Info file: as, -*-Text-*- produced by texinfo-format-buffer from file: gas.texinfo  File: as Node: top, Prev: top, Up: top, Next: Syntax Overview, Usage *************** * Menu: * Syntax:: The (machine independent) syntax that assembly language files must follow. The machine dependent syntax can be found in the machine dependent section of the manual for the machine that you are using. * Segments:: How to use segments and subsegments, and how the assembler and linker will relocate things. * Symbols:: How to set up and manipulate symbols. * Expressions:: And how the assembler deals with them. * PseudoOps:: The assorted machine directives that tell the assembler exactly what to do with its input. * MachineDependent:: Information specific to each machine. * Maintenance:: Keeping the assembler running. * Retargeting:: Teaching the assembler about new machines. This document describes the GNU assembler `as'. This document does *not* describe what an assembler does, or how it works. This document also does *not* describe the opcodes, registers or addressing modes that `as' uses on any paticular computer that `as' runs on. Consult a good book on assemblers or the machine's architecture if you need that information. This document describes the pseudo-ops that `as' understands, and their syntax. This document also describes some of the machine-dependent features of various flavors of the assembler. This document also describes how the assembler works internally, and provides some information that may be useful to people attempting to port the assembler to another machine. Throughout this document, we assume that you are running "GNU", the portable operating system from the "Free Software Foundation, Inc.". This restricts our attention to certain kinds of computer (in paticular, the kinds of computers that GNU can run on); once this assumption is granted examples and definitions need less qualification. Readers should already comprehend: * Central processing unit * registers * memory address * contents of memory address * bit * 8-bit byte * 2's complement arithmetic `as' is part of a team of programs that turn a high-level human-readable series of instructions into a low-level computer-readable series of instructions. Different versions of `as' are used for different kinds of computer. In paticular, at the moment, `as' only works for the DEC Vax, the Motorola 68020, the Intel 80386 and the National Semiconductor 32xxx. Notation ======== GNU and `as' assume the computer that will run the programs it assembles will obey these rules. A (memory) "address" is 32 bits. The lowest address is zero. The "contents" of any memory address is one "byte" of exactly 8 bits. A "word" is 16 bits stored in two bytes of memory. The addresses of the bytes differ by exactly 1. Notice that the interpretation of the bits in a word and of how to address a word depends on which particular computer you are assembling for. A "long word", or "long", is 32 bits composed of four bytes. It is stored in 4 bytes of memory; these bytes have contiguous addresses. Again the interpretation and addressing of those bits is machine dependent. National Semiconductor 32xxx computers say double word where we say long. Numeric quantities are usually unsigned or 2's complement. Bytes, words and longs may store numbers. `as' manipulates integer expressions as 32-bit numbers in 2's complement format. When asked to store an integer in a byte or word, the lowest order bits are stored. The order of bytes in a word or long in memory is determined by what kind of computer will run the assembled program. We won't mention this important caveat again. The meaning of these terms has changed over time. Although byte used to mean any length of contiguous bits, byte now pervasively means exactly 8 contiguous bits. A word of 16 bits made sense for 16-bit computers. Even on 32-bit computers, a word still means 16 bits (to machine language programmers). To many other programmers of GNU a word means 32 bits, so beware. Similarly long means 32 bits: from "long word". National Semiconductor 32xxx machine language calls a 32-bit number a "double word". Names for integers of different sizes: some conventions length as vax 32xxx 68020 GNU C (bits) 8 byte byte byte byte char 16 word word word word short (int) 32 long long(-word) double-word long(-word) long (int) 64 quad quad(-word) 128 octa octa-word as, the GNU Assembler ===================== "As" is an assembler; it is one of the team of programs that `compile' your programs into the binary numbers that a computer uses to `run' your program. Often `as' reads a source program written by a compiler and writes an "object" program for the linker (sometimes referred to as a "loader") `ld' to read. The source program consists of "statements" and comments. Each statement might "assemble" to one (and only one) machine language instruction or to one very simple datum. Mostly you don't have to think about the assembler because the compiler invokes it as needed; in that sense the assembler is just another part of the compiler. If you write your own assembly language program, then you must run the assembler yourself to get an object file suitable for linking. You can read below how to do this. `as' is only intended to assemble the output of the C compiler `cc' for use by the linker `ld'. `as' (vax and 68020 versions) tries to assemble correctly everything that the standard assembler would assemble, with a few exceptions (described in the machine-dependent chapters.) Each version of the assembler knows about just one kind of