The Netwide Assembler: NASM =========================== Chapter 1: Introduction ----------------------- 1.1 What Is NASM? The Netwide Assembler, NASM, is an 80x86 assembler designed for portability and modularity. It supports a range of object file formats, including Linux `a.out' and ELF, NetBSD/FreeBSD, COFF, Microsoft 16-bit OBJ and Win32. It will also output plain binary files. Its syntax is designed to be simple and easy to understand, similar to Intel's but less complex. It supports Pentium, P6 and MMX opcodes, and has macro capability. 1.1.1 Why Yet Another Assembler? The Netwide Assembler grew out of an idea on `comp.lang.asm.x86' (or possibly `alt.lang.asm' - I forget which), which was essentially that there didn't seem to be a good free x86-series assembler around, and that maybe someone ought to write one. (*) `a86' is good, but not free, and in particular you don't get any 32-bit capability until you pay. It's DOS only, too. (*) `gas' is free, and ports over DOS and Unix, but it's not very good, since it's designed to be a back end to `gcc', which always feeds it correct code. So its error checking is minimal. Also, its syntax is horrible, from the point of view of anyone trying to actually _write_ anything in it. Plus you can't write 16-bit code in it (properly). (*) `as86' is Linux-specific, and (my version at least) doesn't seem to have much (or any) documentation. (*) MASM isn't very good, and it's expensive, and it runs only under DOS. (*) TASM is better, but still strives for MASM compatibility, which means millions of directives and tons of red tape. And its syntax is essentially MASM's, with the contradictions and quirks that entails (although it sorts out some of those by means of Ideal mode). It's expensive too. And it's DOS-only. So here, for your coding pleasure, is NASM. At present it's still in prototype stage - we don't promise that it can outperform any of these assemblers. But please, _please_ send us bug reports, fixes, helpful information, and anything else you can get your hands on (and thanks to the many people who've done this already! You all know who you are), and we'll improve it out of all recognition. Again. 1.1.2 Licence Conditions Please see the file `Licence', supplied as part of any NASM distribution archive, for the licence conditions under which you may use NASM. 1.2 Contact Information The current version of NASM (since 0.98) are maintained by H. Peter Anvin, `hpa@zytor.com'. If you want to report a bug, please read section 10.2 first. NASM has a WWW page at `http://www.cryogen.com/Nasm'. The original authors are e-mailable as `jules@earthcorp.com' and `anakin@pobox.com'. New releases of NASM are uploaded to `ftp.kernel.org', `sunsite.unc.edu', `ftp.simtel.net' and `ftp.coast.net'. Announcements are posted to `comp.lang.asm.x86', `alt.lang.asm', `comp.os.linux.announce' and `comp.archives.msdos.announce' (the last one is done automagically by uploading to `ftp.simtel.net'). If you don't have Usenet access, or would rather be informed by e-mail when new releases come out, you can subscribe to the `nasm-announce' email list by sending an email containing the line `subscribe nasm-announce' to `majordomo@linux.kernel.org'. If you want information about NASM beta releases, please subscribe to the `nasm-beta' email list by sending an email containing the line `subscribe nasm-beta' to `majordomo@linux.kernel.org'. 1.3 Installation 1.3.1 Installing NASM under MS-DOS or Windows Once you've obtained the DOS archive for NASM, `nasmXXX.zip' (where `XXX' denotes the version number of NASM contained in the archive), unpack it into its own directory (for example `c:\nasm'). The archive will contain four executable files: the NASM executable files `nasm.exe' and `nasmw.exe', and the NDISASM executable files `ndisasm.exe' and `ndisasmw.exe'. In each case, the file whose name ends in `w' is a Win32 executable, designed to run under Windows 95 or Windows NT Intel, and the other one is a 16-bit DOS executable. The only file NASM needs to run is its own executable, so copy (at least) one of `nasm.exe' and `nasmw.exe' to a directory on your PATH, or alternatively edit `autoexec.bat' to add the `nasm' directory to your `PATH'. (If you're only installing the Win32 version, you may wish to rename it to `nasm.exe'.) That's it - NASM is installed. You don't need the `nasm' directory to be present to run NASM (unless you've added it to your `PATH'), so you can delete it if you need to save space; however, you may want to keep the documentation or test programs. If you've downloaded the DOS source archive, `nasmXXXs.zip', the `nasm' directory will also contain the full NASM source code, and a selection of Makefiles you can (hopefully) use to rebuild your copy of NASM from scratch. The file `Readme' lists the various Makefiles and which compilers they work with. Note that the source files `insnsa.c', `insnsd.c', `insnsi.h' and `insnsn.c' are automatically generated from the master instruction table `insns.dat' by a Perl script; the file `macros.c' is generated from `standard.mac' by another Perl script. Although the NASM 0.98 distribution includes these generated files, you will need to rebuild them (and hence, will need a Perl interpreter) if you change `insns.dat', `standard.mac' or the documentation. It is possible future source distributions may not include these files at all. Ports of Perl for a variety of platforms, including DOS and Windows, are available from www.cpan.org. 1.3.2 Installing NASM under Unix Once you've obtained the Unix source archive for NASM, `nasm-X.XX.tar.gz' (where `X.XX' denotes the version number of NASM contained in the archive), unpack it into a directory such as `/usr/local/src'. The archive, when unpacked, will create its own subdirectory `nasm-X.XX'. NASM is an auto-configuring package: once you've unpacked it, `cd' to the directory it's been unpacked into and type `./configure'. This shell script will find the best C compiler to use for building NASM and set up Makefiles accordingly. Once NASM has auto-configured, you can type `make' to build the `nasm' and `ndisasm' binaries, and then `make install' to install them in `/usr/local/bin' and install the man pages `nasm.1' and `ndisasm.1' in `/usr/local/man/man1'. Alternatively, you can give options such as `--prefix' to the `configure' script (see the file `INSTALL' for more details), or install the programs yourself. NASM also comes with a set of utilities for handling the RDOFF custom object-file format, which are in the `rdoff' subdirectory of the NASM archive. You can build these with `make rdf' and install them with `make rdf_install', if you want them. If NASM fails to auto-configure, you may still be able to make it compile by using the fall-back Unix makefile `Makefile.unx'. Copy or rename that file to `Makefile' and try typing `make'. There is also a `Makefile.unx' file in the `rdoff' subdirectory. 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 This 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 are considered 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' and `win32'), 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', `coff', `elf' and `as86') it will substitute `.o'. For `rdf', it will use `.rdf', 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 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 -h'. 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 4.2.9) on the right. For example: nasm -f elf myfile.asm -l myfile.lst 2.1.4 The `-E' 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 `-E' 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 -E myfile.err -f obj myfile.asm 2.1.5 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 `-E' option, section 2.1.4. 2.1.6 The `-i' Option: Include File Search Directories When NASM sees the `%include' directive in a source file (see section 4.5), 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). NASM, in the interests of complete source-code portability, does not understand the file naming conventions of the OS it is running on; the string you provide as an argument to the `-i' option will be prepended exactly as written to the name of the include file. Therefore the trailing backslash in the above example is necessary. Under Unix, a trailing forward slash is similarly necessary. (You can use this to your advantage, if you're really perverse, by noting that the option `-ifoo' will cause `%include "bar.i"' to search for the file `foobar.i'...) 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 `NASM' environment variable (see section 2.1.13). For Makefile compatibility with many C compilers, this option can also be specified as `-I'. 2.1.7 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. For consistency with the `-I', `-D' and `-U' options, this option can also be specified as `-P'. 2.1.8 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'. 2.1.9 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'. 2.1.10 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. 2.1.11 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. 2.1.12 The `-w' Option: 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 suppressible warning classes are: (*) `macro-params' covers warnings about multi-line macros being invoked with the wrong number of parameters. This warning class is enabled by default; see section 4.2.1 for an example of why you might want to disable it. (*) `orphan-labels' covers warnings about source lines which contain no instruction but define a label without a trailing colon. NASM does not warn about this somewhat obscure condition by default; see section 3.1 for an example of why you might want it to. (*) `number-overflow' covers warnings about numeric constants which don't fit in 32 bits (for example, it's easy to type one too many Fs and produce `0x7ffffffff' by mistake). This warning class is enabled by default. 2.1.13 The `NASM' Environment Variable If you define an environment variable called `NASM', the program will interpret it as a list of extra command-line options, which are processed before the real command line. You can use this to define standard search directories for include files, by putting `-i' options in the `NASM' variable. The value of the variable is split up at white space, so that the value `-s -ic:\nasmlib' will be treated as two separate options. However, that means that the value `-dNAME="my name"' won't do what you might want, because it will be split at the space and the NASM command-line processing will get confused by the two nonsensical words `-dNAME="my' and `name"'. To get round this, NASM provides a feature whereby, if you begin the `NASM' environment variable with some character that isn't a minus sign, then NASM will treat this character as the separator character for options. So setting the `NASM' variable to the value `!-s!-ic:\nasmlib' is equivalent to setting it to `-s -ic:\nasmlib', but `!-dNAME="my name"' will work. 2.2 Quick Start for MASM Users If you're used to writing programs with MASM, or with TASM in MASM- compatible (non-Ideal) mode, or with `a86', this section attempts to outline the major differences between MASM's syntax and NASM's. If you're not already used to MASM, it's probably worth skipping this section. 2.2.1 NASM Is Case-Sensitive One simple difference is that NASM is case-sensitive. It makes a difference whether you call your label `foo', `Foo' or `FOO'. If you're assembling to DOS or OS/2 `.OBJ' files, you can invoke the `UPPERCASE' directive (documented in section 6.2) to ensure that all symbols exported to other code modules are forced to be upper case; but even then, _within_ a single module, NASM will distinguish between labels differing only in case. 2.2.2 NASM Requires Square Brackets For Memory References NASM was designed with simplicity of syntax in mind. One of the design goals of NASM is that it should be possible, as far as is practical, for the user to look at a single line of NASM code and tell what opcode is generated by it. You can't do this in MASM: if you declare, for example, foo equ 1 bar dw 2 then the two lines of code mov ax,foo mov ax,bar generate completely different opcodes, despite having identical- looking syntaxes. NASM avoids this undesirable situation by having a much simpler syntax for memory references. The rule is simply that any access to the _contents_ of a memory location requires square brackets around the address, and any access to the _address_ of a variable doesn't. So an instruction of the form `mov ax,foo' will _always_ refer to a compile-time constant, whether it's an `EQU' or the address of a variable; and to access the _contents_ of the variable `bar', you must code `mov ax,[bar]'. This also means that NASM has no need for MASM's `OFFSET' keyword, since the MASM code `mov ax,offset bar' means exactly the same thing as NASM's `mov ax,bar'. If you're trying to get large amounts of MASM code to assemble sensibly under NASM, you can always code `%idefine offset' to make the preprocessor treat the `OFFSET' keyword as a no-op. This issue is even more confusing in `a86', where declaring a label with a trailing colon defines it to be a `label' as opposed to a `variable' and causes `a86' to adopt NASM-style semantics; so in `a86', `mov ax,var' has different behaviour depending on whether `var' was declared as `var: dw 0' (a label) or `var dw 0' (a word- size variable). NASM is very simple by comparison: _everything_ is a label. NASM, in the interests of simplicity, also does not support the hybrid syntaxes supported by MASM and its clones, such as `mov ax,table[bx]', where a memory reference is denoted by one portion outside square brackets and another portion inside. The correct syntax for the above is `mov ax,[table+bx]'. Likewise, `mov ax,es:[di]' is wrong and `mov ax,[es:di]' is right. 