From: "A.Appleyard" To: dj Date: Thu, 24 Aug 1995 10:04:07 BST Subject: GAS manual in ascii Priority: normal X-Mailer: Pegasus Mail v3.22 DJ-Mail-Sort: djgpp, djgpp Herewith djgpp\docs\gas\as.tex converted to ascii. I have removed the matter that does not seem to refer to the IBM PC. Please check it through first in case I went astray anywhere among a tangle of Tex @if --- @end and suchlike. It is now 1510 lines long. ............................................ [The GNU Assembler for the IBM PC family] [January 1994] This file is a user guide to the Gnu assembler `GAS'. This version of the file describes `GAS' configured to generate code for IBM PC architectures. ================================================= [Overview] Here is a brief summary of how to invoke `GAS'. For details, see Invoking Command Line Options. GAS [ -a[dhlns] ] [ -D ] [ -f ] [ -I ] [ -K ] [ -L ] [ -o ] [ -R ] [ --statistics] [ -v ] [ -W ] [ -Z ] [ -- | ... ] -a[dhlns] Turn on listings, in any of a variety of ways:- -ad omit debugging directives from listing -ah include high-level source -al assembly listing -an no forms processing -as symbols You may combine these options; for example, use `-aln' for assembly listing without forms processing. By itself, `-a' defaults to `-ahls' - that is, all listings turned on. -D This option is accepted only for script compatibility with calls to other assemblers; it has no effect on `GAS'. -f ``fast''---skip whitespace and comment preprocessing (assume source is compiler output) -I Add to the search list for `.include' directives -K According to version, this option may or may not issue warnings when difference tables altered for long displacements. -L Keep (in symbol table) local symbols, starting with `L' -o Name the object-file output from `GAS' -R Fold data section into text section --statistics Display maximum space (in bytes), and total time (in seconds), taken by assembly. -v Announce `as' version -W Suppress warning messages -Z Generate object file even after errors -- Standard input ... Source files to assemble. -------------------------------------------------------- [Structure of this Manual] This manual is intended to describe what you need to know to use Gnu `GAS'. We cover the syntax expected in source files, including notation for symbols, constants, and expressions; the directives that `GAS' understands; and of course how to invoke `GAS'. We also cover special features in the IBM PC configuration of `GAS', including assembler directives. On the other hand, this manual is NOT intended as an introduction to programming in assembly language---let alone programming in general! In a similar vein, we make no attempt to introduce the machine architecture; we do NOT describe the instruction set, standard mnemonics, registers or addressing modes that are standard to a particular architecture. -------------------------------------------------------- [GAS, the GNU Assembler] Gnu `as' is really a family of assemblers. This manual describes `GAS', a member of that family which is configured for the IBM PC architectures. If you use (or have used) the Gnu assembler on one architecture, you should find a fairly similar environment when you use it on another architecture. Each version has much in common with the others, including object file formats, most assembler directives (often called `pseudo-ops') and assembler syntax. `GAS' is primarily intended to assemble the output of the Gnu C compiler `GCC' for use by the linker `LD'. Nevertheless, we've tried to make `GAS' assemble correctly everything that other assemblers for the same machine would assemble. Unlike older assemblers, `GAS' is designed to assemble a source program in one pass of the source file. This has a subtle impact on the `.org' directive (see Org,,`.org'). -------------------------------------------------------- [Object File Formats] The Gnu assembler can be configured to produce several alternative object file formats. For the most part, this does not affect how you write assembly language programs; but directives for debugging symbols are typically different in different file formats. See Symbol Attributes. On the IBM PC, `GAS' is configured to produce `a.out' format object files. -------------------------------------------------------- [Command Line] After the program name `GAS', the command line may contain options and file names. Options may appear in any order, and may be before, after, or between file names. The order of file names is significant. `--' (two hyphens) by itself names the standard input file explicitly, as one of the files for `GAS' to assemble. Except for `--' any command line argument that begins with a hyphen (`-') is an option. Each option changes the behavior of `GAS'. No option changes the way another option works. An option is a `-' followed by one or more letters; the case of the letter is important. All options are optional. Some options expect exactly one file name to follow them. The file name may either immediately follow the option's letter (compatible with older assemblers) or it may be the next command argument (Gnu standard). These two command lines are equivalent: GAS -o my-object-file.o mumble.s GAS -omy-object-file.o mumble.s -------------------------------------------------------- [Input Files] We use the phrase `source program', abbreviated `source', to describe the program input to one run of `GAS'. The program may be in one or more files; how the source is partitioned into files doesn't change the meaning of the source. The source program is a concatenation of the text in all the files, in the order specified. Each time you run `GAS' it assembles exactly one source program. The source program is made up of one or more files. (The standard input is also a file.) You give `GAS' a command line that has zero or more input file names. The input files are read (from left file name to right). A command line argument (in any position) that has no special meaning is taken to be an input file name. If you give `GAS' no file names it attempts to read one input file from the `GAS' standard input, which is normally your terminal. You may have to type `ctrl-D' to tell `GAS' there is no more program to assemble. Use `--' if you need to explicitly name the standard input file in your command line. If the source is empty, `GAS' produces a small, empty object file. -------------------------------------------------------- [Filenames and Line-numbers] There are two ways of locating a line in the input file (or files) and either may be used in reporting error messages. One way refers to a line number in a physical file; the other refers to a line number in a ``logical'' file. See Errors, ,Error and Warning Messages. `Physical files' are those files named in the command line given to `GAS'. `Logical files' are simply names declared explicitly by assembler directives; they bear no relation to physical files. Logical file names help error messages reflect the original source file, when `GAS' source is itself synthesized from other files. See App-File,,`.app-file'. -------------------------------------------------------- [Output (Object) File] Every time you run `GAS' it produces an output file, which is your assembly language program translated into numbers. This file is the object file. Its default name is `a.out'. You can give it another name by using the `-o' option. Conventionally, object file names end with `.o'. The default name is used for historical reasons: older assemblers were capable of assembling self-contained programs directly into a runnable program. (For some formats, this isn't currently possible, but it can be done for the `a.out' format.) The object file is meant for input to the linker `LD'. It contains assembled program code, information to help `LD' integrate the assembled program into a runnable file, and (optionally) symbolic information for the debugger. -------------------------------------------------------- [Error and Warning Messages] `GAS' may write warnings and error messages to the standard error file (usually your terminal). This should not happen when a compiler runs `GAS' automatically. Warnings report an assumption made so that `GAS' could keep assembling a flawed program; errors report a grave problem that stops the assembly. Warning messages have the format file_name::Warning Message Text (where is a line number). If a logical file name has been given (see App-File,,`.app-file') it is used for the filename, otherwise the name of the current input file is used. If a logical line number was given, then it is used to calculate the number printed, otherwise the actual line in the current source file is printed. The message text is intended to be self explanatory (in the grand Unix tradition). Error messages have the format file_name::FATAL:Error Message Text The file name and line number are derived as for warning messages. The actual message text may be rather less explanatory because many of them aren't supposed to happen. ================================================= [Command-Line Options] This chapter describes command-line options available in ALL versions of the Gnu assembler; see Machine Dependencies, for options specific to particular machine architectures. If you are invoking `GAS' via the Gnu C compiler (version 2), you can use the `-Wa' option to pass arguments through to the assembler. The assembler arguments must be separated from each other (and the `-Wa') by commas. For example: gcc -c -g -O -Wa,-alh,-L file.c emits a listing to standard output with high-level and assembly source. Usually you do not need to use this `-Wa' mechanism, since many compiler command-line options are automatically passed to the assembler by the compiler. (You can call the Gnu compiler driver with the `-v' option to see precisely what options it passes to each compilation pass, including the assembler.) -------------------------------------------------------- [Enable Listings: `-a[dhlns]'] These options enable listing output from the assembler. By itself, `-a' requests high-level, assembly, and symbols listing. You can use other letters to select specific options for the list: `-ah' requests a high-level language listing, `-al' requests an output-program assembly listing, and `-as' requests a symbol table