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GNU Info File | 1992-09-30 | 48.9 KB | 1,351 lines

This is Info file as.info, produced by Makeinfo-1.43 from the input file as-680x0.texinfo. This file documents the GNU Assembler "as". Copyright (C) 1991 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the section entitled "GNU General Public License" is included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the section entitled "GNU General Public License" may be included in a translation approved by the Free Software Foundation instead of in the original English. File: as.info, Node: Top, Next: Overview, Prev: (dir), Up: (dir) This file is a user guide to the GNU assembler `as'. This version of the file describes `as' configured to generate code for Motorola 680x0 architectures. * Menu: * Overview:: Overview * Invoking:: Command-Line Options * Syntax:: Syntax * Sections:: Sections and Relocation * Symbols:: Symbols * Expressions:: Expressions * Pseudo Ops:: Assembler Directives * M68K-Dependent:: Machine Dependent Features * Copying:: GNU GENERAL PUBLIC LICENSE * Index:: Index File: as.info, Node: Overview, Next: Invoking, Prev: Top, Up: Top Overview ******** Here is a brief summary of how to invoke `as'. For details, *note Comand-Line Options: Invoking.. as [ -a | -al | -as ] [ -D ] [ -f ] [ -I PATH ] [ -k ] [ -L ] [ -o OBJFILE ] [ -R ] [ -v ] [ -w ] [ -l ] [ -mc68000 | -mc68010 | -mc68020 ] [ -- | FILES ... ] `-a | -al | -as' Turn on assembly listings; `-al', listing only, `-as', symbols only, `-a', everything. `-D' This option is accepted only for script compatibility with calls to other assemblers; it has no effect on `as'. `-f' "fast"--skip preprocessing (assume source is compiler output) `-I PATH' Add PATH to the search list for `.include' directives `-k' Issue warnings when difference tables altered for long displacements. `-L' Keep (in symbol table) local symbols, starting with `L' `-o OBJFILE' Name the object-file output from `as' `-R' Fold data section into text section `-v' Announce `as' version `-W' Suppress warning messages `-l' Shorten references to undefined symbols, to one word instead of two `-mc68000 | -mc68010 | -mc68020' Specify what processor in the 68000 family is the target (default 68020) `-- | FILES ...' Standard input, or source files to assemble * Menu: * Manual:: Structure of this Manual * GNU Assembler:: as, the GNU Assembler * Object Formats:: Object File Formats * Command Line:: Command Line * Input Files:: Input Files * Object:: Output (Object) File * Errors:: Error and Warning Messages File: as.info, Node: Manual, Next: GNU Assembler, Prev: Overview, Up: Overview Structure of this Manual ======================== This manual is intended to describe what you need to know to use GNU `as'. We cover the syntax expected in source files, including notation for symbols, constants, and expressions; the directives that `as' understands; and of course how to invoke `as'. We also cover special features in the Motorola 680x0 configuration of `as', 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. File: as.info, Node: GNU Assembler, Next: Object Formats, Prev: Manual, Up: Overview as, the GNU Assembler ===================== GNU `as' is really a family of assemblers. This manual describes `as', a member of that family which is configured for the Motorola 680x0 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. `as' 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 `as' assemble correctly everything that the native assembler would. This doesn't mean `as' always uses the same syntax as another assembler for the same architecture; for example, we know of several incompatible versions of 680x0 assembly language syntax. Unlike older assemblers, `as' is designed to assemble a source program in one pass of the source file. This has a subtle impact on the `.org' directive (*note `.org': Org.). File: as.info, Node: Object Formats, Next: Command Line, Prev: GNU Assembler, Up: Overview 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. *Note Symbol Attributes: Symbol Attributes. On the Motorola 680x0, `as' is configured to produce `a.out' format object files. File: as.info, Node: Command Line, Next: Input Files, Prev: Object Formats, Up: Overview Command Line ============ After the program name `as', 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 `as' to assemble. Except for `--' any command line argument that begins with a hyphen (`-') is an option. Each option changes the behavior of `as'. 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: as -o my-object-file.o mumble.s as -omy-object-file.o mumble.s File: as.info, Node: Input Files, Next: Object, Prev: Command Line, Up: Overview Input Files =========== We use the phrase "source program", abbreviated "source", to describe the program input to one run of `as'. 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 `as' 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 `as' 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 `as' no file names it attempts to read one input file from the `as' standard input, which is normally your terminal. You may have to type ctl-D to tell `as' 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, `as' will produce 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. *Note Error and Warning Messages: Errors. "Physical files" are those files named in the command line given to `as'. "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 `as' source is itself synthesized from other files. *Note `.app-file': App-File. File: as.info, Node: Object, Next: Errors, Prev: Input Files, Up: Overview Output (Object) File ==================== Every time you run `as' it produces an output file, which is your assembly language program translated into numbers. This file is the object file, named `a.out' unless you tell `as' to give it another name by using the `-o' option. Conventionally, object file names end with `.o'. The default name of `a.out' is used for historical reasons: older assemblers were capable of assembling self-contained programs directly into a runnable program. 