2.2.3 NASM Doesn't Store Variable Types NASM, by design, chooses not to remember the types of variables you declare. Whereas MASM will remember, on seeing `var dw 0', that you declared `var' as a word-size variable, and will then be able to fill in the ambiguity in the size of the instruction `mov var,2', NASM will deliberately remember nothing about the symbol `var' except where it begins, and so you must explicitly code `mov word [var],2'. For this reason, NASM doesn't support the `LODS', `MOVS', `STOS', `SCAS', `CMPS', `INS', or `OUTS' instructions, but only supports the forms such as `LODSB', `MOVSW', and `SCASD', which explicitly specify the size of the components of the strings being manipulated. 2.2.4 NASM Doesn't `ASSUME' As part of NASM's drive for simplicity, it also does not support the `ASSUME' directive. NASM will not keep track of what values you choose to put in your segment registers, and will never _automatically_ generate a segment override prefix. 2.2.5 NASM Doesn't Support Memory Models NASM also does not have any directives to support different 16-bit memory models. The programmer has to keep track of which functions are supposed to be called with a far call and which with a near call, and is responsible for putting the correct form of `RET' instruction (`RETN' or `RETF'; NASM accepts `RET' itself as an alternate form for `RETN'); in addition, the programmer is responsible for coding CALL FAR instructions where necessary when calling _external_ functions, and must also keep track of which external variable definitions are far and which are near. 2.2.6 Floating-Point Differences NASM uses different names to refer to floating-point registers from MASM: where MASM would call them `ST(0)', `ST(1)' and so on, and `a86' would call them simply `0', `1' and so on, NASM chooses to call them `st0', `st1' etc. As of version 0.96, NASM now treats the instructions with `nowait' forms in the same way as MASM-compatible assemblers. The idiosyncratic treatment employed by 0.95 and earlier was based on a misunderstanding by the authors. 2.2.7 Other Differences For historical reasons, NASM uses the keyword `TWORD' where MASM and compatible assemblers use `TBYTE'. NASM does not declare uninitialised storage in the same way as MASM: where a MASM programmer might use `stack db 64 dup (?)', NASM requires `stack resb 64', intended to be read as `reserve 64 bytes'. For a limited amount of compatibility, since NASM treats `?' as a valid character in symbol names, you can code `? equ 0' and then writing `dw ?' will at least do something vaguely useful. `DUP' is still not a supported syntax, however. In addition to all of this, macros and directives work completely differently to MASM. See chapter 4 and chapter 5 for further details. Chapter 3: The NASM Language ---------------------------- 3.1 Layout of a NASM Source Line Like most assemblers, each NASM source line contains (unless it is a macro, a preprocessor directive or an assembler directive: see chapter 4 and chapter 5) some combination of the four fields label: instruction operands ; comment As usual, most of these fields are optional; the presence or absence of any combination of a label, an instruction and a comment is allowed. Of course, the operand field is either required or forbidden by the presence and nature of the instruction field. NASM places no restrictions on white space within a line: labels may have white space before them, or instructions may have no space before them, or anything. The colon after a label is also optional. (Note that this means that if you intend to code `lodsb' alone on a line, and type `lodab' by accident, then that's still a valid source line which does nothing but define a label. Running NASM with the command-line option `-w+orphan-labels' will cause it to warn you if you define a label alone on a line without a trailing colon.) Valid characters in labels are letters, numbers, `_', `$', `#', `@', `~', `.', and `?'. The only characters which may be used as the _first_ character of an identifier are letters, `.' (with special meaning: see section 3.8), `_' and `?'. An identifier may also be prefixed with a `$' to indicate that it is intended to be read as an identifier and not a reserved word; thus, if some other module you are linking with defines a symbol called `eax', you can refer to `$eax' in NASM code to distinguish the symbol from the register. The instruction field may contain any machine instruction: Pentium and P6 instructions, FPU instructions, MMX instructions and even undocumented instructions are all supported. The instruction may be prefixed by `LOCK', `REP', `REPE'/`REPZ' or `REPNE'/`REPNZ', in the usual way. Explicit address-size and operand-size prefixes `A16', `A32', `O16' and `O32' are provided - one example of their use is given in chapter 9. You can also use the name of a segment register as an instruction prefix: coding `es mov [bx],ax' is equivalent to coding `mov [es:bx],ax'. We recommend the latter syntax, since it is consistent with other syntactic features of the language, but for instructions such as `LODSB', which has no operands and yet can require a segment override, there is no clean syntactic way to proceed apart from `es lodsb'. An instruction is not required to use a prefix: prefixes such as `CS', `A32', `LOCK' or `REPE' can appear on a line by themselves, and NASM will just generate the prefix bytes. In addition to actual machine instructions, NASM also supports a number of pseudo-instructions, described in section 3.2. Instruction operands may take a number of forms: they can be registers, described simply by the register name (e.g. `ax', `bp', `ebx', `cr0': NASM does not use the `gas'-style syntax in which register names must be prefixed by a `%' sign), or they can be effective addresses (see section 3.3), constants (section 3.4) or expressions (section 3.5). For floating-point instructions, NASM accepts a wide range of syntaxes: you can use two-operand forms like MASM supports, or you can use NASM's native single-operand forms in most cases. Details of all forms of each supported instruction are given in appendix A. For example, you can code: fadd st1 ; this sets st0 := st0 + st1 fadd st0,st1 ; so does this fadd st1,st0 ; this sets st1 := st1 + st0 fadd to st1 ; so does this Almost any floating-point instruction that references memory must use one of the prefixes `DWORD', `QWORD' or `TWORD' to indicate what size of memory operand it refers to. 3.2 Pseudo-Instructions Pseudo-instructions are things which, though not real x86 machine instructions, are used in the instruction field anyway because that's the most convenient place to put them. The current pseudo- instructions are `DB', `DW', `DD', `DQ' and `DT', their uninitialised counterparts `RESB', `RESW', `RESD', `RESQ' and `REST', the `INCBIN' command, the `EQU' command, and the `TIMES' prefix. 3.2.1 `DB' and friends: Declaring Initialised Data `DB', `DW', `DD', `DQ' and `DT' are used, much as in MASM, to declare initialised data in the output file. They can be invoked in a wide range of ways: db 0x55 ; just the byte 0x55 db 0x55,0x56,0x57 ; three bytes in succession db 'a',0x55 ; character constants are OK db 'hello',13,10,'$' ; so are string constants dw 0x1234 ; 0x34 0x12 dw 'a' ; 0x41 0x00 (it's just a number) dw 'ab' ; 0x41 0x42 (character constant) dw 'abc' ; 0x41 0x42 0x43 0x00 (string) dd 0x12345678 ; 0x78 0x56 0x34 0x12 dd 1.234567e20 ; floating-point constant dq 1.234567e20 ; double-precision float dt 1.234567e20 ; extended-precision float `DQ' and `DT' do not accept numeric constants or string constants as operands. 3.2.2 `RESB' and friends: Declaring Uninitialised Data `RESB', `RESW', `RESD', `RESQ' and `REST' are designed to be used in the BSS section of a module: they declare _uninitialised_ storage space. Each takes a single operand, which is the number of bytes, words, doublewords or whatever to reserve. As stated in section 2.2.7, NASM does not support the MASM/TASM syntax of reserving uninitialised space by writing `DW ?' or similar things: this is what it does instead. The operand to a `RESB'-type pseudo- instruction is a _critical expression_: see section 3.7. For example: buffer: resb 64 ; reserve 64 bytes wordvar: resw 1 ; reserve a word realarray resq 10 ; array of ten reals 3.2.3 `INCBIN': Including External Binary Files `INCBIN' is borrowed from the old Amiga assembler DevPac: it includes a binary file verbatim into the output file. This can be handy for (for example) including graphics and sound data directly into a game executable file. It can be called in one of these three ways: incbin "file.dat" ; include the whole file incbin "file.dat",1024 ; skip the first 1024 bytes incbin "file.dat",1024,512 ; skip the first 1024, and ; actually include at most 512 3.2.4 `EQU': Defining Constants `EQU' defines a symbol to a given constant value: when `EQU' is used, the source line must contain a label. The action of `EQU' is to define the given label name to the value of its (only) operand. This definition is absolute, and cannot change later. So, for example, message db 'hello, world' msglen equ $-message defines `msglen' to be the constant 12. `msglen' may not then be redefined later. This is not a preprocessor definition either: the value of `msglen' is evaluated _once_, using the value of `$' (see section 3.5 for an explanation of `$') at the point of definition, rather than being evaluated wherever it is referenced and using the value of `$' at the point of reference. Note that the operand to an `EQU' is also a critical expression (section 3.7). 3.2.5 `TIMES': Repeating Instructions or Data The `TIMES' prefix causes the instruction to be assembled multiple times. This is partly present as NASM's equivalent of the `DUP' syntax supported by MASM-compatible assemblers, in that you can code zerobuf: times 64 db 0 or similar things; but `TIMES' is more versatile than that. The argument to `TIMES' is not just a numeric constant, but a numeric _expression_, so you can do things like buffer: db 'hello, world' times 64-$+buffer db ' ' which will store exactly enough spaces to make the total length of `buffer' up to 64. Finally, `TIMES' can be applied to ordinary instructions, so you can code trivial unrolled loops in it: times 100 movsb Note that there is no effective difference between `times 100 resb 1' and `resb 100', except that the latter will be assembled about 100 times faster due to the internal structure of the assembler. The operand to `TIMES', like that of `EQU' and those of `RESB' and friends, is a critical expression (section 3.7). Note also that `TIMES' can't be applied to macros: the reason for this is that `TIMES' is processed after the macro phase, which allows the argument to `TIMES' to contain expressions such as `64-$+buffer' as above. To repeat more than one line of code, or a complex macro, use the preprocessor `%rep' directive. 