listing. High-level listings require that a compiler debugging option like `-g' be used, and that assembly listings (`-al') be requested also. Use the `-ad' option to omit debugging directives from the listing. Once you have specified one of these options, you can further control listing output and its appearance using the directives `.list', `.nolist', `.psize', `.eject', `.title', and `.sbttl'. The `-an' option turns off all forms processing. If you do not request listing output with one of the `-a' options, the listing-control directives have no effect. The letters after `-a' may be combined into one option, E.G., `-aln'. -------------------------------------------------------- [`-D'] This option has no effect whatsoever, but it is accepted to make it more likely that scripts written for other assemblers also work with `GAS'. -------------------------------------------------------- [Work Faster: `-f'] `-f' should only be used when assembling programs written by a (trusted) compiler. `-f' stops the assembler from doing whitespace and comment preprocessing on the input file(s) before assembling them. See Preprocessing: if you use `-f' when the files actually need to be preprocessed (if they contain comments, for example), `GAS' does not work correctly. -------------------------------------------------------- [`.include' search path: `-I' ] Use this option to add a to the list of directories `GAS' searches for files specified in `.include' directives (see Include,,`.include'). You may use `-I' as many times as necessary to include a variety of paths. The current working directory is always searched first; after that, `GAS' searches any `-I' directories in the same order as they were specified (left to right) on the command line. -------------------------------------------------------- [Difference Tables: `-K'] According to version (the `DIFF-TBL-KLUGE'), `GAS' may or may not sometimes alter the code emitted for directives of the form `.word -'; see Word,,`.word'. You can use the `-K' option if you want a warning issued when this is done. -------------------------------------------------------- [Include Local Labels: `-L'] Labels beginning with `L' (upper case only) are called `local labels'. See Symbol Names. Normally you do not see such labels when debugging, because they are intended for the use of programs (like compilers) that compose assembler programs, not for your notice. Normally both `GAS' and `LD' discard such labels, so you do not normally debug with them. This option tells `GAS' to retain those `L...' symbols in the object file. Usually if you do this you also tell the linker `LD' to preserve symbols whose names begin with `L'. y default, a local label is any label beginning with `L', but each target is allowed to redefine the local label prefix. -------------------------------------------------------- [Name the Object File: `-o'] There is always one object file output when you run `GAS'. By default it has the name `a.out'. You use this option (which takes exactly one filename) to give the object file a different name. Whatever the object file is called, `GAS' overwrites any existing file of the same name. -------------------------------------------------------- [Join Data and Text Sections: `-R'] `-R' tells `GAS' to write the object file as if all data-section data lives in the text section. This is only done at the very last moment: your binary data are the same, but data section parts are relocated differently. The data section part of your object file is zero bytes long because all its bytes are appended to the text section. (See Sections,,Sections and Relocation.) When you specify `-R' it would be possible to generate shorter address displacements (because we do not have to cross between text and data section). We refrain from doing this simply for compatibility with older versions of `GAS'. In future, `-R' may work this way. -------------------------------------------------------- [Display Assembly Statistics: `--statistics'] Use `--statistics' to display two statistics about the resources used by `GAS': the maximum amount of space allocated during the assembly (in bytes), and the total execution time taken for the assembly (in `cpu' seconds). -------------------------------------------------------- [Announce Version: `-v'] You can find out what version of as is running by including the option `-v' (which you can also spell as `-version') on the command line. -------------------------------------------------------- [Suppress Warnings: `-W'] `GAS' should never give a warning or error message when assembling compiler output. But programs written by people often cause `GAS' to give a warning that a particular assumption was made. All such warnings are directed to the standard error file. If you use this option, no warnings are issued. This option only affects the warning messages: it does not change any particular of how `GAS' assembles your file. Errors, which stop the assembly, are still reported. -------------------------------------------------------- [Generate Object File in Spite of Errors: `-Z'] After an error message, `GAS' normally produces no output. If for some reason you are interested in object file output even after `GAS' gives an error message on your program, use the `-Z' option. If there are any errors, `GAS' continues anyways, and writes an object file after a final warning message of the form ` errors, warnings, generating bad object file.' ================================================= [Syntax] This chapter describes the machine-independent syntax allowed in a source file. `GAS' syntax is similar to what many other assemblers use; it is inspired by the BSD 4.2 assembler. -------------------------------------------------------- [Preprocessing] The `GAS' internal preprocessor:- - Adjusts and removes extra whitespace. It leaves one space or tab before the keywords on a line, and turns any other whitespace on the line into a single space. - Removes all comments, replacing them with a single space, or an appropriate number of newlines. - converts character constants into the appropriate numeric values. It does not do macro processing, include file handling, or anything else you may get from your C compiler's preprocessor. You can do include file processing with the `.include' directive (see Include,,`.include'). You can use the Gnu C compiler driver to get other ``CPP'' style preprocessing, by giving the input file a `.S' suffix. See Overall Options,, Options Controlling the Kind of Output, gcc.info, Using GNU CC. Excess whitespace, comments, and character constants cannot be used in the portions of the input text that are not preprocessed. If the first line of an input file is `#NO_APP' or if you use the `-f' option, whitespace and comments are not removed from the input file. Within an input file, you can ask for whitespace and comment removal in specific portions of the by putting a line that says `#APP' before the text that may contain whitespace or comments, and putting a line that says `#NO_APP' after this text. This feature is mainly intend to support `asm' statements in compilers whose output is otherwise free of comments and whitespace. -------------------------------------------------------- [Whitespace] `Whitespace' is one or more blanks or tabs, in any order. Whitespace is used to separate symbols, and to make programs neater for people to read. Unless within character constants (see Characters,,Character Constants), any whitespace means the same as exactly one space. -------------------------------------------------------- [Comments] There are two ways of rendering comments to `GAS'. In both cases the comment is equivalent to one space. Anything from `/*' to the next `*/' is a comment. This means you may not nest these comments. The only way to include a newline in a comment is to use a /* */ comment. Anything from the `line comment' character to the next newline is considered a comment and is ignored. The line comment character is To be compatible with past assemblers, lines that begin with `#' have a special interpretation. Following the `#' should be an absolute expression (see Expressions): the logical line number of the NEXT line. Then a string (see Strings,, Strings) is allowed: if present it is a new logical file name. The rest of the line, if any, should be whitespace. If the first non-whitespace characters on the line are not numeric, the line is ignored. (Just like a comment.) # This is an ordinary comment. # 42-6 "new_file_name" # New logical file name # This is logical line # 36. This feature is deprecated, and may disappear from future versions of `GAS'. -------------------------------------------------------- [Symbols] No symbol may begin with a digit. Case is significant. There is no length limit: all characters are significant. Symbols are delimited by characters not in that set, or by the beginning of a file (since the source program must end with a newline, the end of a file is not a possible symbol delimiter). See Symbols. -------------------------------------------------------- [Statements] A `statement' ends at a newline character (`\n') or at a semicolon (`;'). The newline or semicolon is considered part of the preceding statement. Newlines and semicolons within character constants are an exception: they do not end statements. It is an error to end any statement with end-of-file: the last character of any input file should be a newline. You may write a statement on more than one line if you put a backslash (`\') immediately in front of any newlines within the statement. When `GAS' reads a backslashed newline both characters are ignored. You can even put backslashed newlines in the middle of symbol names without changing the meaning of your source program. An empty statement is allowed, and may include whitespace. It is ignored. A statement begins with zero or more labels, optionally followed by a key symbol which determines what kind of statement it is. The key symbol determines the syntax of the rest of the statement. If the symbol begins with a dot `.' then the statement is an assembler directive: typically valid for any computer. If the symbol begins with a letter the statement is an assembly language `instruction': it assembles into a machine language instruction. A