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. File: as.info, Node: Errors, Prev: Object, Up: Overview Error and Warning Messages ========================== `as' may write warnings and error messages to the standard error file (usually your terminal). This should not happen when a compiler runs `as' automatically. Warnings report an assumption made so that `as' could keep assembling a flawed program; errors report a grave problem that stops the assembly. Warning messages have the format file_name:NNN:Warning Message Text (where NNN is a line number). If a logical file name has been given (*note `.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 (*note `.line': Line.) 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:NNN: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. File: as.info, Node: Invoking, Next: Syntax, Prev: Overview, Up: Top Command-Line Options ******************** This chapter describes command-line options available in *all* versions of the GNU assembler; *note M68K-Dependent::., for options specific to the Motorola 680x0. Enable Listings: `-a', `-al', `-as' =================================== These options enable listing output from the assembler. `-a' by itself requests all listing output; `-al' requests only the output-program listing, and `-as' requests only a symbol table 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'. If you do not request listing output with one of the `-a' options, the listing-control directives have no effect. `-D' ==== This option has no effect whatsoever, but it is accepted to make it more likely that scripts written for other assemblers will also work with `as'. Work Faster: `-f' ================= `-f' should only be used when assembling programs written by a (trusted) compiler. `-f' stops the assembler from pre-processing the input file(s) before assembling them. *Note Pre-processing: Pre-processing. *Warning:* if the files actually need to be pre-processed (if they contain comments, for example), `as' will not work correctly if `-f' is used. `.include' search path: `-I' PATH ================================= Use this option to add a PATH to the list of directories `as' will search for files specified in `.include' directives (*note `.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, `as' searches any `-I' directories in the same order as they were specified (left to right) on the command line. Difference Tables: `-k' ======================= `as' sometimes alters the code emitted for directives of the form `.word SYM1-SYM2'; *note `.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". *Note Symbol Names::. Normally you don't 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 `as' and `ld' discard such labels, so you don't normally debug with them. This option tells `as' 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'. Name the Object File: `-o' ========================== There is always one object file output when you run `as'. 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, `as' will overwrite any existing file of the same name. Join Data and Text Sections: `-R' ================================= `-R' tells `as' 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 it bytes are appended to the text section. (*Note Sections and Relocation: Sections.) When you specify `-R' it would be possible to generate shorter address displacements (because we don't have to cross between text and data section). We refrain from doing this simply for compatibility with older versions of `as'. In future, `-R' may work this way. 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' ======================= `as' should never give a warning or error message when assembling compiler output. But programs written by people often cause `as' 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 `as' assembles your file. Errors, which stop the assembly, are still reported. File: as.info, Node: Syntax, Next: Sections, Prev: Invoking, Up: Top Syntax ****** This chapter describes the machine-independent syntax allowed in a source file. `as' syntax is similar to what many other assemblers use; it is inspired in BSD 4.2 assembler. * Menu: * Pre-processing:: Pre-processing * Whitespace:: Whitespace * Comments:: Comments * Symbol Intro:: Symbols * Statements:: Statements * Constants:: Constants File: as.info, Node: Pre-processing, Next: Whitespace, Prev: Syntax, Up: Syntax Pre-Processing ============== The pre-processor: * 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. Excess whitespace, comments, and character constants cannot be used in the portions of the input text that are not pre-processed. If the first line of an input file is `#NO_APP' or the `-f' option is given, the input file will not be pre-processed. Within such an input file, parts of the file can be pre-processed by putting a line that says `#APP' before the text that should be pre-processed, and putting a line that says `#NO_APP' after them. This feature is mainly intend to support `asm' statements in compilers whose output normally does not need to be pre-processed. File: as.info, Node: Whitespace, Next: Comments, Prev: Pre-processing, Up: Syntax 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 (*note Character Constants: Characters.), any whitespace means the same as exactly one space. File: as.info, Node: Comments, Next: Symbol Intro, Prev: Whitespace, Up: Syntax Comments ======== There are two ways of rendering comments to `as'. In both cases the comment is equivalent to one space. Anything from `/*' through the next `*/' is a comment. This means you may not nest these comments. /* The only way to include a newline ('\n') in a comment is to use this sort of comment. */ /* This sort of comment does not nest. */ Anything from the "line comment" character to the next newline is considered a comment and is ignored. The line comment character is `|' on the 680x0; *note M68K-Dependent::.. To be compatible with past assemblers, a special interpretation is given to lines that begin with `#'. Following the `#' an absolute expression (*note Expressions::.) is expected: this will be the logical line number of the next line. Then a string (*Note 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 `as'. File: as.info, Node: Symbol Intro, Next: Statements, Prev: Comments, Up: Syntax Symbols ======= A "symbol" is one or more characters chosen from the set of all letters (both upper and lower case), digits and the three characters `_.$' 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). *Note Symbols::. File: as.info, Node: Statements, Next: Constants, Prev: Symbol Intro, Up: Syntax 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 don't 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 `as' 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 will assemble 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. *Note Labels::. label: .directive followed by something another_label: # This is an empty statement. instruction operand_1, operand_2, ... File: as.info, Node: Constants, Prev: Statements, Up: Syntax 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. * Menu: * Characters:: Character Constants * Numbers:: Number Constants File: as.info, Node: Characters, Next: Numbers, Prev: Constants, Up: Constants 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. * Menu: * Strings:: Strings * Chars:: Characters File: as.info, Node: Strings, Next: Chars, Prev: Characters, Up: Characters 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 `as' to interpret the second character literally as a backslash (which prevents `as' 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. `\ DIGIT DIGIT DIGIT' 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. `\ ANYTHING-ELSE' Any other character when escaped by `\' will give a warning, but assemble 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 `as' has no other interpretation, so `as' 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, don't use an escape sequence. File: as.info, Node: Chars, Prev: Strings, Up: Characters 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. `as' assumes your character code is ASCII: `'A' means 65, `'B' means 66, and so on. File: as.info, Node: Numbers, Prev: Characters, Up: Constants Number Constants ---------------- `as' 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. * Menu: * Integers:: Integers * Bignums:: Bignums * Flonums:: Flonums File: as.info, Node: Integers, Next: Bignums, Prev: Numbers, Up: Numbers 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 (*note Prefix Operators: Prefix Ops.). File: as.info, Node: Bignums, Next: Flonums, Prev: Integers, Up: Numbers 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. File: as.info, Node: Flonums, Prev: Bignums, Up: Numbers Flonums ....... A "flonum" represents a floating point number. The translation is indirect: a decimal floating point number from the text is converted by `as' 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 `as' specialized to that computer. A flonum is written by writing (in order) * The digit `0'. * A letter, to tell `as' the rest of the number is a flonum. * 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. `as' does all processing using integers. Flonums are computed independently of any floating point hardware in the computer running `as'. File: as.info, Node: Sections, Next: Symbols, Prev: Syntax, Up: Top Sections and Relocation *********************** * Menu: * Secs Background:: Background * ld Sections:: ld Sections * as Sections:: as Internal Sections * Sub-Sections:: Sub-Sections * bss:: bss Section File: as.info, Node: Secs Background, Next: ld Sections, Prev: Sections, Up: Sections 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 `as' emits an object file, the partial program is assumed to start at address 0. `ld' will assign the final addresses the partial program occupies, so that different partial programs don't overlap. This is actually an over-simplification, but it will suffice to explain how `as' 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 `as' 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 will change when the sections are relocated, and how to change that data, `as' 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 (ADDRESS) - (START-ADDRESS OF SECTION)? * Is the reference to an address "Program-Counter relative"? In fact, every address `as' ever uses is expressed as (SECTION) + (OFFSET INTO SECTION) Further, every expression `as' computes is of this section-relative nature. "Absolute expression" means an expression with section "absolute" (*note ld Sections::.). A "pass1 expression" means an expression with section "pass1" (*note as Internal Sections: as Sections.). In this manual we use the notation {SECNAME N} to mean "offset N into section SECNAME". 