3.3 Effective Addresses An effective address is any operand to an instruction which references memory. Effective addresses, in NASM, have a very simple syntax: they consist of an expression evaluating to the desired address, enclosed in square brackets. For example: wordvar dw 123 mov ax,[wordvar] mov ax,[wordvar+1] mov ax,[es:wordvar+bx] Anything not conforming to this simple system is not a valid memory reference in NASM, for example `es:wordvar[bx]'. More complicated effective addresses, such as those involving more than one register, work in exactly the same way: mov eax,[ebx*2+ecx+offset] mov ax,[bp+di+8] NASM is capable of doing algebra on these effective addresses, so that things which don't necessarily _look_ legal are perfectly all right: mov eax,[ebx*5] ; assembles as [ebx*4+ebx] mov eax,[label1*2-label2] ; ie [label1+(label1-label2)] Some forms of effective address have more than one assembled form; in most such cases NASM will generate the smallest form it can. For example, there are distinct assembled forms for the 32-bit effective addresses `[eax*2+0]' and `[eax+eax]', and NASM will generally generate the latter on the grounds that the former requires four bytes to store a zero offset. NASM has a hinting mechanism which will cause `[eax+ebx]' and `[ebx+eax]' to generate different opcodes; this is occasionally useful because `[esi+ebp]' and `[ebp+esi]' have different default segment registers. However, you can force NASM to generate an effective address in a particular form by the use of the keywords `BYTE', `WORD', `DWORD' and `NOSPLIT'. If you need `[eax+3]' to be assembled using a double- word offset field instead of the one byte NASM will normally generate, you can code `[dword eax+3]'. Similarly, you can force NASM to use a byte offset for a small value which it hasn't seen on the first pass (see section 3.7 for an example of such a code fragment) by using `[byte eax+offset]'. As special cases, `[byte eax]' will code `[eax+0]' with a byte offset of zero, and `[dword eax]' will code it with a double-word offset of zero. The normal form, `[eax]', will be coded with no offset field. Similarly, NASM will split `[eax*2]' into `[eax+eax]' because that allows the offset field to be absent and space to be saved; in fact, it will also split `[eax*2+offset]' into `[eax+eax+offset]'. You can combat this behaviour by the use of the `NOSPLIT' keyword: `[nosplit eax*2]' will force `[eax*2+0]' to be generated literally. 3.4 Constants NASM understands four different types of constant: numeric, character, string and floating-point. 3.4.1 Numeric Constants A numeric constant is simply a number. NASM allows you to specify numbers in a variety of number bases, in a variety of ways: you can suffix `H', `Q' and `B' for hex, octal and binary, or you can prefix `0x' for hex in the style of C, or you can prefix `$' for hex in the style of Borland Pascal. Note, though, that the `$' prefix does double duty as a prefix on identifiers (see section 3.1), so a hex number prefixed with a `$' sign must have a digit after the `$' rather than a letter. Some examples: mov ax,100 ; decimal mov ax,0a2h ; hex mov ax,$0a2 ; hex again: the 0 is required mov ax,0xa2 ; hex yet again mov ax,777q ; octal mov ax,10010011b ; binary 3.4.2 Character Constants A character constant consists of up to four characters enclosed in either single or double quotes. The type of quote makes no difference to NASM, except of course that surrounding the constant with single quotes allows double quotes to appear within it and vice versa. A character constant with more than one character will be arranged with little-endian order in mind: if you code mov eax,'abcd' then the constant generated is not `0x61626364', but `0x64636261', so that if you were then to store the value into memory, it would read `abcd' rather than `dcba'. This is also the sense of character constants understood by the Pentium's `CPUID' instruction (see section A.22). 3.4.3 String Constants String constants are only acceptable to some pseudo-instructions, namely the `DB' family and `INCBIN'. A string constant looks like a character constant, only longer. It is treated as a concatenation of maximum-size character constants for the conditions. So the following are equivalent: db 'hello' ; string constant db 'h','e','l','l','o' ; equivalent character constants And the following are also equivalent: dd 'ninechars' ; doubleword string constant dd 'nine','char','s' ; becomes three doublewords db 'ninechars',0,0,0 ; and really looks like this Note that when used as an operand to `db', a constant like `'ab'' is treated as a string constant despite being short enough to be a character constant, because otherwise `db 'ab'' would have the same effect as `db 'a'', which would be silly. Similarly, three-character or four-character constants are treated as strings when they are operands to `dw'. 3.4.4 Floating-Point Constants Floating-point constants are acceptable only as arguments to `DD', `DQ' and `DT'. They are expressed in the traditional form: digits, then a period, then optionally more digits, then optionally an `E' followed by an exponent. The period is mandatory, so that NASM can distinguish between `dd 1', which declares an integer constant, and `dd 1.0' which declares a floating-point constant. Some examples: dd 1.2 ; an easy one dq 1.e10 ; 10,000,000,000 dq 1.e+10 ; synonymous with 1.e10 dq 1.e-10 ; 0.000 000 000 1 dt 3.141592653589793238462 ; pi NASM cannot do compile-time arithmetic on floating-point constants. This is because NASM is designed to be portable - although it always generates code to run on x86 processors, the assembler itself can run on any system with an ANSI C compiler. Therefore, the assembler cannot guarantee the presence of a floating-point unit capable of handling the Intel number formats, and so for NASM to be able to do floating arithmetic it would have to include its own complete set of floating-point routines, which would significantly increase the size of the assembler for very little benefit. 3.5 Expressions Expressions in NASM are similar in syntax to those in C. NASM does not guarantee the size of the integers used to evaluate expressions at compile time: since NASM can compile and run on 64- bit systems quite happily, don't assume that expressions are evaluated in 32-bit registers and so try to make deliberate use of integer overflow. It might not always work. The only thing NASM will guarantee is what's guaranteed by ANSI C: you always have _at least_ 32 bits to work in. NASM supports two special tokens in expressions, allowing calculations to involve the current assembly position: the `$' and `$$' tokens. `$' evaluates to the assembly position at the beginning of the line containing the expression; so you can code an infinite loop using `JMP $'. `$$' evaluates to the beginning of the current section; so you can tell how far into the section you are by using `($-$$)'. The arithmetic operators provided by NASM are listed here, in increasing order of precedence. 3.5.1 `|': Bitwise OR Operator The `|' operator gives a bitwise OR, exactly as performed by the `OR' machine instruction. Bitwise OR is the lowest-priority arithmetic operator supported by NASM. 3.5.2 `^': Bitwise XOR Operator `^' provides the bitwise XOR operation. 3.5.3 `&': Bitwise AND Operator `&' provides the bitwise AND operation. 3.5.4 `<<' and `>>': Bit Shift Operators `<<' gives a bit-shift to the left, just as it does in C. So `5<<3' evaluates to 5 times 8, or 40. `>>' gives a bit-shift to the right; in NASM, such a shift is _always_ unsigned, so that the bits shifted in from the left-hand end are filled with zero rather than a sign- extension of the previous highest bit. 3.5.5 `+' and `-': Addition and Subtraction Operators The `+' and `-' operators do perfectly ordinary addition and subtraction. 3.5.6 `*', `/', `//', `%' and `%%': Multiplication and Division `*' is the multiplication operator. `/' and `//' are both division operators: `/' is unsigned division and `//' is signed division. Similarly, `%' and `%%' provide unsigned and signed modulo operators respectively. NASM, like ANSI C, provides no guarantees about the sensible operation of the signed modulo operator. Since the `%' character is used extensively by the macro preprocessor, you should ensure that both the signed and unsigned modulo operators are followed by white space wherever they appear. 3.5.7 Unary Operators: `+', `-', `~' and `SEG' The highest-priority operators in NASM's expression grammar are those which only apply to one argument. `-' negates its operand, `+' does nothing (it's provided for symmetry with `-'), `~' computes the one's complement of its operand, and `SEG' provides the segment address of its operand (explained in more detail in section 3.6). 3.6 `SEG' and `WRT' When writing large 16-bit programs, which must be split into multiple segments, it is often necessary to be able to refer to the segment part of the address of a symbol. NASM supports the `SEG' operator to perform this function. The `SEG' operator returns the _preferred_ segment base of a symbol, defined as the segment base relative to which the offset of the symbol makes sense. So the code mov ax,seg symbol mov es,ax mov bx,symbol will load `ES:BX' with a valid pointer to the symbol `symbol'. Things can be more complex than this: since 16-bit segments and groups may overlap, you might occasionally want to refer to some symbol using a different segment base from the preferred one. NASM lets you do this, by the use of the `WRT' (With Reference To) keyword. So you can do things like mov ax,weird_seg ; weird_seg is a segment base mov es,ax mov bx,symbol wrt weird_seg to load `ES:BX' with a different, but functionally equivalent, pointer to the symbol `symbol'. NASM supports far (inter-segment) calls and jumps by means of the syntax `call segment:offset', where `segment' and `offset' both represent immediate values. So to call a far procedure, you could code either of call (seg procedure):procedure call weird_seg:(procedure wrt weird_seg) (The parentheses are included for clarity, to show the intended parsing of the above instructions. They are not necessary in practice.) NASM supports the syntax `call far procedure' as a synonym for the first of the above usages. `JMP' works identically to `CALL' in these examples. To declare a far pointer to a data item in a data segment, you must code dw symbol, seg symbol NASM supports no convenient synonym for this, though you can always invent one using the macro processor. 3.7 Critical Expressions A limitation of NASM is that it is a two-pass assembler; unlike TASM and others, it will always do exactly two assembly passes. Therefore it is unable to cope with source files that are complex enough to require three or more passes. The first pass is used to determine the size of all the assembled code and data, so that the second pass, when generating all the code, knows all the symbol addresses the code refers to. So one thing NASM can't handle is code whose size depends on the value of a symbol declared after the code in question. For example, times (label-$) db 0 label: db 'Where am I?' The argument to `TIMES' in this case could equally legally evaluate to anything at all; NASM will reject this example because it cannot tell the size of the `TIMES' line when it first sees it. It will just as firmly reject the slightly paradoxical code times (label-$+1) db 0 label: db 'NOW where am I?' in which _any_ value for the `TIMES' argument is by definition wrong! NASM rejects these examples by means of a concept called a _critical expression_, which is defined to be an expression whose value is required to be computable in the first pass, and which must therefore depend only on symbols defined before it. The argument to the `TIMES' prefix is a critical expression; for the same reason, the arguments to the `RESB' family of pseudo-instructions are also critical expressions. Critical expressions can crop up in other contexts as well: consider the following code. mov ax,symbol1 symbol1 equ symbol2 symbol2: On the first pass, NASM cannot determine the value of `symbol1', because `symbol1' is defined to be equal to `symbol2' which NASM hasn't seen yet. On the second pass, therefore, when it encounters the line `mov ax,symbol1', it is unable to generate the code for it because it still doesn't know the value of `symbol1'. On the next line, it would see the `EQU' again and be able to determine the value of `symbol1', but by then it would be too late. NASM avoids this problem by defining the right-hand side of an `EQU' statement to be a critical expression, so the definition of `symbol1' would be rejected in the first pass. There is a related issue involving forward references: consider this code fragment. mov eax,[ebx+offset] offset equ 10 NASM, on pass one, must calculate the size of the instruction `mov eax,[ebx+offset]' without knowing the value of `offset'. It has no way of knowing that `offset' is small enough to fit into a one- byte offset field and that it could therefore get away with generating a shorter form of the effective-address encoding; for all it knows, in pass one, `offset' could be a symbol in the code segment, and it might need the full four-byte form. So it is forced to compute the size of the instruction to accommodate a four-byte address part. In pass two, having made this decision, it is now forced to honour it and keep the instruction large, so the code generated in this case is not as small as it could have been. This problem can be solved by defining `offset' before using it, or by forcing byte size in the effective address by coding `[byte ebx+offset]'. 