label is a symbol immediately followed by a colon (`:'). Whitespace before a label or after a colon is permitted, but you may not have whitespace between a label's symbol and its colon. See Labels. label: .directive followed by something another_label: # This is an empty statement. instruction operand_1, operand_2, ... -------------------------------------------------------- [Constants] A constant is a number, written so that its value is known by inspection, without knowing any context. Like this: .byte 74, 0112, 092, 0x4A, 0X4a, 'J, '\J # All the same value. .ascii "Ring the bell\7" # A string constant. .octa 0x123456789abcdef0123456789ABCDEF0 # A bignum. .float 0f-314159265358979323846264338327\ 95028841971.693993751E-40 # - pi, a flonum. .............................................. [Character Constants] There are two kinds of character constants. A `character' stands for one character in one byte and its value may be used in numeric expressions. String constants (properly called string LITERALS) are potentially many bytes and their values may not be used in arithmetic expressions. . . . . . . . . . . . . . . . . . . . . . . . [Strings] A `string' is written between double-quotes. It may contain double-quotes or null characters. The way to get special characters into a string is to `escape' these characters: precede them with a backslash `\' character. For example `\\' represents one backslash: the first `\' is an escape which tells `GAS' to interpret the second character literally as a backslash (which prevents `GAS' from recognizing the second `\' as an escape character). The complete list of escapes follows. \b Mnemonic for backspace; for ASCII this is octal code 010. \f Mnemonic for FormFeed; for ASCII this is octal code 014. \n Mnemonic for newline; for ASCII this is octal code 012. \r Mnemonic for carriage-Return; for ASCII this is octal code 015. \t Mnemonic for horizontal Tab; for ASCII this is octal code 011. \ An octal character code. The numeric code is 3 octal digits. For compatibility with other Unix systems, 8 and 9 are accepted as digits: for example, `\008' has the value 010, and `\009' the value 011. \\ Represents one `\' character. \" Represents one `"' character. Needed in strings to represent this character, because an unescaped `"' would end the string. \ Any other character when escaped by `\' gives a warning, but assembles as if the `\' was not present. The idea is that if you used an escape sequence you clearly didn't want the literal interpretation of the following character. However `GAS' has no other interpretation, so `GAS' knows it is giving you the wrong code and warns you of the fact. Which characters are escapable, and what those escapes represent, varies widely among assemblers. The current set is what we think the BSD 4.2 assembler recognizes, and is a subset of what most C compilers recognize. If you are in doubt, do not use an escape sequence. . . . . . . . . . . . . . . . . . . . . . . . [Characters] A single character may be written as a single quote immediately followed by that character. The same escapes apply to characters as to strings. So if you want to write the character backslash, you must write `'\\' where the first `\' escapes the second `\'. As you can see, the quote is an acute accent, not a grave accent. A newline (or semicolon `;') immediately following an acute accent is taken as a literal character and does not count as the end of a statement. The value of a character constant in a numeric expression is the machine's byte-wide code for that character. `GAS' assumes your character code is ASCII: `'A' means 65, `'B' means 66, and so on. .............................................. [Number Constants] `GAS' distinguishes three kinds of numbers according to how they are stored in the target machine. INTEGERS are numbers that would fit into an `int' in the C language. BIGNUMS are integers, but they are stored in more than 32 bits. FLONUMS are floating point numbers, described below. . . . . . . . . . . . . . . . . . . . . . . . [Integers] A binary integer is `0b' or `0B' followed by zero or more of the binary digits `01'. An octal integer is `0' followed by zero or more of the octal digits (`01234567'). A decimal integer starts with a non-zero digit followed by zero or more digits (`0123456789'). A hexadecimal integer is `0x' or `0X' followed by one or more hexadecimal digits chosen from `0123456789abcdefABCDEF'. Integers have the usual values. To denote a negative integer, use the prefix operator `-' discussed under expressions (see Prefix Ops,,Prefix Operators). . . . . . . . . . . . . . . . . . . . . . . . [Bignums] A `bignum' has the same syntax and semantics as an integer except that the number (or its negative) takes more than 32 bits to represent in binary. The distinction is made because in some places integers are permitted while bignums are not. . . . . . . . . . . . . . . . . . . . . . . . [Flonums] A `flonum' represents a floating point number. The translation is indirect: a decimal floating point number from the text is converted by `GAS' to a generic binary floating point number of more than sufficient precision. This generic floating point number is converted to a particular computer's floating point format (or formats) by a portion of `GAS' specialized to that computer. A flonum is written by writing (in order) The digit `0'. A letter, to tell `GAS' the rest of the number is a flonum. `e' is recommended. Case is not important. An optional sign: either `+' or `-'. An optional `integer part': zero or more decimal digits. An optional `fractional part': `.' followed by zero or more decimal digits. An optional exponent, consisting of: An `E' or `e'. Optional sign: either `+' or `-'. One or more decimal digits. At least one of the integer part or the fractional part must be present. The floating point number has the usual base-10 value. `GAS' does all processing using integers. Flonums are computed independently of any floating point hardware in the computer running `GAS'. ================================================= [Sections and Relocation] -------------------------------------------------------- [Background] Roughly, a section is a range of addresses, with no gaps; all data ``in'' those addresses is treated the same for some particular purpose. For example there may be a ``read only'' section. The linker `LD' reads many object files (partial programs) and combines their contents to form a runnable program. When `GAS' emits an object file, the partial program is assumed to start at address 0. `LD' assigns the final addresses for the partial program, so that different partial programs do not overlap. This is actually an oversimplification, but it suffices to explain how `GAS' uses sections. `LD' moves blocks of bytes of your program to their run-time addresses. These blocks slide to their run-time addresses as rigid units; their length does not change and neither does the order of bytes within them. Such a rigid unit is called a SECTION. Assigning run-time addresses to sections is called `relocation'. It includes the task of adjusting mentions of object-file addresses so they refer to the proper run-time addresses. An object file written by `GAS' has at least three sections, any of which may be empty. These are named `text', `data' and `bss' sections. Within the object file, the text section starts at address `0', the data section follows, and the bss section follows the data section. To let `LD' know which data changes when the sections are relocated, and how to change that data, `GAS' also writes to the object file details of the relocation needed. To perform relocation `LD' must know, each time an address in the object file is mentioned: Where in the object file is the beginning of this reference to an address? How long (in bytes) is this reference? Which section does the address refer to? What is the numeric value of (
) - ()? Is the reference to an address ``Program-Counter relative''? In fact, every address `GAS' ever uses is expressed as (
) + () Further, most expressions `GAS' computes have this section-relative nature. In this manual we use the notation { } to mean ``offset into section .'' Apart from text, data and bss sections you need to know about the `absolute' section. When `LD' mixes partial programs, addresses in the absolute section remain unchanged. For example, address `{absolute 0}' is ``relocated'' to run-time address 0 by `LD'. Although the linker never arranges two partial programs' data sections with overlapping addresses after linking, BY DEFINITION their absolute sections must overlap. Address `{absolute 239}' in one part of a program is always the same address when the program is running as address `{absolute 239}' in any other part of the program. The idea of sections is extended to the `undefined' section. Any address whose section is unknown at assembly time is by definition rendered {undefined }---where is filled in later. Since numbers are always defined, the only way to generate an undefined address is to mention an undefined symbol. A reference to a named common block would be such a symbol: its value is unknown at assembly time so it has section UNDEFINED. By analogy the word SECTION is used to describe groups of sections in the linked program. `LD' puts all partial programs' text sections in contiguous addresses in the linked program. It is customary to refer to the TEXT SECTION of a program, meaning all the addresses of all partial programs' text sections. Likewise for data and bss sections. Some sections are manipulated by `LD'; others are invented for use of `GAS' and have no meaning except during assembly. -------------------------------------------------------- [LD Sections] `LD' deals with just four kinds of sections, summarized below. These sections hold your program. `GAS' and `LD' treat them as separate but equal sections. Anything you can say of one section is true another. bss section: This section contains zeroed bytes when your program begins running. It is used to hold unitialized variables or common storage. The length of each partial program's bss section is important, but because it starts out containing zeroed bytes there is no need to store explicit zero bytes in the object file. The bss section was invented