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 two partial programs' data sections will not overlap addresses after linking, *by definition* their absolute sections will overlap. Address `{absolute 239}' in one partial program will always be the same address when the program is running as address `{absolute 239}' in any other partial 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 U}--where U will be 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 program's text sections. Likewise for data and bss sections. Some sections are manipulated by `ld'; others are invented for use of `as' and have no meaning except during assembly. File: as.info, Node: ld Sections, Next: as Sections, Prev: Secs Background, Up: Sections ld Sections =========== `ld' deals with just four kinds of sections, summarized below. *text section* *data section* These sections hold your program. `as' and `ld' treat them as separate but equal sections. Anything you can say of one section is true another. When the program is running, however, it is customary for the text section to be unalterable. The text section is often shared among processes: it will contain instructions, constants and the like. The data section of a running program is usually alterable: for example, C variables would be stored in the data section. *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 don't change during relocation. *undefined section* This "section" is a catch-all for address references to objects not in the preceding sections. 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 ... File: as.info, Node: as Sections, Next: Sub-Sections, Prev: ld Sections, Up: Sections as Internal Sections ==================== These sections are meant only for the internal use of `as'. They have no meaning at run-time. You don't really need to know about these sections for most purposes; but they can be mentioned in `as' warning messages, so it might be helpful to have an idea of their meanings to `as'. These sections are used to permit the value of every expression in your assembly language program to be a section-relative address. absent An expression was expected and none was found. ASSEMBLER-INTERNAL-LOGIC-ERROR! An internal assembler logic error has been found. This means there is a bug in the assembler. bignum/flonum If a number can't be written as a C `int' constant (a bignum or a flonum, but not an integer), it is recorded as belonging to this "section". `as' has to remember that a flonum or a bignum does not fit into 32 bits, and cannot be an argument (*note Arguments::.) in an expression: this is done by making a flonum or bignum be in a separate internal section. This is purely for internal `as' convenience; bignum/flonum section behaves similarly to absolute section. pass1 section The expression was impossible to evaluate in the first pass. The assembler will attempt a second pass (second reading of the source) to evaluate the expression. Your expression mentioned an undefined symbol in a way that defies the one-pass (section + offset in section) assembly process. No compiler need emit such an expression. *Warning:* the second pass is currently not implemented. `as' will abort with an error message if one is required. difference section As an assist to the C compiler, expressions of the forms (UNDEFINED SYMBOL) - (EXPRESSION) SOMETHING - (UNDEFINED SYMBOL) (UNDEFINED SYMBOL) - (UNDEFINED SYMBOL) are permitted, and belong to the difference section. `as' re-evaluates such expressions after the source file has been read and the symbol table built. If by that time there are no undefined symbols in the expression then the expression assumes a new section. The intention is to permit statements like `.word label - base_of_table' to be assembled in one pass where both `label' and `base_of_table' are undefined. This is useful for compiling C and Algol switch statements, Pascal case statements, FORTRAN computed goto statements and the like. File: as.info, Node: Sub-Sections, Next: bss, Prev: as Sections, Up: Sections Sub-Sections ============ Assembled bytes fall into two sections: text and data. You may have separate groups of text or data that you want to end up near to each other in the object file, even though they are not contiguous in the assembler source. `as' 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 will be grouped with other objects in the same subsection when they are all put into the object file. 