3.8 Local Labels NASM gives special treatment to symbols beginning with a period. A label beginning with a single period is treated as a _local_ label, which means that it is associated with the previous non-local label. So, for example: label1 ; some code .loop ; some more code jne .loop ret label2 ; some code .loop ; some more code jne .loop ret In the above code fragment, each `JNE' instruction jumps to the line immediately before it, because the two definitions of `.loop' are kept separate by virtue of each being associated with the previous non-local label. This form of local label handling is borrowed from the old Amiga assembler DevPac; however, NASM goes one step further, in allowing access to local labels from other parts of the code. This is achieved by means of _defining_ a local label in terms of the previous non-local label: the first definition of `.loop' above is really defining a symbol called `label1.loop', and the second defines a symbol called `label2.loop'. So, if you really needed to, you could write label3 ; some more code ; and some more jmp label1.loop Sometimes it is useful - in a macro, for instance - to be able to define a label which can be referenced from anywhere but which doesn't interfere with the normal local-label mechanism. Such a label can't be non-local because it would interfere with subsequent definitions of, and references to, local labels; and it can't be local because the macro that defined it wouldn't know the label's full name. NASM therefore introduces a third type of label, which is probably only useful in macro definitions: if a label begins with the special prefix `..@', then it does nothing to the local label mechanism. So you could code label1: ; a non-local label .local: ; this is really label1.local ..@foo: ; this is a special symbol label2: ; another non-local label .local: ; this is really label2.local jmp ..@foo ; this will jump three lines up NASM has the capacity to define other special symbols beginning with a double period: for example, `..start' is used to specify the entry point in the `obj' output format (see section 6.2.6). Chapter 4: The NASM Preprocessor -------------------------------- NASM contains a powerful macro processor, which supports conditional assembly, multi-level file inclusion, two forms of macro (single- line and multi-line), and a `context stack' mechanism for extra macro power. Preprocessor directives all begin with a `%' sign. 4.1 Single-Line Macros 4.1.1 The Normal Way: `%define' Single-line macros are defined using the `%define' preprocessor directive. The definitions work in a similar way to C; so you can do things like %define ctrl 0x1F & %define param(a,b) ((a)+(a)*(b)) mov byte [param(2,ebx)], ctrl 'D' which will expand to mov byte [(2)+(2)*(ebx)], 0x1F & 'D' When the expansion of a single-line macro contains tokens which invoke another macro, the expansion is performed at invocation time, not at definition time. Thus the code %define a(x) 1+b(x) %define b(x) 2*x mov ax,a(8) will evaluate in the expected way to `mov ax,1+2*8', even though the macro `b' wasn't defined at the time of definition of `a'. Macros defined with `%define' are case sensitive: after `%define foo bar', only `foo' will expand to `bar': `Foo' or `FOO' will not. By using `%idefine' instead of `%define' (the `i' stands for `insensitive') you can define all the case variants of a macro at once, so that `%idefine foo bar' would cause `foo', `Foo', `FOO', `fOO' and so on all to expand to `bar'. There is a mechanism which detects when a macro call has occurred as a result of a previous expansion of the same macro, to guard against circular references and infinite loops. If this happens, the preprocessor will only expand the first occurrence of the macro. Hence, if you code %define a(x) 1+a(x) mov ax,a(3) the macro `a(3)' will expand once, becoming `1+a(3)', and will then expand no further. This behaviour can be useful: see section 8.1 for an example of its use. You can overload single-line macros: if you write %define foo(x) 1+x %define foo(x,y) 1+x*y the preprocessor will be able to handle both types of macro call, by counting the parameters you pass; so `foo(3)' will become `1+3' whereas `foo(ebx,2)' will become `1+ebx*2'. However, if you define %define foo bar then no other definition of `foo' will be accepted: a macro with no parameters prohibits the definition of the same name as a macro _with_ parameters, and vice versa. This doesn't prevent single-line macros being _redefined_: you can perfectly well define a macro with %define foo bar and then re-define it later in the same source file with %define foo baz Then everywhere the macro `foo' is invoked, it will be expanded according to the most recent definition. This is particularly useful when defining single-line macros with `%assign' (see section 4.1.3). You can pre-define single-line macros using the `-d' option on the NASM command line: see section 2.1.8. 4.1.2 Undefining macros: `%undef' Single-line macros can be removed with the `%undef' command. For example, the following sequence: %define foo bar %undef foo mov eax, foo will expand to the instruction `mov eax, foo', since after `%undef' the macro `foo' is no longer defined. Macros that would otherwise be pre-defined can be undefined on the command-line using the `-u' option on the NASM command line: see section 2.1.9. 4.1.3 Preprocessor Variables: `%assign' An alternative way to define single-line macros is by means of the `%assign' command (and its case sensitivecase-insensitive counterpart `%iassign', which differs from `%assign' in exactly the same way that `%idefine' differs from `%define'). `%assign' is used to define single-line macros which take no parameters and have a numeric value. This value can be specified in the form of an expression, and it will be evaluated once, when the `%assign' directive is processed. Like `%define', macros defined using `%assign' can be re-defined later, so you can do things like %assign i i+1 to increment the numeric value of a macro. `%assign' is useful for controlling the termination of `%rep' preprocessor loops: see section 4.4 for an example of this. Another use for `%assign' is given in section 7.4 and section 8.1. The expression passed to `%assign' is a critical expression (see section 3.7), and must also evaluate to a pure number (rather than a relocatable reference such as a code or data address, or anything involving a register). 4.2 Multi-Line Macros: `%macro' Multi-line macros are much more like the type of macro seen in MASM and TASM: a multi-line macro definition in NASM looks something like this. %macro prologue 1 push ebp mov ebp,esp sub esp,%1 %endmacro This defines a C-like function prologue as a macro: so you would invoke the macro with a call such as myfunc: prologue 12 which would expand to the three lines of code myfunc: push ebp mov ebp,esp sub esp,12 The number `1' after the macro name in the `%macro' line defines the number of parameters the macro `prologue' expects to receive. The use of `%1' inside the macro definition refers to the first parameter to the macro call. With a macro taking more than one parameter, subsequent parameters would be referred to as `%2', `%3' and so on. Multi-line macros, like single-line macros, are case-sensitive, unless you define them using the alternative directive `%imacro'. If you need to pass a comma as _part_ of a parameter to a multi-line macro, you can do that by enclosing the entire parameter in braces. So you could code things like %macro silly 2 %2: db %1 %endmacro silly 'a', letter_a ; letter_a: db 'a' silly 'ab', string_ab ; string_ab: db 'ab' silly {13,10}, crlf ; crlf: db 13,10 4.2.1 Overloading Multi-Line Macros As with single-line macros, multi-line macros can be overloaded by defining the same macro name several times with different numbers of parameters. This time, no exception is made for macros with no parameters at all. So you could define %macro prologue 0 push ebp mov ebp,esp %endmacro to define an alternative form of the function prologue which allocates no local stack space. Sometimes, however, you might want to `overload' a machine instruction; for example, you might want to define %macro push 2 push %1 push %2 %endmacro so that you could code push ebx ; this line is not a macro call push eax,ecx ; but this one is Ordinarily, NASM will give a warning for the first of the above two lines, since `push' is now defined to be a macro, and is being invoked with a number of parameters for which no definition has been given. The correct code will still be generated, but the assembler will give a warning. This warning can be disabled by the use of the `-w-macro-params' command-line option (see section 2.1.12). 4.2.2 Macro-Local Labels NASM allows you to define labels within a multi-line macro definition in such a way as to make them local to the macro call: so calling the same macro multiple times will use a different label each time. You do this by prefixing `%%' to the label name. So you can invent an instruction which executes a `RET' if the `Z' flag is set by doing this: %macro retz 0 jnz %%skip ret %%skip: %endmacro You can call this macro as many times as you want, and every time you call it NASM will make up a different `real' name to substitute for the label `%%skip'. The names NASM invents are of the form `..@2345.skip', where the number 2345 changes with every macro call. The `..@' prefix prevents macro-local labels from interfering with the local label mechanism, as described in section 3.8. You should avoid defining your own labels in this form (the `..@' prefix, then a number, then another period) in case they interfere with macro- local labels. 4.2.3 Greedy Macro Parameters Occasionally it is useful to define a macro which lumps its entire command line into one parameter definition, possibly after extracting one or two smaller parameters from the front. An example might be a macro to write a text string to a file in MS-DOS, where you might want to be able to write writefile [filehandle],"hello, world",13,10 NASM allows you to define the last parameter of a macro to be _greedy_, meaning that if you invoke the macro with more parameters than it expects, all the spare parameters get lumped into the last defined one along with the separating commas. So if you code: %macro writefile 2+ jmp %%endstr %%str: db %2 %%endstr: mov dx,%%str mov cx,%%endstr-%%str mov bx,%1 mov ah,0x40 int 0x21 %endmacro then the example call to `writefile' above will work as expected: the text before the first comma, `[filehandle]', is used as the first macro parameter and expanded when `%1' is referred to, and all the subsequent text is lumped into `%2' and placed after the `db'. The greedy nature of the macro is indicated to NASM by the use of the `+' sign after the parameter count on the `%macro' line. If you define a greedy macro, you are effectively telling NASM how it should expand the macro given _any_ number of parameters from the actual number specified up to infinity; in this case, for example, NASM now knows what to do when it sees a call to `writefile' with 2, 3, 4 or more parameters. NASM will take this into account when overloading macros, and will not allow you to define another form of `writefile' taking 4 parameters (for example). Of course, the above macro could have been implemented as a non- greedy macro, in which case the call to it would have had to look like writefile [filehandle], {"hello, world",13,10} NASM provides both mechanisms for putting commas in macro parameters, and you choose which one you prefer for each macro definition. See section 5.2.1 for a better way to write the above macro. 4.2.4 Default Macro Parameters NASM also allows you to define a multi-line macro with a _range_ of allowable parameter counts. If you do this, you can specify defaults for omitted parameters. So, for example: %macro die 0-1 "Painful program death has occurred." writefile 2,%1 mov ax,0x4c01 int 0x21 %endmacro This macro (which makes use of the `writefile' macro defined in section 4.2.3) can be called with an explicit error message, which it will display on the error output stream before exiting, or it can be called with no parameters, in which case it will use the default error message supplied in the macro definition. In general, you supply a minimum and maximum number of parameters for a macro of this type; the minimum number of parameters are then required in the macro call, and then you provide defaults for the optional ones. So if a macro definition began with the line %macro foobar 1-3 eax,[ebx+2] then it could be called with between one and three parameters, and `%1' would always be taken from the macro call. `%2', if not specified by the macro call, would default to `eax', and `%3' if not specified would default to `[ebx+2]'. You may omit parameter defaults from the macro definition, in which case the parameter default is taken to be blank. This can be useful for macros which can take a variable number of parameters, since the `%0' token (see section 4.2.5) allows you to determine how many parameters were really passed to the macro call. This defaulting mechanism can be combined with the greedy-parameter mechanism; so the `die' macro above could be made more powerful, and more useful, by changing the first line of the definition to %macro die 0-1+ "Painful program death has occurred.",13,10 The maximum parameter count can be infinite, denoted by `*'. In this case, of course, it is impossible to provide a _full_ set of default parameters. Examples of this usage are shown in section 4.2.6. 