to eliminate those explicit zeros from object files. absolute section: Address 0 of this section is always ``relocated'' to runtime address 0. This is useful if you want to refer to an address that `LD' must not change when relocating. In this sense we speak of absolute addresses being ``unrelocatable'': they do not change during relocation. undefined section: This ``section'' is a catch-all for address references to objects not in the preceding sections. @end table An idealized example of three relocatable sections follows. Memory addresses are on the horizontal axis. +-----+----+--+ partial program # 1: |ttttt|dddd|00| +-----+----+--+ text data bss seg. seg. seg. +---+---+---+ partial program # 2: |TTT|DDD|000| +---+---+---+ +--+---+-----+--+----+---+-----+~~ linked program: | |TTT|ttttt| |dddd|DDD|00000| +--+---+-----+--+----+---+-----+~~ addresses: 0 ... -------------------------------------------------------- [GAS Internal Sections] These sections are meant only for the internal use of `GAS'. They have no meaning at run-time. You do not really need to know about these sections for most purposes; but they can be mentioned in `GAS' warning messages, so it might be helpful to have an idea of their meanings to `GAS'. These sections are used to permit the value of every expression in your assembly language program to be a section-relative address. ASSEMBLER-INTERNAL-LOGIC-ERROR!: An internal assembler logic error has been found. This means there is a bug in the assembler. expr section: The assembler stores complex expression internally as combinations of symbols. When it needs to represent an expression as a symbol, it puts it in the expr section. -------------------------------------------------------- [Sub-Sections] You may have separate groups of data in named sections that you want to end up near to each other in the object file, even though they are not contiguous in the assembler source. `GAS' allows you to use `subsections' for this purpose. Within each section, there can be numbered subsections with values >from 0 to 8192. Objects assembled into the same subsection go into the object file together with other objects in the same subsection. For example, a compiler might want to store constants in the text section, but might not want to have them interspersed with the program being assembled. In this case, the compiler could issue a `.text 0' before each section of code being output, and a `.text 1' before each group of constants being output. Subsections are optional. If you do not use subsections, everything goes in subsection number zero. Subsections appear in your object file in numeric order, lowest numbered to highest. (All this to be compatible with other people's assemblers.) The object file contains no representation of subsections; `LD' and other programs that manipulate object files see no trace of them. They just see all your text subsections as a text section, and all your data subsections as a data section. To specify which subsection you want subsequent statements assembled into, use a numeric argument to specify it, in a `.text ' or a `.data ' statement. You can also use an extra subsection argument with arbitrary named sections: `.section , '. should be an absolute expression. (See Expressions.) If you just say `.text' then `.text 0' is assumed. Likewise `.data' means `.data 0'. Assembly begins in `text 0'. For instance: .text 0 # The default subsection is text 0 anyway. .ascii "This lives in the first text subsection. *" .text 1 .ascii "But this lives in the second text subsection." .data 0 .ascii "This lives in the data section," .ascii "in the first data subsection." .text 0 .ascii "This lives in the first text section," .ascii "immediately following the asterisk (*)." Each section has a `location counter' incremented by one for every byte assembled into that section. Because subsections are merely a convenience restricted to `GAS' there is no concept of a subsection location counter. There is no way to directly manipulate a location counter---but the `.align' directive changes it, and any label definition captures its current value. The location counter of the section where statements are being assembled is said to be the `active' location counter. -------------------------------------------------------- [bss Section] The bss section is used for local common variable storage. You may allocate address space in the bss section, but you may not dictate data to load into it before your program executes. When your program starts running, all the contents of the bss section are zeroed bytes. Addresses in the bss section are allocated with special directives; you may not assemble anything directly into the bss section. Hence there are no bss subsections. See Comm,,`.comm', see Lcomm,,`.lcomm'. ================================================= [Symbols] Symbols are a central concept: the programmer uses symbols to name things, the linker uses symbols to link, and the debugger uses symbols to debug. WARNING: `GAS' does not place symbols in the object file in the same order they were declared. This may break some debuggers. -------------------------------------------------------- [Labels] `label' is written as a symbol immediately followed by a colon `:'. The symbol then represents the current value of the active location counter, and is, for example, a suitable instruction operand. You are warned if you use the same symbol to represent two different locations: the first definition overrides any other definitions. -------------------------------------------------------- [Giving Symbols Other Values] A symbol can be given an arbitrary value by writing a symbol, followed by an equals sign `=', followed by an expression (see Expressions). This is equivalent to using the `.set' directive. See Set,,`.set'. -------------------------------------------------------- [Symbol Names] Symbol names begin with a letter or with one of `._'. On most machines, you can also use `$' in symbol names; exceptions are noted in @ref{Machine Dependencies}. That character may be followed by any string of digits, letters, dollar signs (unless otherwise noted in `Machine Dependencies'), and underscores. Case of letters is significant: `foo' is a different symbol name than `Foo'. Each symbol has exactly one name. Each name in an assembly language program refers to exactly one symbol. You may use that symbol name any number of times in a program. .............................................. [Local Symbol Names] Local symbols help compilers and programmers use names temporarily. There are ten local symbol names, which are re-used throughout the program. You may refer to them using the names `0' `1' ... `9'. To define a local symbol, write a label of the form `:' (where represents any digit). To refer to the most recent previous definition of that symbol write `'b, using the same digit as when you defined the label. To refer to the next definition of a local label, write `'f---where gives you a choice of 10 forward references. The `b' stands for ``backwards'' and the `f' stands for ``forwards''. Local symbols are not emitted by the current Gnu C compiler. There is no restriction on how you can use these labels, but remember that at any point in the assembly you can refer to at most 10 prior local labels and to at most 10 forward local labels. Local symbol names are only a notation device. They are immediately transformed into more conventional symbol names before the assembler uses them. The symbol names stored in the symbol table, appearing in error messages and optionally emitted to the object file have these parts: L : All local labels begin with `L'. Normally both `GAS' and `LD' forget symbols that start with `L'. These labels are used for symbols you are never intended to see. If you use the `-L' option then `GAS' retains these symbols in the object file. If you also instruct `LD' to retain these symbols, you may use them in debugging. : If the label is written `0:' then the digit is `0'. If the label is written `1:' then the digit is `1'. And so on up through `9:'. : This unusual character is included so you do not accidentally invent a symbol of the same name. The character has ASCII value `\001'. ORDINAL NUMBER : This is a serial number to keep the labels distinct. The first `0:' gets the number `1'; The 15th `0:' gets the number `15'; ETC.. Likewise for the other labels `1:' through `9:'. @end table For instance, the first `1:' is named `L11', the 44th `3:' is named `L344'. -------------------------------------------------------- [The Special Dot Symbol] The special symbol `.' refers to the current address that `GAS' is assembling into. Thus, the expression `melvin: .long .' defines `melvin' to contain its own address. Assigning a value to `.' is treated the same as a `.org' directive. Thus, the expression `.=.