machine language, but much is common between the versions, including object file formats, (most) assembler directives (often called "pseudo-ops)" and assembler syntax. Unlike older assemblers, `as' tries to assemble a source program in one pass of the source file. This subtly changes the meaning of the `.org' directive (*Note Org::.). If you want to write assembly language programs, you must tell `as' what numbers should be in a computer's memory, and which addresses should contain them, so that the program may be executed by the computer. Using symbols will prevent many bookkeeping mistakes that can occur if you use raw numbers. Command Line Synopsis ===================== as [ options ] [ -G GDB_symbol_file ] [ -o object_file ][ input1 ... ] After the program name `as' the command line may contain switches and file names in any order. The order of switches doesn't matter but the order of file names is significant. Only the assembler's name `as' is compulsory and it must (of course) be first. Switches -------- Except for `--' any command line argument that begins with a hyphen (`-') is a switch. Each switch changes the behavior of `as'. No switch changes the way another switch works. A switch is a `-' followed by a letter; the case of the letter is important. No switch (letter) should be used twice on the same command line. (Nobody has decided what two copies of the same switch should mean.) All switches are optional. Some switches expect exactly one file name to follow them. The file name may either immediately follow the switch'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 as -omy-object-file.o mumble Always, `--' (that's two hyphens, not one) by itself names the standard input file. Input File(s) ============= We use the words "source program", abbreviated "source", to describe the program input to one run of `as'. The program may be in one or more GNU files; how the source is partitioned into files doesn't change the meaning of the source. The source text is a catenation of the text in each file. Each time you run `as' it assembles exactly one source program. A source program text is made of one or more GNU 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 `as' is given no file names it attempts to read one input file from `as''s standard input. Use `--' if you need to explicitly name the standard input file in your command line. It is OK to assemble an empty source. You get a small harmless object (output) file. If you try to assemble no files then `as' will try to read standard input, which is normally your terminal. You may have to type ctl-D to tell `as' there is no more program to assemble. Input Filenames and Line-numbers -------------------------------- A line is text up to and including the next newline. The first line of a file is numbered 1, the next 2 and so on. There are two ways of locating a line in the input file(s) and both are 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. "Physical files" are those files named in the command line given to `as'. "Logical files" are "pretend" files which bear no relation to physical files. Logical file names help error messages reflect the proper source file. Often they are used when `as'' source is itself synthesized from other files. 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' switch. 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. This may still work, but hasn't been tested. The object file is for input to the linker `ld'. It contains assembled program code, information to help `ld' to integrate the assembled program into a runnable file and (optionally) symbolic information for the debugger. The precise format of object files is described elsewhere. Error and Warning Messages ========================== `as' may write warnings and error messages to the standard error file (usually your terminal). This should not happen when `as' is run automatically by a compiler. Error messages are useful for those (few) people who still write in assembly language. 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:line_number:Warning Message Text If a logical file name has been given (*Note 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::.) 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 UN*X tradition). Error messages have the format file_name:line_number:FATAL:Error Message Text The file name and line number are derived the same as for warning messages. The actual message text may be rather less explanatory because many of them aren't supposed to happen. Optional Switches ================= -f Works Faster --------------- `-f' should only be used when assembling programs written by a (trusted) compiler. `-f' causes the assembler to not bother pre-processing the input file(s) before assembling them. Needless to say, if the files actually need to be pre-processed (if the contain comments, for example), `as' will not work correctly if `-f' is used. -G Includes GDB Symbolic Information ------------------------------------ (This option is depreciated, and may stop working without warning. GNU is abandoning the GDB symbolic information. It doesn't speed things up by much, and is difficult to maintain.) The C compiler may produce (apart from an assembler source file of your program) symbolic information for the `gdb' program, in a file. Certain assembler statements manipulate this information, and `as' can include the symbolic information in the object file that is the result of your assembly. Use this switch to say which file contains the symbolic information. The switch needs exactly one