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 don't use subsections, everything will be stored 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 will 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 EXPRESSION' or a `.data EXPRESSION' statement. EXPRESSION should be an absolute expression. (*Note 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 `as' there is no concept of a subsection location counter. There is no way to directly manipulate a location counter--but the `.align' directive will change it, and any label definition will capture its current value. The location counter of the section that statements are being assembled into is said to be the "active" location counter. File: as.info, Node: bss, Prev: Sub-Sections, Up: Sections 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. *Note `.comm': Comm, *note `.lcomm': Lcomm.. File: as.info, Node: Symbols, Next: Expressions, Prev: Sections, Up: Top 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:* `as' does not place symbols in the object file in the same order they were declared. This may break some debuggers. * Menu: * Labels:: Labels * Setting Symbols:: Giving Symbols Other Values * Symbol Names:: Symbol Names * Dot:: The Special Dot Symbol * Symbol Attributes:: Symbol Attributes File: as.info, Node: Labels, Next: Setting Symbols, Prev: Symbols, Up: Symbols Labels ====== A "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. File: as.info, Node: Setting Symbols, Next: Symbol Names, Prev: Labels, Up: Symbols 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 (*note Expressions::.). This is equivalent to using the `.set' directive. *Note `.set': Set. File: as.info, Node: Symbol Names, Next: Dot, Prev: Setting Symbols, Up: Symbols Symbol Names ============ Symbol names begin with a letter or with one of `_.$' That character may be followed by any string of digits, letters, underscores and dollar signs. 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 `N:' (where N represents any digit). To refer to the most recent previous definition of that symbol write `Nb', using the same digit as when you defined the label. To refer to the next definition of a local label, write `Nf'--where N 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 `as' and `ld' forget symbols that start with `L'. These labels are used for symbols you are never intended to see. If you give the `-L' option then `as' will retain these symbols in the object file. If you also instruct `ld' to retain these symbols, you may use them in debugging. `DIGIT' 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:'. A' This unusual character is included so you don't 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:'. For instance, the first `1:' is named `LA1', the 44th `3:' is named `LA44'. File: as.info, Node: Dot, Next: Symbol Attributes, Prev: Symbol Names, Up: Symbols The Special Dot Symbol ====================== The special symbol `.' refers to the current address that `as' is assembling into. Thus, the expression `melvin: .long .' will cause `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'. File: as.info, Node: Symbol Attributes, Prev: Dot, Up: Symbols 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, `as' 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. * Menu: * Symbol Value:: Value * Symbol Type:: Type * a.out Symbols:: Symbol Attributes: `a.out' File: as.info, Node: Symbol Value, Next: Symbol Type, Prev: Symbol Attributes, Up: Symbol Attributes 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 program, and `ld' will try to determine its value from other programs it is linked with. 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. File: as.info, Node: Symbol Type, Next: a.out Symbols, Prev: Symbol Value, Up: Symbol Attributes 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. File: as.info, Node: a.out Symbols, Prev: Symbol Type, Up: Symbol Attributes Symbol Attributes: `a.out' -------------------------- * Menu: * Symbol Desc:: Descriptor * Symbol Other:: Other File: as.info, Node: Symbol Desc, Next: Symbol Other, Prev: a.out Symbols, Up: a.out Symbols Descriptor .......... This is an arbitrary 16-bit value. You may establish a symbol's descriptor value by using a `.desc' statement (*note `.desc': Desc.). A descriptor value means nothing to `as'. File: as.info, Node: Symbol Other, Prev: Symbol Desc, Up: a.out Symbols Other ..... This is an arbitrary 8-bit value. It means nothing to `as'. File: as.info, Node: Expressions, Next: Pseudo Ops, Prev: Symbols, Up: Top Expressions *********** An "expression" specifies an address or numeric value. Whitespace may precede and/or follow an expression. * Menu: * Empty Exprs:: Empty Expressions * Integer Exprs:: Integer Expressions File: as.info, Node: Empty Exprs, Next: Integer Exprs, Prev: Expressions, Up: Expressions 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 `as' will assume a value of (absolute) 0. This is compatible with other assemblers. File: as.info, Node: Integer Exprs, Prev: Empty Exprs, Up: Expressions Integer Expressions =================== An "integer expression" is one or more *arguments* delimited by *operators*. * Menu: * Arguments:: Arguments * Operators:: Operators * Prefix Ops:: Prefix Operators * Infix Ops:: Infix Operators File: as.info, Node: Arguments, Next: Operators, Prev: Integer Exprs, Up: Integer Exprs 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 {SECTION NNN} where SECTION is one of text, data, bss, absolute, or undefined. NNN 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 `as' 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. File: as.info, Node: Operators, Next: Prefix Ops, Prev: Arguments, Up: Integer Exprs 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. File: as.info, Node: Prefix Ops, Next: Infix Ops, Prev: Operators, Up: Integer Exprs Prefix Operator --------------- `as' has the following "prefix operators". They each take one argument, which must be absolute. `-' "Negation". Two's complement negation. `~' "Complementation". Bitwise not.