+4' is the same as saying `.space 4'. -------------------------------------------------------- [Symbol Attributes] Every symbol has, as well as its name, the attributes ``Value'' and ``Type''. Depending on output format, symbols can also have auxiliary attributes. If you use a symbol without defining it, `GAS' assumes zero for all these attributes, and probably won't warn you. This makes the symbol an externally defined symbol, which is generally what you would want. .............................................. [Value] The value of a symbol is (usually) 32 bits. For a symbol which labels a location in the text, data, bss or absolute sections the value is the number of addresses from the start of that section to the label. Naturally for text, data and bss sections the value of a symbol changes as `LD' changes section base addresses during linking. Absolute symbols' values do not change during linking: that is why they are called absolute. The value of an undefined symbol is treated in a special way. If it is 0 then the symbol is not defined in this assembler source file, and `LD' tries to determine its value from other files linked into the same program. You make this kind of symbol simply by mentioning a symbol name without defining it. A non-zero value represents a `.comm' common declaration. The value is how much common storage to reserve, in bytes (addresses). The symbol refers to the first address of the allocated storage. .............................................. [Type] The type attribute of a symbol contains relocation (section) information, any flag settings indicating that a symbol is external, and (optionally), other information for linkers and debuggers. The exact format depends on the object-code output format in use. . . . . . . . . . . . . . . . . . . . . . . . [Descriptor] This is an arbitrary 16-bit value. You may establish a symbol's descriptor value by using a `.desc' statement (see Desc,,`.desc'). A descriptor value means nothing to `GAS'. . . . . . . . . . . . . . . . . . . . . . . . [Other] This is an arbitrary 8-bit value. It means nothing to `GAS'. ================================================= [Expressions] An `expression' specifies an address or numeric value. Whitespace may precede and/or follow an expression. The result of an expression must be an absolute number, or else an offset into a particular section. If an expression is not absolute, and there is not enough information when `GAS' sees the expression to know its section, a second pass over the source program might be necessary to interpret the expression---but the second pass is currently not implemented. `GAS' aborts with an error message in this situation. -------------------------------------------------------- [Empty Expressions] An empty expression has no value: it is just whitespace or null. Wherever an absolute expression is required, you may omit the expression, and `GAS' assumes a value of (absolute) 0. This is compatible with other assemblers. -------------------------------------------------------- [Integer Expressions] An `integer expression' is one or more ARGUMENTS delimited by OPERATORS. .............................................. [Arguments] `Arguments' are symbols, numbers or subexpressions. In other contexts arguments are sometimes called ``arithmetic operands''. In this manual, to avoid confusing them with the ``instruction operands'' of the machine language, we use the term ``argument'' to refer to parts of expressions only, reserving the word ``operand'' to refer only to machine instruction operands. Symbols are evaluated to yield {
} where
is one of text, data, bss, absolute, or undefined. is a signed, 2's complement 32 bit integer. Numbers are usually integers. A number can be a flonum or bignum. In this case, you are warned that only the low order 32 bits are used, and `GAS' pretends these 32 bits are an integer. You may write integer-manipulating instructions that act on exotic constants, compatible with other assemblers. Subexpressions are a left parenthesis `(' followed by an integer expression, followed by a right parenthesis `)'; or a prefix operator followed by an argument. .............................................. [Operators] `Operators' are arithmetic functions, like `+' or `%'. Prefix operators are followed by an argument. Infix operators appear between their arguments. Operators may be preceded and/or followed by whitespace. .............................................. [Prefix Operator] `GAS' has the following `prefix operators'. They each take one argument, which must be absolute. - `Negation'. Two's complement negation. ~ `Complementation'. Bitwise not. .............................................. [Infix Operators] `Infix operators' take two arguments, one on either side. Operators have precedence, but operations with equal precedence are performed left to right. Apart from `+' or `-', both arguments must be absolute, and the result is absolute. Highest Precedence * `Multiplication'. / `Division'. Truncation is the same as the C operator `/' % `Remainder'. < `Shift Left'. Same as the C operator `<<'. << `Shift Left'. Same as the C operator `<<'. > `Shift Right'. Same as the C operator `>>'. >> `Shift Right'. Same as the C operator `>>'. Intermediate precedence | `Bitwise Inclusive Or'. & `Bitwise And'. ^ `Bitwise Exclusive Or'. ! `Bitwise Or Not'. Lowest Precedence + `Addition'. If either argument is absolute, the result has the section of the other argument. You may not add together arguments from different sections. - `Subtraction'. If the right argument is absolute, the result has the section of the left argument. If both arguments are in the same section, the result is absolute. You may not subtract arguments from different sections. In short, it's only meaningful to add or subtract the OFFSETS in an address; you can only have a defined section in one of the two arguments. ================================================= [Assembler Directives] All assembler directives have names that begin with a period (`.'). The rest of the name is letters, usually in lower case. This chapter discusses directives that are available regardless of the target machine configuration for the Gnu assembler. -------------------------------------------------------- [`.abort'] This directive stops the assembly immediately. It is for compatibility with other assemblers. The original idea was that the assembly language source would be piped into the assembler. If the sender of the source quit, it could use this directive tells `GAS' to quit also. One day `.abort' will not be supported. -------------------------------------------------------- [`.align , '] Pad the location counter (in the current subsection) to a particular storage boundary. The first expression (which must be absolute) is the number of low-order zero bits the location counter must have after advancement. For example `.align 3' advances the location counter until it a multiple of 8. If the location counter is already a multiple of 8, no change is needed. The second expression (also absolute) gives the value to be stored in the padding bytes. It (and the comma) may be omitted. If it is omitted, the padding bytes are zero. -------------------------------------------------------- [`.app-file '] `.app-file' (which may also be spelled `.file') tells `GAS' that we are about to start a new logical file. is the new file name. In general, the filename is recognized whether or not it is surrounded by quotes `"'; but if you wish to specify an empty file name is permitted, you must give the quotes--`""'. This statement may go away in future: it is only recognized to be compatible with old `GAS' programs. -------------------------------------------------------- [`.ascii ""'...] `.ascii' expects zero or more string literals (see Strings) separated by commas. It assembles each string (with no automatic trailing zero byte) into consecutive addresses. -------------------------------------------------------- [`.asciz ""'...] `.asciz' is just like `.ascii', but each string is followed by a zero byte. -------------------------------------------------------- [`.byte '] `.byte' expects zero or more expressions, separated by commas. Each expression is assembled into the next byte. -------------------------------------------------------- [`.comm }] `.comm' declares a named common area in the bss section. Normally `LD' reserves memory addresses for it during linking, so no partial program defines the location of the symbol. Use `.comm' to tell `LD' that it must be at least bytes long. `LD' allocates space for each `.comm' symbol that is at least as long as the longest `.comm' request in any of the partial programs linked. is an absolute expression. -------------------------------------------------------- [`.data '] `.data' tells `GAS' to assemble the following statements onto the end of the data subsection numbered (which is an absolute expression). If is omitted, it defaults to zero. -------------------------------------------------------- [`.double '] `.double' expects zero or more flonums, separated by commas. It assembles floating point numbers. On the IBM PC family `.double' emits 64-bit floating-point numbers in `ieee' format. -------------------------------------------------------- [`.eject'] Force a page break at this point, when generating assembly listings. -------------------------------------------------------- [`.else'] `.else' is part of the `GAS' support for conditional assembly; see If,,`.if'. It marks the beginning of a section of code to be assembled if the condition for the preceding `.if' was false. -------------------------------------------------------- [`.endif'] `.endif' is part of the `GAS' support for conditional assembly; it marks the end of a block of code that is only assembled conditionally. See If,,`.if'. -------------------------------------------------------- [`.equ , '] This directive sets the value of to . It is synonymous with `.set'; see Set,,`.set'. -------------------------------------------------------- [`.extern'] `.extern' is accepted in the source program---for compatibility with other assemblers---but it is ignored. `GAS' treats all undefined symbols as external. -------------------------------------------------------- [`.file '] `.file' (which may also be spelled `.app-file') tells `GAS' that we are about to start a new logical file. is the new file name. In general, the filename is recognized whether or not it is surrounded by quotes `"'; but if you wish to specify an empty file name, you must give the quotes--`""'. This statement may go away in future: it is only recognized to be compatible with old `GAS' programs. -------------------------------------------------------- [`.fill , , '] , and are absolute expressions. This emits copies of bytes. may be zero or more. may be zero or more, but if it is more than 8, then it is deemed to have the value 8, compatible with other people's assemblers. The contents of each bytes is taken from an 8-byte number. The highest order 4 bytes are zero. The lowest order 4 bytes are rendered in the byte-order of an integer on the computer `GAS' is assembling for. Each bytes in a repetition is taken >from the lowest order bytes of this number. Again, this bizarre behavior is compatible with other people's assemblers. and are optional. If the second comma and are absent, is assumed zero. If the first comma and following tokens are absent, is assumed to be 1. -------------------------------------------------------- [`.float '] This directive assembles zero or more flonums, separated by commas. It has the same effect as `.single'. On the IBM PC family, `.float' emits 32-bit floating point numbers in `ieee' format. -------------------------------------------------------- [`.global ', `.globl '] `.global' makes the symbol visible to `LD'. If you define in your partial program, its value is made available to other partial programs that are linked with it. Otherwise, takes its attributes from a symbol of the same name from another file linked into the same program. Both spellings (`.globl' and `.global') are accepted, for compatibility with other assemblers. -------------------------------------------------------- [`.hword '] This expects zero or more , and emits a 16 bit number for each. -------------------------------------------------------- [`.ident'] This directive is used by some assemblers to place tags in object files. `GAS' simply accepts the directive for source-file compatibility with such assemblers, but does not actually emit anything for it. -------------------------------------------------------- [`.if '] `.if' marks the beginning of a section of code which is only considered part of the source program being assembled if the argument (which must be an ) is non-zero. The end of the conditional section of code must be marked by `.endif' (see Endif,,`.endif'); optionally, you may include code for the alternative condition, flagged by `.else' (see Else,,`.else'. The following variants of `.if' are also supported: .ifdef Assembles the following section of code if the specified has been defined. .ifndef .ifnotdef Assembles the following section of code if the specified has not been defined. Both spelling variants are equivalent. -------------------------------------------------------- [`.include ""'] This directive provides a way to include supporting files at specified points in your source program. The code from is assembled as if it followed the point of the `.include'; when the end of the included file is reached, assembly of the original file continues. You can control the search paths used with the `-I' command-line option (see Invoking,,Command-Line Options). Quotation marks are required around . -------------------------------------------------------- [`.int '] Expect zero or more , of any section, separated by commas. For each expression, emit a number that, at run time, is the value of that expression. The byte order and bit size of the number depends on what kind of target the assembly is for. -------------------------------------------------------- [`.lcomm , '] Reserve (an absolute expression) bytes for a local common denoted by . The section and value of are those of the new local common. The addresses are allocated in the bss section, so that at run-time the bytes start off zeroed. is not declared global (see Global), so is normally not visible to `LD'. -------------------------------------------------------- [`.lflags'] `GAS' accepts this directive, for compatibility with other assemblers, but ignores it. -------------------------------------------------------- [`.line '] Even though this is a directive associated with the `a.out' or `b.out' object-code formats, `GAS' still recognizes it when producing COFF output, and treats `.line' as though it were the COFF `.ln' IF it is found outside a `.def'/`.endef' pair. Inside a `.def', `.line' is, instead, one of the directives used by compilers to generate auxiliary symbol information for debugging. -------------------------------------------------------- [`.ln '] `.ln' is a synonym for `.line'. -------------------------------------------------------- [`.list'] Control (in conjunction with the `.nolist' directive) whether or not assembly listings are generated. These two directives maintain an internal counter (which is zero initially). `.list' increments the counter, and `.nolist' decrements it. Assembly listings are generated whenever the counter is greater than zero. By default, listings are disabled. When you enable them (with the `-a' command line option; see Invoking,,Command-Line Options), the initial value of the listing counter is one. -------------------------------------------------------- [`.long '] `.long' is the same as `.int', see Int,,`.int'. -------------------------------------------------------- [`.nolist'] Control (in conjunction with the `.list' directive) whether or not assembly listings are generated. These two directives maintain an internal counter (which is zero initially). `.list' increments the counter, and `.nolist' decrements it. Assembly listings are generated whenever the counter is greater than zero. -------------------------------------------------------- [`.octa '] This directive expects zero or more bignums, separated by commas. For each bignum, it emits a 16-byte integer. The term ``octa'' comes from contexts in which a ``word'' is two bytes; hence OCTA-word for 16 bytes. -------------------------------------------------------- [`.org , '] Advance the location counter of the current section to . is either an absolute expression or an expression with the same section as the current subsection. That is, you can't use `.org' to cross sections: if has the wrong section, the `.org' directive is ignored. To be compatible with former assemblers, if the section of is absolute, `GAS' issues a warning, then pretends the section of is the same as the current subsection. `.org' may only increase the location counter, or leave it unchanged; you cannot use `.org' to move the location counter backwards. Because `GAS' tries to assemble programs in one pass, may not be undefined. If you really detest this restriction we eagerly await a chance to share your improved assembler. Beware that the origin is relative to the start of the section, not to the start of the subsection. This is compatible with other people's assemblers. When the location counter (of the current subsection) is advanced, the intervening bytes are filled with which should be an absolute expression. If the comma and are omitted, defaults to zero. -------------------------------------------------------- [`.psize , '] Use this directive to declare the number of lines - and, optionally, the number of columns - to use for each page, when generating listings. If you do not use `.psize', listings use a default line-count of 60. You may omit the comma and specification; the default width is 200 columns. `GAS' generates formfeeds whenever the specified number of lines is exceeded (or whenever you explicitly request one, using `.eject'). If you specify as `0', no formfeeds are generated save those explicitly specified with `.eject'. -------------------------------------------------------- [`.quad '] `.quad' expects zero or more bignums, separated by commas. For each bignum, it emits an 8-byte integer. If the bignum won't fit in 8 bytes, it prints a warning message; and just takes the lowest order 8 bytes of the bignum. The term ``quad'' comes from contexts in which a ``word'' is two bytes; hence QUAD-word for 8 bytes. -------------------------------------------------------- [`.sbttl ""'] Use as the title (third line, immediately after the title line) when generating assembly listings. This directive affects subsequent pages, as well as the current page if it appears within ten lines of the top of a page. -------------------------------------------------------- [`.scl '] Set the storage-class value for a symbol. This directive may only be used inside a `.def'/`.endef' pair. Storage class may flag whether a symbol is static or external, or it may record further symbolic debugging information. -------------------------------------------------------- [`.set , '] Set the value of to . This changes 's value and type to conform to . If was flagged as external, it remains flagged. (See Symbol Attributes.) You may `.set' a symbol many times in the same assembly. If you `.set' a global symbol, the value stored in the object file is the last value stored into it. -------------------------------------------------------- [`.short '] `.short' is normally the same as `.word'. See Word. In some configurations, however, `.short' and `.word' generate numbers of different lengths; see Machine Dependencies. -------------------------------------------------------- [`.single '] This directive assembles zero or more flonums, separated by commas. It has the same effect as `.float'. On the IBM PC family, `.single' emits 32-bit floating point numbers in `ieee' format. -------------------------------------------------------- [`.space }] This directive emits bytes, each of value . Both and are absolute expressions. If the comma and are omitted, is assumed to be zero. -------------------------------------------------------- [`.stabd, .stabn, .stabs'] There are three directives that begin `.stab'. All emit symbols (see Symbols), for use by symbolic debuggers. The symbols are not entered in the `GAS' hash table: they cannot be referenced elsewhere in the source file. Up to five fields are required: string This is the symbol's name. It may contain any character except `\000', so is more general than ordinary symbol names. Some debuggers used to code arbitrarily complex structures into symbol names using this field. type An absolute expression. The symbol's type is set to the low 8 bits of this expression. Any bit pattern is permitted, but `LD' and debuggers choke on silly bit patterns. other An absolute expression. The symbol's ``other'' attribute is set to the low 8 bits of this expression. desc An absolute expression. The symbol's descriptor is set to the low 16 bits of this expression. value An absolute expression which becomes the symbol's value. If a warning is detected while reading a `.stabd', `.stabn', or `.stabs' statement, the symbol has probably already been created; you get a half-formed symbol in your object file. This is compatible with earlier assemblers! .stabd , , The ``name'' of the symbol generated is not even an empty string. It is a null pointer, for compatibility. Older assemblers used a null pointer so they didn't waste space in object files with empty strings. The symbol's value is set to the location counter, relocatably. When your program is linked, the value of this symbol is the address of the location counter when the `.stabd' was assembled. .stabn , , , The name of the symbol is set to the empty string `""'. .stabs , , , , All five fields are specified. -------------------------------------------------------- [`.string' ""] Copy