4.2.5 `%0': Macro Parameter Counter For a macro which can take a variable number of parameters, the parameter reference `%0' will return a numeric constant giving the number of parameters passed to the macro. This can be used as an argument to `%rep' (see section 4.4) in order to iterate through all the parameters of a macro. Examples are given in section 4.2.6. 4.2.6 `%rotate': Rotating Macro Parameters Unix shell programmers will be familiar with the `shift' shell command, which allows the arguments passed to a shell script (referenced as `$1', `$2' and so on) to be moved left by one place, so that the argument previously referenced as `$2' becomes available as `$1', and the argument previously referenced as `$1' is no longer available at all. NASM provides a similar mechanism, in the form of `%rotate'. As its name suggests, it differs from the Unix `shift' in that no parameters are lost: parameters rotated off the left end of the argument list reappear on the right, and vice versa. `%rotate' is invoked with a single numeric argument (which may be an expression). The macro parameters are rotated to the left by that many places. If the argument to `%rotate' is negative, the macro parameters are rotated to the right. So a pair of macros to save and restore a set of registers might work as follows: %macro multipush 1-* %rep %0 push %1 %rotate 1 %endrep %endmacro This macro invokes the `PUSH' instruction on each of its arguments in turn, from left to right. It begins by pushing its first argument, `%1', then invokes `%rotate' to move all the arguments one place to the left, so that the original second argument is now available as `%1'. Repeating this procedure as many times as there were arguments (achieved by supplying `%0' as the argument to `%rep') causes each argument in turn to be pushed. Note also the use of `*' as the maximum parameter count, indicating that there is no upper limit on the number of parameters you may supply to the `multipush' macro. It would be convenient, when using this macro, to have a `POP' equivalent, which _didn't_ require the arguments to be given in reverse order. Ideally, you would write the `multipush' macro call, then cut-and-paste the line to where the pop needed to be done, and change the name of the called macro to `multipop', and the macro would take care of popping the registers in the opposite order from the one in which they were pushed. This can be done by the following definition: %macro multipop 1-* %rep %0 %rotate -1 pop %1 %endrep %endmacro This macro begins by rotating its arguments one place to the _right_, so that the original _last_ argument appears as `%1'. This is then popped, and the arguments are rotated right again, so the second-to-last argument becomes `%1'. Thus the arguments are iterated through in reverse order. 4.2.7 Concatenating Macro Parameters NASM can concatenate macro parameters on to other text surrounding them. This allows you to declare a family of symbols, for example, in a macro definition. If, for example, you wanted to generate a table of key codes along with offsets into the table, you could code something like %macro keytab_entry 2 keypos%1 equ $-keytab db %2 %endmacro keytab: keytab_entry F1,128+1 keytab_entry F2,128+2 keytab_entry Return,13 which would expand to keytab: keyposF1 equ $-keytab db 128+1 keyposF2 equ $-keytab db 128+2 keyposReturn equ $-keytab db 13 You can just as easily concatenate text on to the other end of a macro parameter, by writing `%1foo'. If you need to append a _digit_ to a macro parameter, for example defining labels `foo1' and `foo2' when passed the parameter `foo', you can't code `%11' because that would be taken as the eleventh macro parameter. Instead, you must code `%{1}1', which will separate the first `1' (giving the number of the macro parameter) from the second (literal text to be concatenated to the parameter). This concatenation can also be applied to other preprocessor in-line objects, such as macro-local labels (section 4.2.2) and context- local labels (section 4.6.2). In all cases, ambiguities in syntax can be resolved by enclosing everything after the `%' sign and before the literal text in braces: so `%{%foo}bar' concatenates the text `bar' to the end of the real name of the macro-local label `%%foo'. (This is unnecessary, since the form NASM uses for the real names of macro-local labels means that the two usages `%{%foo}bar' and `%%foobar' would both expand to the same thing anyway; nevertheless, the capability is there.) 4.2.8 Condition Codes as Macro Parameters NASM can give special treatment to a macro parameter which contains a condition code. For a start, you can refer to the macro parameter `%1' by means of the alternative syntax `%+1', which informs NASM that this macro parameter is supposed to contain a condition code, and will cause the preprocessor to report an error message if the macro is called with a parameter which is _not_ a valid condition code. Far more usefully, though, you can refer to the macro parameter by means of `%-1', which NASM will expand as the _inverse_ condition code. So the `retz' macro defined in section 4.2.2 can be replaced by a general conditional-return macro like this: %macro retc 1 j%-1 %%skip ret %%skip: %endmacro This macro can now be invoked using calls like `retc ne', which will cause the conditional-jump instruction in the macro expansion to come out as `JE', or `retc po' which will make the jump a `JPE'. The `%+1' macro-parameter reference is quite happy to interpret the arguments `CXZ' and `ECXZ' as valid condition codes; however, `%-1' will report an error if passed either of these, because no inverse condition code exists. 4.2.9 Disabling Listing Expansion When NASM is generating a listing file from your program, it will generally expand multi-line macros by means of writing the macro call and then listing each line of the expansion. This allows you to see which instructions in the macro expansion are generating what code; however, for some macros this clutters the listing up unnecessarily. NASM therefore provides the `.nolist' qualifier, which you can include in a macro definition to inhibit the expansion of the macro in the listing file. The `.nolist' qualifier comes directly after the number of parameters, like this: %macro foo 1.nolist Or like this: %macro bar 1-5+.nolist a,b,c,d,e,f,g,h 4.3 Conditional Assembly Similarly to the C preprocessor, NASM allows sections of a source file to be assembled only if certain conditions are met. The general syntax of this feature looks like this: %if ; some code which only appears if is met %elif ; only appears if is not met but is %else ; this appears if neither nor was met %endif The `%else' clause is optional, as is the `%elif' clause. You can have more than one `%elif' clause as well. 4.3.1 `%ifdef': Testing Single-Line Macro Existence Beginning a conditional-assembly block with the line `%ifdef MACRO' will assemble the subsequent code if, and only if, a single-line macro called `MACRO' is defined. If not, then the `%elif' and `%else' blocks (if any) will be processed instead. For example, when debugging a program, you might want to write code such as ; perform some function %ifdef DEBUG writefile 2,"Function performed successfully",13,10 %endif ; go and do something else Then you could use the command-line option `-dDEBUG' to create a version of the program which produced debugging messages, and remove the option to generate the final release version of the program. You can test for a macro _not_ being defined by using `%ifndef' instead of `%ifdef'. You can also test for macro definitions in `%elif' blocks by using `%elifdef' and `%elifndef'. 4.3.2 `%ifctx': Testing the Context Stack The conditional-assembly construct `%ifctx ctxname' will cause the subsequent code to be assembled if and only if the top context on the preprocessor's context stack has the name `ctxname'. As with `%ifdef', the inverse and `%elif' forms `%ifnctx', `%elifctx' and `%elifnctx' are also supported. For more details of the context stack, see section 4.6. For a sample use of `%ifctx', see section 4.6.5. 4.3.3 `%if': Testing Arbitrary Numeric Expressions The conditional-assembly construct `%if expr' will cause the subsequent code to be assembled if and only if the value of the numeric expression `expr' is non-zero. An example of the use of this feature is in deciding when to break out of a `%rep' preprocessor loop: see section 4.4 for a detailed example. The expression given to `%if', and its counterpart `%elif', is a critical expression (see section 3.7). `%if' extends the normal NASM expression syntax, by providing a set of relational operators which are not normally available in expressions. The operators `=', `<', `>', `<=', `>=' and `<>' test equality, less-than, greater-than, less-or-equal, greater-or-equal and not-equal respectively. The C-like forms `==' and `!=' are supported as alternative forms of `=' and `<>'. In addition, low- priority logical operators `&&', `^^' and `||' are provided, supplying logical AND, logical XOR and logical OR. These work like the C logical operators (although C has no logical XOR), in that they always return either 0 or 1, and treat any non-zero input as 1 (so that `^^', for example, returns 1 if exactly one of its inputs is zero, and 0 otherwise). The relational operators also return 1 for true and 0 for false. 4.3.4 `%ifidn' and `%ifidni': Testing Exact Text Identity The construct `%ifidn text1,text2' will cause the subsequent code to be assembled if and only if `text1' and `text2', after expanding single-line macros, are identical pieces of text. Differences in white space are not counted. `%ifidni' is similar to `%ifidn', but is case-insensitive. For example, the following macro pushes a register or number on the stack, and allows you to treat `IP' as a real register: %macro pushparam 1 %ifidni %1,ip call %%label %%label: %else push %1 %endif %endmacro Like most other `%if' constructs, `%ifidn' has a counterpart `%elifidn', and negative forms `%ifnidn' and `%elifnidn'. Similarly, `%ifidni' has counterparts `%elifidni', `%ifnidni' and `%elifnidni'. 4.3.5 `%ifid', `%ifnum', `%ifstr': Testing Token Types Some macros will want to perform different tasks depending on whether they are passed a number, a string, or an identifier. For example, a string output macro might want to be able to cope with being passed either a string constant or a pointer to an existing string. The conditional assembly construct `%ifid', taking one parameter (which may be blank), assembles the subsequent code if and only if the first token in the parameter exists and is an identifier. `%ifnum' works similarly, but tests for the token being a numeric constant; `%ifstr' tests for it being a string. For example, the `writefile' macro defined in section 4.2.3 can be extended to take advantage of `%ifstr' in the following fashion: %macro writefile 2-3+ %ifstr %2 jmp %%endstr %if %0 = 3 %%str: db %2,%3 %else %%str: db %2 %endif %%endstr: mov dx,%%str mov cx,%%endstr-%%str %else mov dx,%2 mov cx,%3 %endif mov bx,%1 mov ah,0x40 int 0x21 %endmacro Then the `writefile' macro can cope with being called in either of the following two ways: writefile [file], strpointer, length writefile [file], "hello", 13, 10 In the first, `strpointer' is used as the address of an already- declared string, and `length' is used as its length; in the second, a string is given to the macro, which therefore declares it itself and works out the address and length for itself. Note the use of `%if' inside the `%ifstr': this is to detect whether the macro was passed two arguments (so the string would be a single string constant, and `db %2' would be adequate) or more (in which case, all but the first two would be lumped together into `%3', and `db %2,%3' would be required). The usual `%elifXXX', `%ifnXXX' and `%elifnXXX' versions exist for each of `%ifid', `%ifnum' and `%ifstr'. 4.3.6 `%error': Reporting User-Defined Errors The preprocessor directive `%error' will cause NASM to report an error if it occurs in assembled code. So if other users are going to try to assemble your source files, you can ensure that they define the right macros by means of code like this: %ifdef SOME_MACRO ; do some setup %elifdef SOME_OTHER_MACRO ; do some different setup %else %error Neither SOME_MACRO nor SOME_OTHER_MACRO was defined. %endif Then any user who fails to understand the way your code is supposed to be assembled will be quickly warned of their mistake, rather than having to wait until the program crashes on being run and then not knowing what went wrong. 