEBUG'. For Makefile compatibility with many C compilers, this option can also be specified as `-D'. 2.1.20 The `-u' Option: Undefine a Macro The `-u' option undefines a macro that would otherwise have been pre-defined, either automatically or by a `-p' or `-d' option specified earlier on the command lines. For example, the following command line: nasm myfile.asm -dFOO=100 -uFOO would result in `FOO' _not_ being a predefined macro in the program. This is useful to override options specified at a different point in a Makefile. For Makefile compatibility with many C compilers, this option can also be specified as `-U'. 2.1.21 The `-E' Option: Preprocess Only NASM allows the preprocessor to be run on its own, up to a point. Using the `-E' option (which requires no arguments) will cause NASM to preprocess its input file, expand all the macro references, remove all the comments and preprocessor directives, and print the resulting file on standard output (or save it to a file, if the `-o' option is also used). This option cannot be applied to programs which require the preprocessor to evaluate expressions which depend on the values of symbols: so code such as %assign tablesize ($-tablestart) will cause an error in preprocess-only mode. For compatiblity with older version of NASM, this option can also be written `-e'. `-E' in older versions of NASM was the equivalent of the current `-Z' option, section 2.1.15. 2.1.22 The `-a' Option: Don't Preprocess At All If NASM is being used as the back end to a compiler, it might be desirable to suppress preprocessing completely and assume the compiler has already done it, to save time and increase compilation speeds. The `-a' option, requiring no argument, instructs NASM to replace its powerful preprocessor with a stub preprocessor which does nothing. 2.1.23 The `-O' Option: Specifying Multipass Optimization Using the `-O' option, you can tell NASM to carry out different levels of optimization. Multiple flags can be specified after the `-O' options, some of which can be combined in a single option, e.g. `-Oxv'. (*) `-O0': No optimization. All operands take their long forms, if a short form is not specified, except conditional jumps. This is intended to match NASM 0.98 behavior. (*) `-O1': Minimal optimization. As above, but immediate operands which will fit in a signed byte are optimized, unless the long form is specified. Conditional jumps default to the long form unless otherwise specified. (*) `-Ox' (where `x' is the actual letter `x'): Multipass optimization. Minimize branch offsets and signed immediate bytes, overriding size specification unless the `strict' keyword has been used (see section 3.7). For compatibility with earlier releases, the letter `x' may also be any number greater than one. This number has no effect on the actual number of passes. (*) `-Ov': At the end of assembly, print the number of passes actually executed. The `-Ox' mode is recommended for most uses, and is the default since NASM 2.09. Note that this is a capital `O', and is different from a small `o', which is used to specify the output file name. See section 2.1.1. 2.1.24 The `-t' Option: Enable TASM Compatibility Mode NASM includes a limited form of compatibility with Borland's `TASM'. When NASM's `-t' option is used, the following changes are made: (*) local labels may be prefixed with `@@' instead of `.' (*) size override is supported within brackets. In TASM compatible mode, a size override inside square brackets changes the size of the operand, and not the address type of the operand as it does in NASM syntax. E.g. `mov eax,[DWORD val]' is valid syntax in TASM compatibility mode. Note that you lose the ability to override the default address type for the instruction. (*) unprefixed forms of some directives supported (`arg', `elif', `else', `endif', `if', `ifdef', `ifdifi', `ifndef', `include', `local') 2.1.25 The `-w' and `-W' Options: Enable or Disable Assembly Warnings NASM can observe many conditions during the course of assembly which are worth mentioning to the user, but not a sufficiently severe error to justify NASM refusing to generate an output file. These conditions are reported like errors, but come up with the word `warning' before the message. Warnings do not prevent NASM from generating an output file and returning a success status to the operating system. Some conditions are even less severe than that: they are only sometimes worth mentioning to the user. Therefore NASM supports the `-w' command-line option, which enables or disables certain classes of assembly warning. Such warning classes are described by a name, for example `orphan-labels'; you can enable warnings of this class by the command-line option `-w+orphan-labels' and disable it by `-w-orphan-labels'. The current warning classes are: (*) `bad-pragma' warns