the characters in to the object file. You may specify more than one string to copy, separated by commas. Unless otherwise specified for a particular machine, the assembler marks the end of each string with a 0 byte. You can use any of the escape sequences described in @ref{Strings,,Strings}. -------------------------------------------------------- [`.text '] Tells `GAS' to assemble the following statements onto the end of the text subsection numbered , which is an absolute expression. If is omitted, subsection number zero is used. -------------------------------------------------------- [`.title ""'] Use as the title (second line, immediately after the source file name and pagenumber) when generating assembly listings. This directive affects subsequent pages, as well as the current page if it appears within ten lines of the top of a page. -------------------------------------------------------- [`.word '] This directive expects zero or more , of any section, separated by commas. For each expression, `GAS' emits a 16-bit number. WARNING: SPECIAL TREATMENT TO SUPPORT COMPILERS: Machines with a 32-bit address space, but that do less than 32-bit addressing, require the following special treatment. If the machine of interest to you does 32-bit addressing (or doesn't require it; see Machine Dependencies), you can ignore this issue. In order to assemble compiler output into something that works, `GAS' occasionlly does strange things to `.word' directives. Directives of the form `.word sym1-sym2' are often emitted by compilers as part of jump tables. Therefore, when `GAS' assembles a directive of the form `.word sym1-sym2', and the difference between `sym1' and `sym2' does not fit in 16 bits, `GAS' creates a `secondary jump table', immediately before the next label. This secondary jump table is preceded by a short-jump to the first byte after the secondary table. This short-jump prevents the flow of control from accidentally falling into the new table. Inside the table is a long-jump to `sym2'. The original `.word' contains `sym1' minus the address of the long-jump to `sym2'. If there were several occurrences of `.word sym1-sym2' before the secondary jump table, all of them are adjusted. If there was a `.word sym3-sym4', that also did not fit in sixteen bits, a long-jump to `sym4' is included in the secondary jump table, and the `.word' directives are adjusted to contain `sym3' minus the address of the long-jump to `sym4'; and so on, for as many entries in the original jump table as necessary. THIS FEATURE MAY BE DISABLED BY COMPILING `GAS' WITH THE `-dworking_dot_word' OPTION. This feature is likely to confuse assembly language programmers. -------------------------------------------------------- [Deprecated Directives] One day these directives won't work. They are included for compatibility with older assemblers. .abort .app-file .line ================================================= [80386 Dependent Features] -------------------------------------------------------- [Options] The 80386 has no machine dependent options. -------------------------------------------------------- [AT&T Syntax versus Intel Syntax] In order to maintain compatibility with the output of `GCC', `GAS' supports AT&T System V/386 assembler syntax. This is quite different from Intel syntax. We mention these differences because almost all 80386 documents used only Intel syntax. Notable differences between the two syntaxes are: - AT&T immediate operands are preceded by `$'; Intel immediate operands are undelimited (Intel `push 4' is AT&T `pushl $4'). AT&T register operands are preceded by `%'; Intel register operands are undelimited. AT&T absolute (as opposed to PC relative) jump/call operands are prefixed by `*'; they are undelimited in Intel syntax. - AT&T and Intel syntax use the opposite order for source and destination operands. Intel `add eax, 4' is `addl $4, %eax'. The `source, dest' convention is maintained for compatibility with previous Unix assemblers. - In AT&T syntax the size of memory operands is determined from the last character of the opcode name. Opcode suffixes of `b', `w', and `l' specify byte (8-bit), word (16-bit), and long (32-bit) memory references. Intel syntax accomplishes this by prefixes memory operands (NOT the opcodes themselves) with `byte ptr', `word ptr', and `dword ptr'. Thus, Intel `mov al, byte ptr ' is `movb , %al' in AT&T syntax. - Immediate form long jumps and calls are `lcall/ljmp $
, $' in AT&T syntax; the Intel syntax is `call/jmp far
:'. Also, the far return instruction is `lret $' in AT&T syntax; Intel syntax is `ret far '. - The AT&T assembler does not provide support for multiple section programs. Unix style systems expect all programs to be single sections. @end itemize -------------------------------------------------------- [Opcode Naming] Opcode names are suffixed with one character modifiers which specify the size of operands. The letters `b', `w', and `l' specify byte, word, and long operands. If no suffix is specified by an instruction and it contains no memory operands then `GAS' tries to fill in the missing suffix based on the destination register operand (the last one by convention). Thus, `mov %ax, %bx' is equivalent to `movw %ax, %bx'; also, `mov $1, %bx' is equivalent to `movw $1, %bx'. Note that this is incompatible with the AT&T Unix assembler which assumes that a missing opcode suffix implies long operand size. (This incompatibility does not affect compiler output since compilers always explicitly specify the opcode suffix.) Almost all opcodes have the same names in AT&T and Intel format. There are a few exceptions. The sign extend and zero extend instructions need two sizes to specify them. They need a size to sign/zero extend FROM and a size to zero extend TO. This is accomplished by using two opcode suffixes in AT&T syntax. Base names for sign extend and zero extend are `movs...' and `movz...' in AT&T syntax (`movsx' and `movzx' in Intel syntax). The opcode suffixes are tacked on to this base name, the FROM suffix before the TO suffix. Thus, `movsbl %al, %edx' is AT&T syntax for ``move sign extend FROM %al TO %edx.'' Possible suffixes, thus, are `bl' (from byte to long), `bw' (from byte to word), and `wl' (from word to long). The Intel-syntax conversion instructions `cbw' --- sign-extend byte in `%al' to word in `%ax', `cwde' --- sign-extend word in `%ax' to long in `%eax', `cwd' --- sign-extend word in `%ax' to long in `%dx:%ax', `cdq' --- sign-extend dword in `%eax' to quad in `%edx:%eax', are called `cbtw', `cwtl', `cwtd', and `cltd' in AT&T naming. `GAS' accepts either naming for these instructions. Far call/jump instructions are `lcall' and `ljmp' in AT&T syntax, but are `call far' and `jump far' in Intel convention. -------------------------------------------------------- [Register Naming] Register operands are always prefixed with `%'. The 80386 registers consist of - the 8 32-bit registers `%eax' (the accumulator), `%ebx', `%ecx', `%edx', `%edi', `%esi', `%ebp' (the frame pointer), and `%esp' (the stack pointer). - the 8 16-bit low-ends of these: `%ax', `%bx', `%cx', `%dx', `%di', `%si', `%bp', and `%sp'. - the 8 8-bit registers: `%ah', `%al', `%bh', `%bl', `%ch', `%cl', `%dh', and `%dl' (These are the high-bytes and low-bytes of `%ax', `%bx', `%cx', and `%dx') - the 6 section registers `%cs' (code section), `%ds' (data section), `%ss' (stack section), `%es', `%fs', and `%gs'. - the 3 processor control registers `%cr0', `%cr2', and `%cr3'. - the 6 debug registers `%db0', `%db1', `%db2', `%db3', `%db6', and `%db7'. - the 2 test registers `%tr6' and `%tr7'. - the 8 floating point register stack `%st' or equivalently `%st(0)', `%st(1)', `%st(2)', `%st(3)', `%st(4)', `%st(5)', `%st(6)', and `%st(7)'. -------------------------------------------------------- [Opcode Prefixes] Opcode prefixes are used to modify the following opcode. They are used to repeat string instructions, to provide section overrides, to perform bus lock operations, and to give operand and address size (16-bit operands are specified in an instruction by prefixing what would normally be 32-bit operands with a ``operand size'' opcode prefix). Opcode prefixes are usually given as single-line instructions with no operands, and must directly precede the instruction they act upon. For example, the `scas' (scan string) instruction is repeated with: repne scas Here is a list of opcode prefixes: - Section override prefixes `cs', `ds', `ss', `es', `fs', `gs'. These are automatically added by specifying using the
: form for memory references. - Operand/Address size prefixes `data16' and `addr16' change 32-bit operands/addresses into 16-bit operands/addresses. Note that 16-bit addressing modes (i.e. 8086 and 80286 addressing modes) are not supported (yet). - The bus lock prefix `lock' inhibits interrupts during execution of the instruction it precedes. (This is only valid with certain instructions; see a 80386 manual for details). - The wait for coprocessor prefix `wait' waits for the coprocessor to complete the current instruction. This should never be needed for the 80386/80387 combination. - The `rep', `repe', and `repne' prefixes are added to string instructions to make them repeat `%ecx' times. -------------------------------------------------------- [Memory References] An Intel syntax indirect memory reference of the form
:[ + * + ] is translated into the AT&T syntax
:(, , ) where and are the optional 32-bit base and index registers, is the optional displacement, and , taking the values 1, 2, 4, and 8, multiplies to calculate the address of the operand. If no is specified, is taken to be 1.
specifies the optional section register for the memory operand, and may override the default section register (see a 80386 manual for section register defaults). Note that section overrides in AT&T syntax MUST have be preceded by a `%'. If you specify a section override which coincides with the default section register, `GAS' does NOT output any section register override prefixes to assemble the given instruction. Thus, section overrides can be specified to emphasize which section register is used for a given memory operand. Here are some examples of Intel and AT&T style memory references: AT&T: `-4(%ebp)', Intel: `[ebp - 4]' is `%ebp'; is `-4'.