4.4 Preprocessor Loops: `%rep' NASM's `TIMES' prefix, though useful, cannot be used to invoke a multi-line macro multiple times, because it is processed by NASM after macros have already been expanded. Therefore NASM provides another form of loop, this time at the preprocessor level: `%rep'. The directives `%rep' and `%endrep' (`%rep' takes a numeric argument, which can be an expression; `%endrep' takes no arguments) can be used to enclose a chunk of code, which is then replicated as many times as specified by the preprocessor: %assign i 0 %rep 64 inc word [table+2*i] %assign i i+1 %endrep This will generate a sequence of 64 `INC' instructions, incrementing every word of memory from `[table]' to `[table+126]'. For more complex termination conditions, or to break out of a repeat loop part way along, you can use the `%exitrep' directive to terminate the loop, like this: fibonacci: %assign i 0 %assign j 1 %rep 100 %if j > 65535 %exitrep %endif dw j %assign k j+i %assign i j %assign j k %endrep fib_number equ ($-fibonacci)/2 This produces a list of all the Fibonacci numbers that will fit in 16 bits. Note that a maximum repeat count must still be given to `%rep'. This is to prevent the possibility of NASM getting into an infinite loop in the preprocessor, which (on multitasking or multi- user systems) would typically cause all the system memory to be gradually used up and other applications to start crashing. 4.5 Including Other Files Using, once again, a very similar syntax to the C preprocessor, NASM's preprocessor lets you include other source files into your code. This is done by the use of the `%include' directive: %include "macros.mac" will include the contents of the file `macros.mac' into the source file containing the `%include' directive. Include files are searched for in the current directory (the directory you're in when you run NASM, as opposed to the location of the NASM executable or the location of the source file), plus any directories specified on the NASM command line using the `-i' option. The standard C idiom for preventing a file being included more than once is just as applicable in NASM: if the file `macros.mac' has the form %ifndef MACROS_MAC %define MACROS_MAC ; now define some macros %endif then including the file more than once will not cause errors, because the second time the file is included nothing will happen because the macro `MACROS_MAC' will already be defined. You can force a file to be included even if there is no `%include' directive that explicitly includes it, by using the `-p' option on the NASM command line (see section 2.1.7). 4.6 The Context Stack Having labels that are local to a macro definition is sometimes not quite powerful enough: sometimes you want to be able to share labels between several macro calls. An example might be a `REPEAT' ... `UNTIL' loop, in which the expansion of the `REPEAT' macro would need to be able to refer to a label which the `UNTIL' macro had defined. However, for such a macro you would also want to be able to nest these loops. NASM provides this level of power by means of a _context stack_. The preprocessor maintains a stack of _contexts_, each of which is characterised by a name. You add a new context to the stack using the `%push' directive, and remove one using `%pop'. You can define labels that are local to a particular context on the stack. 4.6.1 `%push' and `%pop': Creating and Removing Contexts The `%push' directive is used to create a new context and place it on the top of the context stack. `%push' requires one argument, which is the name of the context. For example: %push foobar This pushes a new context called `foobar' on the stack. You can have several contexts on the stack with the same name: they can still be distinguished. The directive `%pop', requiring no arguments, removes the top context from the context stack and destroys it, along with any labels associated with it. 4.6.2 Context-Local Labels Just as the usage `%%foo' defines a label which is local to the particular macro call in which it is used, the usage `%$foo' is used to define a label which is local to the context on the top of the context stack. So the `REPEAT' and `UNTIL' example given above could be implemented by means of: %macro repeat 0 %push repeat %$begin: %endmacro %macro until 1 j%-1 %$begin %pop %endmacro and invoked by means of, for example, mov cx,string repeat add cx,3 scasb until e which would scan every fourth byte of a string in search of the byte in `AL'. If you need to define, or access, labels local to the context _below_ the top one on the stack, you can use `%$$foo', or `%$$$foo' for the context below that, and so on. 4.6.3 Context-Local Single-Line Macros NASM also allows you to define single-line macros which are local to a particular context, in just the same way: %define %$localmac 3 will define the single-line macro `%$localmac' to be local to the top context on the stack. Of course, after a subsequent `%push', it can then still be accessed by the name `%$$localmac'. 4.6.4 `%repl': Renaming a Context If you need to change the name of the top context on the stack (in order, for example, to have it respond differently to `%ifctx'), you can execute a `%pop' followed by a `%push'; but this will have the side effect of destroying all context-local labels and macros associated with the context that was just popped. NASM provides the directive `%repl', which _replaces_ a context with a different name, without touching the associated macros and labels. So you could replace the destructive code %pop %push newname with the non-destructive version `%repl newname'. 4.6.5 Example Use of the Context Stack: Block IFs This example makes use of almost all the context-stack features, including the conditional-assembly construct `%ifctx', to implement a block IF statement as a set of macros. %macro if 1 %push if j%-1 %$ifnot %endmacro %macro else 0 %ifctx if %repl else jmp %$ifend %$ifnot: %else %error "expected `if' before `else'" %endif %endmacro %macro endif 0 %ifctx if %$ifnot: %pop %elifctx else %$ifend: %pop %else %error "expected `if' or `else' before `endif'" %endif %endmacro This code is more robust than the `REPEAT' and `UNTIL' macros given in section 4.6.2, because it uses conditional assembly to check that the macros are issued in the right order (for example, not calling `endif' before `if') and issues a `%error' if they're not. In addition, the `endif' macro has to be able to cope with the two distinct cases of either directly following an `if', or following an `else'. It achieves this, again, by using conditional assembly to do different things depending on whether the context on top of the stack is `if' or `else'. The `else' macro has to preserve the context on the stack, in order to have the `%$ifnot' referred to by the `if' macro be the same as the one defined by the `endif' macro, but has to change the context's name so that `endif' will know there was an intervening `else'. It does this by the use of `%repl'. A sample usage of these macros might look like: cmp ax,bx if ae cmp bx,cx if ae mov ax,cx else mov ax,bx endif else cmp ax,cx if ae mov ax,cx endif endif The block-`IF' macros handle nesting quite happily, by means of pushing another context, describing the inner `if', on top of the one describing the outer `if'; thus `else' and `endif' always refer to the last unmatched `if' or `else'. 4.7 Standard Macros NASM defines a set of standard macros, which are already defined when it starts to process any source file. If you really need a program to be assembled with no pre-defined macros, you can use the `%clear' directive to empty the preprocessor of everything. Most user-level assembler directives (see chapter 5) are implemented as macros which invoke primitive directives; these are described in chapter 5. The rest of the standard macro set is described here. 4.7.1 `__NASM_MAJOR__' and `__NASM_MINOR__': NASM Version The single-line macros `__NASM_MAJOR__' and `__NASM_MINOR__' expand to the major and minor parts of the version number of NASM being used. So, under NASM 0.96 for example, `__NASM_MAJOR__' would be defined to be 0 and `__NASM_MINOR__' would be defined as 96. 4.7.2 `__FILE__' and `__LINE__': File Name and Line Number Like the C preprocessor, NASM allows the user to find out the file name and line number containing the current instruction. The macro `__FILE__' expands to a string constant giving the name of the current input file (which may change through the course of assembly if `%include' directives are used), and `__LINE__' expands to a numeric constant giving the current line number in the input file. These macros could be used, for example, to communicate debugging information to a macro, since invoking `__LINE__' inside a macro definition (either single-line or multi-line) will return the line number of the macro _call_, rather than _definition_. So to determine where in a piece of code a crash is occurring, for example, one could write a routine `stillhere', which is passed a line number in `EAX' and outputs something like `line 155: still here'. You could then write a macro %macro notdeadyet 0 push eax mov eax,__LINE__ call stillhere pop eax %endmacro and then pepper your code with calls to `notdeadyet' until you find the crash point. 4.7.3 `STRUC' and `ENDSTRUC': Declaring Structure Data Types The core of NASM contains no intrinsic means of defining data structures; instead, the preprocessor is sufficiently powerful that data structures can be implemented as a set of macros. The macros `STRUC' and `ENDSTRUC' are used to define a structure data type. `STRUC' takes one parameter, which is the name of the data type. This name is defined as a symbol with the value zero, and also has the suffix `_size' appended to it and is then defined as an `EQU' giving the size of the structure. Once `STRUC' has been issued, you are defining the structure, and should define fields using the `RESB' family of pseudo-instructions, and then invoke `ENDSTRUC' to finish the definition. For example, to define a structure called `mytype' containing a longword, a word, a byte and a string of bytes, you might code struc mytype mt_long: resd 1 mt_word: resw 1 mt_byte: resb 1 mt_str: resb 32 endstruc The above code defines six symbols: `mt_long' as 0 (the offset from the beginning of a `mytype' structure to the longword field), `mt_word' as 4, `mt_byte' as 6, `mt_str' as 7, `mytype_size' as 39, and `mytype' itself as zero. The reason why the structure type name is defined at zero is a side effect of allowing structures to work with the local label mechanism: if your structure members tend to have the same names in more than one structure, you can define the above structure like this: struc mytype .long: resd 1 .word: resw 1 .byte: resb 1 .str: resb 32 endstruc This defines the offsets to the structure fields as `mytype.long', `mytype.word', `mytype.byte' and `mytype.str'. NASM, since it has no _intrinsic_ structure support, does not support any form of period notation to refer to the elements of a structure once you have one (except the above local-label notation), so code such as `mov ax,[mystruc.mt_word]' is not valid. `mt_word' is a constant just like any other constant, so the correct syntax is `mov ax,[mystruc+mt_word]' or `mov ax,[mystruc+mytype.word]'. 