about a malformed or otherwise unparsable `%pragma' directive. Disabled by default. (*) `bnd' warns about ineffective use of the `BND' prefix when the `JMP' instruction is converted to the `SHORT' form. This should be extremely rare since the short `JMP' only is applicable to jumps inside the same module, but if it is legitimate, it may be necessary to use `BND JMP DWORD'... Enabled by default. (*) `float-denorm' warns about denormal floating point constants. Disabled by default. (*) `float-overflow' warns about floating point underflow. Enabled by default. (*) `float-toolong' warns about too many digits in floating-point numbers. Enabled by default. (*) `float-underflow' warns about floating point underflow (a nonzero constant rounded to zero.) Disabled by default. (*) `gnu-elf-extensions' warns if 8-bit or 16-bit relocations are used in the `elf32' output format. The GNU extensions allow this. Disabled by default. (*) `hle' warns about invalid use of the HLE `XACQUIRE' or `XRELEASE' prefixes. Enabled by default. (*) `label-redef' warns if a label is defined more than once, but the value is identical. It is an unconditional error to define the same label more than once to _different_ values. Disabled by default. (*) `label-redef-late' the value of a label changed during the final, code-generation pass. This may be the result of strange use of the preprocessor. This is very likely to produce incorrect code and may end up being an unconditional error in a future version of NASM. Enabled and promoted to error by default. (*) `lock' warns about `LOCK' prefixes on unlockable instructions. Enabled by default. (*) `macro-defaults' warns when a macro has more default parameters than optional parameters. See section 4.3.5 for why might want to disable this warning. Enabled by default. (*) `macro-params' covers warnings about multi-line macros being invoked with the wrong number of parameters. See section 4.3.1 for an example of why you might want to disable this warning. Enabled by default. (*) `negative-rep' warns about negative counts given to the `%rep' preprocessor directive. Enabled by default. (*) `not-my-pragma' warns about a `%pragma' directive which is not applicable to this particular assembly session. This is not yet implemented. Disabled by default. (*) `number-overflow' covers warnings about numeric constants which don't fit in 64 bits. Enabled by default. (*) `orphan-labels' warns about source lines which contain no instruction but define a label without a trailing colon. This is most likely indicative of a typo, but is technically correct NASM syntax (see section 3.1.) Enabled by default. (*) `phase' warns about symbols having changed values during the second-to-last assembly pass. This is not inherently fatal, but may be a source of bugs. Disabled by default. (*) `ptr' warns about keywords used in other assemblers that might indicate a mistake in the source code. Currently only the MASM `PTR' keyword is recognized. Enabled by default. (*) `unknown-pragma' warns about an unknown `%pragma' directive. This is not yet implemented for most cases. Disabled by default. (*) `unknown-warning' warns about a `-w' or `-W' option or a `[WARNING]' directive that contains an unknown warning name or is otherwise not possible to process. Disabled by default. (*) `user' controls output of `%warning' directives (see section 4.9). Enabled by default. (*) `zext-reloc' warns that a relocation has been zero-extended due to limitations in the output format. Enabled by default. (*) `other' specifies any warning not included in any specific warning class. Enabled by default. (*) `all' is an alias for _all_ suppressible warning classes. Thus, `-w+all' enables all available warnings, and `-w-all' disables warnings entirely (since NASM 2.13). Since version 2.00, NASM has also supported the `gcc'-like syntax `-Wwarning-class' and `-Wno-warning-class' instead of `-w+warning-class' and `-w-warning-class', respectively; both syntaxes work identically. The option `-w+error' or `-Werror' can be used to treat warnings as errors. This can be controlled on a per warning class basis (`-w+error='_warning-class_ or `-Werror='_warning-class_); if no _warning-class_ is specified NASM treats it as `-w+error=all'; the same applies to `-w-error' or `-Wno-error', of course. In addition, you can control warnings in the source code itself, using the `[WARNING]' directive. See section 6.13. 2.1.26 The `-v' Option: Display Version Info Typing `NASM -v' will display the version of NASM which you are using, and the date on which it was compiled. You will need the version number if you report a bug. For command-line compatibility with Yasm, the form `--v' is also accepted for this option starting in NASM version 2.11.05. 2.1.27 The `-y' Option: Display Available Debug Info Formats Typing `nasm -f