is missing, and the default section is used (`%ss' for addressing with `%ebp' as the base register). , are both missing. AT&T: `foo(,%eax,4)', Intel: `[foo + eax*4]' is `%eax' (scaled by a 4); is `foo'. All other fields are missing. The section register here defaults to `%ds'. AT&T: `foo(,1)'; Intel `[foo]' This uses the value pointed to by `foo' as a memory operand. Note that and are both missing, but there is only ONE `,'. This is a syntactic exception. AT&T: `%gs:foo'; Intel `gs:foo' This selects the contents of the variable `foo' with section register
being `%gs'. Absolute (as opposed to PC relative) call and jump operands must be prefixed with `*'. If no `*' is specified, `GAS' always chooses PC relative addressing for jump/call labels. Any instruction that has a memory operand MUST specify its size (byte, word, or long) with an opcode suffix (`b', `w', or `l', respectively). -------------------------------------------------------- [Handling of Jump Instructions] Jump instructions are always optimized to use the smallest possible displacements. This is accomplished by using byte (8-bit) displacement jumps whenever the target is sufficiently close. If a byte displacement is insufficient a long (32-bit) displacement is used. We do not support word (16-bit) displacement jumps (i.e. prefixing the jump instruction with the `addr16' opcode prefix), since the 80386 insists upon masking `%eip' to 16 bits after the word displacement is added. Note that the `jcxz', `jecxz', `loop', `loopz', `loope', `loopnz' and `loopne' instructions only come in byte displacements, so that if you use these instructions (`GCC' does not use them) you may get an error message (and incorrect code). The AT&T 80386 assembler tries to get around this problem by expanding `jcxz foo' to jcxz cx_zero jmp cx_nonzero cx_zero: jmp foo cx_nonzero: -------------------------------------------------------- [Floating Point] All 80387 floating point types except packed BCD are supported. (BCD support may be added without much difficulty). These data types are 16-, 32-, and 64- bit integers, and single (32-bit), double (64-bit), and extended (80-bit) precision floating point. Each supported type has an opcode suffix and a constructor associated with it. Opcode suffixes specify operand's data types. Constructors build these data types into memory. - Floating point constructors are `.float' or `.single', `.double', and `.tfloat' for 32-, 64-, and 80-bit formats. These correspond to opcode suffixes `s', `l', and `t'. `t' stands for temporary real, and that the 80387 only supports this format via the `fldt' (load temporary real to stack top) and `fstpt' (store temporary real and pop stack) instructions. - Integer constructors are `.word', `.long' or `.int', and `.quad' for the 16-, 32-, and 64-bit integer formats. The corresponding opcode suffixes are `s' (single), `l' (long), and `q' (quad). As with the temporary real format the 64-bit `q' format is only present in the `fildq' (load quad integer to stack top) and `fistpq' (store quad integer and pop stack) instructions. Register to register operations do not require opcode suffixes, so that `fst %st, %st(1)' is equivalent to `fstl %st, %st(1)'. Since the 80387 automatically synchronizes with the 80386 `fwait' instructions are almost never needed (this is not the case for the 80286/80287 and 8086/8087 combinations). Therefore, `GAS' suppresses the `fwait' instruction whenever it is implicitly selected by one of the `fn...' instructions. For example, `fsave' and `fnsave' are treated identically. In general, all the `fn...' instructions are made equivalent to `f...' instructions. If `fwait' is desired it must be explicitly coded. -------------------------------------------------------- [Notes] There is some trickery concerning the `mul' and `imul' instructions that deserves mention. The 16-, 32-, and 64-bit expanding multiplies (base opcode `0xf6'; extension 4 for `mul' and 5 for `imul') can be output only in the one operand form. Thus, `imul %ebx, %eax' does NOT select the expanding multiply; the expanding multiply would clobber the `%edx' register, and this would confuse `GCC' output. Use `imul %ebx' to get the 64-bit product in `%edx:%eax'. We have added a two operand form of `imul' when the first operand is an immediate mode expression and the second operand is a register. This is just a shorthand, so that, multiplying `%eax' by 69, for example, can be done with `imul $69, %eax' rather than `imul $69, %eax, %eax'. ================================================= [Acknowledgements] If you have contributed to `GAS' and your name isn't listed here, it is not meant as a slight. We just don't know about it. Send mail to the maintainer, and we'll correct the situation. Currently (January 1994), the maintainer is Ken Raeburn (email address `raeburn@cygnus.com'). Dean Elsner wrote the original Gnu assembler for the VAX. Jay Fenlason maintained GAS for a while, adding support for GDB-specific debug information and the 68k series machines, most of the preprocessing pass, and extensive changes in `messages.c', `input-file.c', `write.c'. K. Richard Pixley maintained GAS for a while, adding various enhancements and many bug fixes, including merging support for several processors, breaking GAS up to handle multiple object file format back ends (including heavy rewrite, testing, an integration of the coff and b.out back ends), adding configuration including heavy testing and verification of cross assemblers and file splits and renaming, converted GAS to strictly ANSI C including full prototypes, added support for m680[34]0 and cpu32, did considerable work on i960 including a COFF port (including considerable amounts of reverse engineering), a SPARC opcode file rewrite, DECstation, rs6000, and hp300hpux host ports, updated ``know'' assertions and made them work, much other reorganization, cleanup, and lint. Ken Raeburn wrote the high-level BFD interface code to replace most of the code in format-specific I/O modules. The original VMS support was contributed by David L. Kashtan. Eric Youngdale has done much work with it since. The Intel 80386 machine description was written by Eliot Dresselhaus. Minh Tran-Le at IntelliCorp contributed some AIX 386 support. The Motorola 88k machine description was contributed by Devon Bowen of Buffalo University and Torbjorn Granlund of the Swedish Institute of Computer Science. Keith Knowles at the Open Software Foundation wrote the original MIPS back end (`tc-mips.c', `tc-mips.h'), and contributed Rose format support (which hasn't been merged in yet). Ralph Campbell worked with the MIPS code to support a.out format. Support for the Zilog Z8k and Hitachi H8/300 and H8/500 processors (tc-z8k, tc-h8300, tc-h8500), and IEEE 695 object file format (obj-ieee), was written by Steve Chamberlain of Cygnus Support. Steve also modified the COFF back end to use BFD for some low-level operations, for use with the H8/300 and AMD 29k targets. John Gilmore built the AMD 29000 support, added `.include' support, and simplified the configuration of which versions accept which directives. He updated the 68k machine description so that Motorola's opcodes always produced fixed-size instructions (e.g. `jsr'), while synthetic instructions remained shrinkable (`jbsr'). John fixed many bugs, including true tested cross-compilation support, and one bug in relaxation that took a week and required the proverbial one-bit fix. Ian Lance Taylor of Cygnus Support merged the Motorola and MIT syntax for the 68k, completed support for some COFF targets (68k, i386 SVR3, and SCO Unix), added support for MIPS ECOFF and ELF targets, and made a few other minor patches. Steve Chamberlain made `GAS' able to generate listings. Hewlett-Packard contributed support for the HP9000/300. Jeff Law wrote GAS and BFD support for the native HPPA object format (SOM) along with a fairly extensive HPPA testsuite (for both SOM and ELF object formats). This work was supported by both the Center for Software Science at the University of Utah and Cygnus Support. Support for ELF format files has been worked on by Mark Eichin of Cygnus Support (original, incomplete implementation for SPARC), Pete Hoogenboom and Jeff Law at the University of Utah (HPPA mainly), Michael Meissner of the Open Software Foundation (i386 mainly), and Ken Raeburn of Cygnus Support (sparc, and some initial 64-bit support). Several engineers at Cygnus Support have also provided many small bug fixes and configuration enhancements. Many others have contributed large or small bugfixes and enhancements. If you have contributed significant work and are not mentioned on this list, and want to be, let us know. Some of the history has been lost; we are not intentionally leaving anyone out.