4.7.4 `ISTRUC', `AT' and `IEND': Declaring Instances of Structures Having defined a structure type, the next thing you typically want to do is to declare instances of that structure in your data segment. NASM provides an easy way to do this in the `ISTRUC' mechanism. To declare a structure of type `mytype' in a program, you code something like this: mystruc: istruc mytype at mt_long, dd 123456 at mt_word, dw 1024 at mt_byte, db 'x' at mt_str, db 'hello, world', 13, 10, 0 iend The function of the `AT' macro is to make use of the `TIMES' prefix to advance the assembly position to the correct point for the specified structure field, and then to declare the specified data. Therefore the structure fields must be declared in the same order as they were specified in the structure definition. If the data to go in a structure field requires more than one source line to specify, the remaining source lines can easily come after the `AT' line. For example: at mt_str, db 123,134,145,156,167,178,189 db 190,100,0 Depending on personal taste, you can also omit the code part of the `AT' line completely, and start the structure field on the next line: at mt_str db 'hello, world' db 13,10,0 4.7.5 `ALIGN' and `ALIGNB': Data Alignment The `ALIGN' and `ALIGNB' macros provides a convenient way to align code or data on a word, longword, paragraph or other boundary. (Some assemblers call this directive `EVEN'.) The syntax of the `ALIGN' and `ALIGNB' macros is align 4 ; align on 4-byte boundary align 16 ; align on 16-byte boundary align 8,db 0 ; pad with 0s rather than NOPs align 4,resb 1 ; align to 4 in the BSS alignb 4 ; equivalent to previous line Both macros require their first argument to be a power of two; they both compute the number of additional bytes required to bring the length of the current section up to a multiple of that power of two, and then apply the `TIMES' prefix to their second argument to perform the alignment. If the second argument is not specified, the default for `ALIGN' is `NOP', and the default for `ALIGNB' is `RESB 1'. So if the second argument is specified, the two macros are equivalent. Normally, you can just use `ALIGN' in code and data sections and `ALIGNB' in BSS sections, and never need the second argument except for special purposes. `ALIGN' and `ALIGNB', being simple macros, perform no error checking: they cannot warn you if their first argument fails to be a power of two, or if their second argument generates more than one byte of code. In each of these cases they will silently do the wrong thing. `ALIGNB' (or `ALIGN' with a second argument of `RESB 1') can be used within structure definitions: struc mytype2 mt_byte: resb 1 alignb 2 mt_word: resw 1 alignb 4 mt_long: resd 1 mt_str: resb 32 endstruc This will ensure that the structure members are sensibly aligned relative to the base of the structure. A final caveat: `ALIGN' and `ALIGNB' work relative to the beginning of the _section_, not the beginning of the address space in the final executable. Aligning to a 16-byte boundary when the section you're in is only guaranteed to be aligned to a 4-byte boundary, for example, is a waste of effort. Again, NASM does not check that the section's alignment characteristics are sensible for the use of `ALIGN' or `ALIGNB'. Chapter 5: Assembler Directives ------------------------------- NASM, though it attempts to avoid the bureaucracy of assemblers like MASM and TASM, is nevertheless forced to support a _few_ directives. These are described in this chapter. NASM's directives come in two types: user-level directives_user-level_ directives and primitive directives_primitive_ directives. Typically, each directive has a user-level form and a primitive form. In almost all cases, we recommend that users use the user-level forms of the directives, which are implemented as macros which call the primitive forms. Primitive directives are enclosed in square brackets; user-level directives are not. In addition to the universal directives described in this chapter, each object file format can optionally supply extra directives in order to control particular features of that file format. These format-specific directives_format-specific_ directives are documented along with the formats that implement them, in chapter 6. 5.1 `BITS': Specifying Target Processor Mode The `BITS' directive specifies whether NASM should generate code designed to run on a processor operating in 16-bit mode, or code designed to run on a processor operating in 32-bit mode. The syntax is `BITS 16' or `BITS 32'. In most cases, you should not need to use `BITS' explicitly. The `aout', `coff', `elf' and `win32' object formats, which are designed for use in 32-bit operating systems, all cause NASM to select 32-bit mode by default. The `obj' object format allows you to specify each segment you define as either `USE16' or `USE32', and NASM will set its operating mode accordingly, so the use of the `BITS' directive is once again unnecessary. The most likely reason for using the `BITS' directive is to write 32-bit code in a flat binary file; this is because the `bin' output format defaults to 16-bit mode in anticipation of it being used most frequently to write DOS `.COM' programs, DOS `.SYS' device drivers and boot loader software. You do _not_ need to specify `BITS 32' merely in order to use 32-bit instructions in a 16-bit DOS program; if you do, the assembler will generate incorrect code because it will be writing code targeted at a 32-bit platform, to be run on a 16-bit one. When NASM is in `BITS 16' state, instructions which use 32-bit data are prefixed with an 0x66 byte, and those referring to 32-bit addresses have an 0x67 prefix. In `BITS 32' state, the reverse is true: 32-bit instructions require no prefixes, whereas instructions using 16-bit data need an 0x66 and those working in 16-bit addresses need an 0x67. The `BITS' directive has an exactly equivalent primitive form, `[BITS 16]' and `[BITS 32]'. The user-level form is a macro which has no function other than to call the primitive form. 5.2 `SECTION' or `SEGMENT': Changing and Defining Sections The `SECTION' directive (`SEGMENT' is an exactly equivalent synonym) changes which section of the output file the code you write will be assembled into. In some object file formats, the number and names of sections are fixed; in others, the user may make up as many as they wish. Hence `SECTION' may sometimes give an error message, or may define a new section, if you try to switch to a section that does not (yet) exist. The Unix object formats, and the `bin' object format, all support the standardised section names `.text', `.data' and `.bss' for the code, data and uninitialised-data sections. The `obj' format, by contrast, does not recognise these section names as being special, and indeed will strip off the leading period of any section name that has one. 5.2.1 The `__SECT__' Macro The `SECTION' directive is unusual in that its user-level form functions differently from its primitive form. The primitive form, `[SECTION xyz]', simply switches the current target section to the one given. The user-level form, `SECTION xyz', however, first defines the single-line macro `__SECT__' to be the primitive `[SECTION]' directive which it is about to issue, and then issues it. So the user-level directive SECTION .text expands to the two lines %define __SECT__ [SECTION .text] [SECTION .text] Users may find it useful to make use of this in their own macros. For example, the `writefile' macro defined in section 4.2.3 can be usefully rewritten in the following more sophisticated form: %macro writefile 2+ [section .data] %%str: db %2 %%endstr: __SECT__ mov dx,%%str mov cx,%%endstr-%%str mov bx,%1 mov ah,0x40 int 0x21 %endmacro This form of the macro, once passed a string to output, first switches temporarily to the data section of the file, using the primitive form of the `SECTION' directive so as not to modify `__SECT__'. It then declares its string in the data section, and then invokes `__SECT__' to switch back to _whichever_ section the user was previously working in. It thus avoids the need, in the previous version of the macro, to include a `JMP' instruction to jump over the data, and also does not fail if, in a complicated `OBJ' format module, the user could potentially be assembling the code in any of several separate code sections. 5.3 `ABSOLUTE': Defining Absolute Labels The `ABSOLUTE' directive can be thought of as an alternative form of `SECTION': it causes the subsequent code to be directed at no physical section, but at the hypothetical section starting at the given absolute address. The only instructions you can use in this mode are the `RESB' family. `ABSOLUTE' is used as follows: absolute 0x1A kbuf_chr resw 1 kbuf_free resw 1 kbuf resw 16 This example describes a section of the PC BIOS data area, at segment address 0x40: the above code defines `kbuf_chr' to be 0x1A, `kbuf_free' to be 0x1C, and `kbuf' to be 0x1E. The user-level form of `ABSOLUTE', like that of `SECTION', redefines the `__SECT__' macro when it is invoked. `STRUC' and `ENDSTRUC' are defined as macros which use `ABSOLUTE' (and also `__SECT__'). `ABSOLUTE' doesn't have to take an absolute constant as an argument: it can take an expression (actually, a critical expression: see section 3.7) and it can be a value in a segment. For example, a TSR can re-use its setup code as run-time BSS like this: org 100h ; it's a .COM program jmp setup ; setup code comes last ; the resident part of the TSR goes here setup: ; now write the code that installs the TSR here absolute setup runtimevar1 resw 1 runtimevar2 resd 20 tsr_end: This defines some variables `on top of' the setup code, so that after the setup has finished running, the space it took up can be re-used as data storage for the running TSR. The symbol `tsr_end' can be used to calculate the total size of the part of the TSR that needs to be made resident. 5.4 `EXTERN': Importing Symbols from Other Modules `EXTERN' is similar to the MASM directive `EXTRN' and the C keyword `extern': it is used to declare a symbol which is not defined anywhere in the module being assembled, but is assumed to be defined in some other module and needs to be referred to by this one. Not every object-file format can support external variables: the `bin' format cannot. The `EXTERN' directive takes as many arguments as you like. Each argument is the name of a symbol: extern _printf extern _sscanf,_fscanf Some object-file formats provide extra features to the `EXTERN' directive. In all cases, the extra features are used by suffixing a colon to the symbol name followed by object-format specific text. For example, the `obj' format allows you to declare that the default segment base of an external should be the group `dgroup' by means of the directive extern _variable:wrt dgroup The primitive form of `EXTERN' differs from the user-level form only in that it can take only one argument at a time: the support for multiple arguments is implemented at the preprocessor level. You can declare the same variable as `EXTERN' more than once: NASM will quietly ignore the second and later redeclarations. You can't declare a variable as `EXTERN' as well as something else, though. 5.5 `GLOBAL': Exporting Symbols to Other Modules `GLOBAL' is the other end of `EXTERN': if one module declares a symbol as `EXTERN' and refers to it, then in order to prevent linker errors, some other module must actually _define_ the symbol and declare it as `GLOBAL'. Some assemblers use the name `PUBLIC' for this purpose. The `GLOBAL' directive applying to a symbol must appear _before_ the definition of the symbol. `GLOBAL' uses the same syntax as `EXTERN', except that it must refer to symbols which _are_ defined in the same module as the `GLOBAL' directive. For example: global _main _main: ; some code `GLOBAL', like `EXTERN', allows object formats to define private extensions by means of a colon. The `elf' object format, for example, lets you specify whether global data items are functions or data: global hashlookup:function, hashtable:data Like `EXTERN', the primitive form of `GLOBAL' differs from the user- level form only in that it can take only one argument at a time. 5.6 `COMMON': Defining Common Data Areas The `COMMON' directive is used to declare _common variables_. A common variable is much like a global variable declared in the uninitialised data section, so that common intvar 4 is similar in function to global intvar section .bss intvar resd 1 The difference is that if more than one module defines the same common variable, then at link time those variables will be _merged_, and references to `intvar' in all modules will point at the same piece of memory. Like `GLOBAL' and `EXTERN', `COMMON' supports object-format specific extensions. For example, the `obj' format allows common variables to be NEAR or FAR, and the `elf' format allows you to specify the alignment requirements of a common variable: common commvar 4:near ; works in OBJ common intarray 100:4 ; works in ELF: 4 byte aligned Once again, like `EXTERN' and `GLOBAL', the primitive form of `COMMON' differs from the user-level form only in that it can take only one argument at a time. Chapter 6: Output Formats ------------------------- NASM is a portable assembler, designed to be able to compile on any ANSI C-supporting platform and produce output to run on a variety of Intel x86 operating systems. For this reason, it has a large number of available output formats, selected using the `-f' option on the NASM command line. Each of these formats, along with its extensions to the base NASM syntax, is detailed in this chapter. As stated in section 2.1.1, NASM chooses a default name for your output file based on the input file name and the chosen output format. This will be generated by removing the extension (`.asm', `.s', or whatever you like to use) from the input file name, and substituting an extension defined by the output format. The extensions are given with each format below. 6.1 `bin': Flat-Form Binary Output The `bin' format does not produce object files: it generates nothing in the output file except the code you wrote. Such `pure binary' files are used by MS-DOS: `.COM' executables and `.SYS' device drivers are pure binary files. Pure binary output is also useful for operating-system and boot loader development. `bin' supports the three standardised section names `.text', `.data' and `.bss' only. The file NASM outputs will contain the contents of the `.text' section first, followed by the contents of the `.data' section, aligned on a four-byte boundary. The `.bss' section is not stored in the output file at all, but is assumed to appear directly after the end of the `.data' section, again aligned on a four-byte boundary. If you specify no explicit `SECTION' directive, the code you write will be directed by default into the `.text' section. Using the `bin' format puts NASM by default into 16-bit mode (see section 5.1). In order to use `bin' to write 32-bit code such as an OS kernel, you need to explicitly issue the `BITS 32' directive. `bin' has no default output file name extension: instead, it leaves your file name as it is once the original extension has been removed. Thus, the default is for NASM to assemble `binprog.asm' into a binary file called `binprog'. 6.1.1 `ORG': Binary File Program Origin The `bin' format provides an additional directive to the list given in chapter 5: `ORG'. The function of the `ORG' directive is to specify the origin address which NASM will assume the program begins at when it is loaded into memory. For example, the following code will generate the longword `0x00000104': org 0x100 dd label label: Unlike the `ORG' directive provided by MASM-compatible assemblers, which allows you to jump around in the object file and overwrite code you have already generated, NASM's `ORG' does exactly what the directive says: _origin_. Its sole function is to specify one offset which is added to all internal address references within the file; it does not permit any of the trickery that MASM's version does. See section 10.1.3 for further comments. 6.1.2 `bin' Extensions to the `SECTION' Directive The `bin' output format extends the `SECTION' (or `SEGMENT') directive to allow you to specify the alignment requirements of segments. This is done by appending the `ALIGN' qualifier to the end of the section-definition line. For example, section .data align=16 switches to the section `.data' and also specifies that it must be aligned on a 16-byte boundary. The parameter to `ALIGN' specifies how many low bits of the section start address must be forced to zero. The alignment value given may be any power of two. 6.2 `obj': Microsoft OMF Object Files The `obj' file format (NASM calls it `obj' rather than `omf' for historical reasons) is the one produced by MASM and TASM, which is typically fed to 16-bit DOS linkers to produce `.EXE' files. It is also the format used by OS/2. `obj' provides a default output file-name extension of `.obj'. `obj' is not exclusively a 16-bit format, though: NASM has full support for the 32-bit extensions to the format. In particular, 32- bit `obj' format files are used by Borland's Win32 compilers, instead of using Microsoft's newer `win32' object file format. The `obj' format does not define any special segment names: you can call your segments anything you like. Typical names for segments in `obj' format files are `CODE', `DATA' and `BSS'. If your source file contains code before specifying an explicit `SEGMENT' directive, then NASM will invent its own segment called `__NASMDEFSEG' for you. When you define a segment in an `obj' file, NASM defines the segment name as a symbol as well, so that you can access the segment address of the segment. So, for example: segment data dvar: dw 1234 segment code function: mov ax,data ; get segment address of data mov ds,ax ; and move it into DS inc word [dvar] ; now this reference will work ret The `obj' format also enables the use of the `SEG' and `WRT' operators, so that you can write code which does things like extern foo mov ax,seg foo ; get preferred segment of foo mov ds,ax mov ax,data ; a different segment mov es,ax mov ax,[ds:foo] ; this accesses `foo' mov [es:foo wrt data],bx ; so does this 6.2.1 `obj' Extensions to the `SEGMENT' Directive The `obj' output format extends the `SEGMENT' (or `SECTION') directive to allow you to specify various properties of the segment you are defining. This is done by appending extra qualifiers to the end of the segment-definition line. For example, segment code private align=16 defines the segment `code', but also declares it to be a private segment, and requires that the portion of it described in this code module must be aligned on a 16-byte boundary. The available qualifiers are: (*) `PRIVATE', `PUBLIC', `COMMON' and `STACK' specify the combination characteristics of the segment. `PRIVATE' segments do not get combined with any others by the linker; `PUBLIC' and `STACK' segments get concatenated together at link time; and `COMMON' segments all get overlaid on top of each other rather than stuck end-to-end. (*) `ALIGN' is used, as shown above, to specify how many low bits of the segment start address must be forced to zero. The alignment value given may be any power of two from 1 to 4096; in reality, the only values supported are 1, 2, 4, 16, 256 and 4096, so if 8 is specified it will be rounded up to 16, and 32, 64 and 128 will all be rounded up to 256, and so on. Note that alignment to 4096-byte boundaries is a PharLap extension to the format and may not be supported by all linkers. (*) `CLASS' can be used to specify the segment class; this feature indicates to the linker that segments of the same class should be placed near each other in the output file. The class name can be any word, e.g. `CLASS=CODE'. (*) `OVERLAY', like `CLASS', is specified with an arbitrary word as an argument, and provides overlay information to an overlay- capable linker. (*) Segments can be declared as `USE16' or `USE32', which has the effect of recording the choice in the object file and also ensuring that NASM's default assembly mode when assembling in that segment is 16-bit or 32-bit respectively. (*) When writing OS/2 object files, you should declare 32-bit segments as `FLAT', which causes the default segment base for anything in the segment to be the special group `FLAT', and also defines the group if it is not already defined. (*) The `obj' file format also allows segments to be declared as having a pre-defined absolute segment address, although no linkers are currently known to make sensible use of this feature; nevertheless, NASM allows you to declare a segment such as `SEGMENT SCREEN ABSOLUTE=0xB800' if you need to. The `ABSOLUTE' and `ALIGN' keywords are mutually exclusive. NASM's default segment attributes are `PUBLIC', `ALIGN=1', no class, no overlay, and `USE16'. 6.2.2 `GROUP': Defining Groups of Segments The `obj' format also allows segments to be grouped, so that a single segment register can be used to refer to all the segments in a group. NASM therefore supplies the `GROUP' directive, whereby you can code segment data ; some data segment bss ; some uninitialised data group dgroup data bss which will define a group called `dgroup' to contain the segments `data' and `bss'. Like `SEGMENT', `GROUP' causes the group name to be defined as a symbol, so that you can refer to a variable `var' in the `data' segment as `var wrt data' or as `var wrt dgroup', depending on which segment value is currently in your segment register. If you just refer to `var', however, and `var' is declared in a segment which is part of a group, then NASM will default to giving you the offset of `var' from the beginning of the _group_, not the _segment_. Therefore `SEG var', also, will return the group base rather than the segment base. NASM will allow a segment to be part of more than one group, but will generate a warning if you do this. Variables declared in a segment which is part of more than one group will default to being relative to the first group that was defined to contain the segment. A group does not have to contain any segments; you can still make `WRT' references to a group which does not contain the variable you are referring to. OS/2, for example, defines the special group `FLAT' with no segments in it. 6.2.3 `UPPERCASE': Disabling Case Sensitivity in Output Although NASM itself is case sensitive, some OMF linkers are not; therefore it can be useful for NASM to output single-case object files. The `UPPERCASE' format-specific directive causes all segment, group and symbol names that are written to the object file to be forced to upper case just before being written. Within a source file, NASM is still case-sensitive; but the object file can be written entirely in upper case if desired. `UPPERCASE' is used alone on a line; it requires no parameters. 6.2.4 `IMPORT': Importing DLL Symbols The `IMPORT' format-specific directive defines a symbol to be imported from a DLL, for use if you are writing a DLL's import library in NASM. You still need to declare the symbol as `EXTERN' as well as using the `IMPORT' directive. The `IMPORT' directive takes two required parameters, separated by white space, which are (respectively) the name of the symbol you wish to import and the name of the library you wish to import it from. For example: import WSAStartup wsock32.dll A third optional parameter gives the name by which the symbol is known in the library you are importing it from, in case this is not the same as the name you wish the symbol to be known by to your code once you have imported it. For example: import asyncsel wsock32.dll WSAAsyncSelect 6.2.5 `EXPORT': Exporting DLL Symbols The `EXPORT' format-specific directive defines a global symbol to be exported as a DLL symbol, for use if you are writing a DLL in NASM. You still need to declare the symbol as `GLOBAL' as well as using the `EXPORT' directive. `EXPORT' takes one required parameter, which is the name of the symbol you wish to export, as it was defined in your source file. An optional second parameter (separated by white space from the first) gives the _external_ name of the symbol: the name by which you wish the symbol to be known to programs using the DLL. If this name is the same as the internal name, you may leave the second parameter off. Further parameters can be given to define attributes of the exported symbol. These parameters, like the second, are separated by white space. If further parameters are given, the external name must also be specified, even if it is the same as the internal name. The available attributes are: (*) `resident' indicates that the exported name is to be kept resident by the system loader. This is an optimisation for frequently used symbols imported by name. (*) `nodata' indicates that the exported symbol is a function which does not make use of any initialised data. (*) `parm=NNN', where `NNN' is an integer, sets the number of parameter words for the case in which the symbol is a call gate between 32-bit and 16-bit segments. (*) An attribute which is just a number indicates that the symbol should be exported with an identifying number (ordinal), and gives the desired number. For example: export myfunc export myfunc TheRealMoreFormalLookingFunctionName export myfunc myfunc 1234 ; export by ordinal export myfunc myfunc resident parm=23 nodata 6.2.6 `..start': Defining the Program Entry Point OMF linkers require exactly one of the object files being linked to define the program entry point, where execution will begin when the program is run. If the object file that defines the entry point is assembled using NASM, you specify the entry point by declaring the special symbol `..start' at the point where you wish execution to begin. 6.2.7 `obj' Extensions to the `EXTERN' Directive If you declare an external symbol with the directive extern foo then references such as `mov ax,foo' will give you the offset of `foo' from its preferred segment base (as specified in whichever module `foo' is actually defined in). So to access the contents of `foo' you will usually need to do something like mov ax,seg foo ; get preferred segment base mov es,ax ; move it into ES mov ax,[es:foo] ; and use offset `foo' from it This is a little unwieldy, particularly if you know that an external is going to be accessible from a given segment or group, say `dgroup'. So if `DS' already contained `dgroup', you could simply code mov ax,[foo wrt dgroup] However, having to type this every time you want to access `foo' can be a pain; so NASM allows you to declare `foo' in the alternative form extern foo:wrt dgroup This form causes NASM to pretend that the preferred segment base of `foo' is in fact `dgroup'; so the expression `seg foo' will now return `dgroup', and the expression `foo' is equivalent to `foo wrt dgroup'. This default-`WRT' mechanism can be used to make externals appear to be relative to any group or segment in your program. It can also be applied to common variables: see section 6.2.8. 6.2.8 `obj' Extensions to the `COMMON' Directive The `obj' format allows common variables to be either near or far; NASM allows you to specify which your variables should be by the use of the syntax common nearvar 2:near ; `nearvar' is a near common common farvar 10:far ; and `farvar' is far Far common variables may be greater in size than 64Kb, and so the OMF specification says that they are declared as a number of _elements_ of a given size. So a 10-byte far common variable could be declared as ten one-byte elements, five two-byte elements, two five-byte elements or one ten-byte element. Some OMF linkers require the element size, as well as the variable size, to match when resolving common v