filename. `as' directives that begin with `.gdb...' manipulate this `gdb' symbolic information. Unless you use a `-G' switch all `.gdb...' assembler statements are ignored. The `gdb' notes file is described elsewhere. -l Shortens Long Undefined Symbols ---------------------------------- If this switch is not given, references to undefined symbols will be a full long (32 bits) wide. (Since `as' cannot know where these symbols will end up being, `as' can only allocate space for the linker to fill in later. Since `as' doesn't know how far away these symbols will be, it allocates as much space as it can.) If this option is given, the references will only be one word wide (16 bits). This may be useful if you want the object file to be as small as possible, and you know that the relevant symbols will be less than 17 bits away. This switch only works with the MC68020 version of `as'. -L Includes Local Labels ------------------------ For historical reasons, labels beginning with `L' (upper case only) are called "local labels". Normally you don't see such labels 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 switch 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'. -m{c}680{0,1,2}0 Different Kinds of 68000 ----------------------------------------- The 68020 version of `as' is usually used to assemble programs for the Motorola MC68020 microprocessor. Occasionally it is used to assemble programs for the mostly-similar-but-slightly-different MC68000 or MC68010 microprocessors. You can give `as' the switches `-m68000', `-mc68000', `-m68010', `-mc68010', `-m68020', and `-mc68020' to tell it what processor it should be assembling for. Unfortunately, these switches are essentially ignored. -o Names the Object File ------------------------ There is always one object file output when you run `as'. By default it has the name `a.out'. You use this switch (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. -R Folds Data Segment into Text Segment --------------------------------------- `-R' tells `as' to write the object file as if all data-segment data lives in the text segment. This is only done at the very last moment: your binary data are the same, but data segment parts are relocated differently. The data segment part of your object file is zero bytes long because all it bytes are appended to the text segment. (*Note Segments::.) When you use `-R' it would be nice to generate shorter address displacements (possible because we don't have to cross segments) between text and data segment. We don't do this simply for compatibility with older versions of `as'. `-R' may work this way in future. -W Represses Warnings --------------------- `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 switch, any warning is repressed. This switch only affects warning messages: it cannot change any detail of how `as' assembles your file. Errors, which stop the assembly, are still reported. Useless (but Compatible) Switches --------------------------------- `As' accepts any of these switches, gives a warning message that the switch was ignored and proceeds. These switches are for compatibility with scripts designed for other people's assemblers. `-D' (Debug) `-S' (Symbol Table) `-T' (Token Trace) Obsolete switches used to debug old assemblers. `-V' (Virtualize Interpass Temporary File) Other assemblers use a temporary file. This switch commanded them to keep the information in active memory rather than in a disk file. `as' always does this, so this switch is redundant. `-J' (JUMPify Longer Branches) Many 32-bit computers permit a variety of branch instructions to do the same job. Some of these instructions are short (and fast) but have a limited range; others are long (and slow) but can branch anywhere in virtual memory. Often there are 3 flavors of branch: short, medium and long. Other assemblers would emit short and medium branches, unless told by this switch to emit short and long branches. This is an archaic machine-dependent switch. `-d' (Displacement size for JUMPs) Like the `-J' switch, this is archaic. It expects a number following the `-d'. Like switches that expect filenames, the number may immediately follow the `-d' (old standard) or constitute the whole of the command line argument that follows `-d' (GNU standard). `-t' (Temporary File Directory) Other assemblers may use a temporary file, and this switch takes a filename being the directory to site the temporary file. `as' does not use a temporary disk file, so this switch makes no difference. `-t' needs exactly one filename. Special Features to support Compilers ===================================== In order to assemble compiler output into something that will work, `as' will occasionlly do strange things to `.word' pseudo-ops. In particular, when `gas' assembles a pseudo-op of the form `.word sym1-sym2', and the difference between `sym1' and `sym2' does not fit in 16 bits, `as' will create a "secondary jump table", immediately before the next label. This SECONDARY JUMP TABLE will be preceeded by a short-jump to the first byte after the table. The short-jump prevents the flow-of-control from accidentally falling into the table. Inside the table will be a long-jump to `sym2'. The original `.word' will contain `sym1' minus (the address of the long-jump to sym2) If there were several `.word sym1-sym2' before the secondary jump table, all of them will be adjusted. If ther was a `.word sym3-sym4', that also did not fit in sixteen bits, a long-jump to `sym4' will be included in the secondary jump table, and the `.word'(s), will be adjusted to contain `sym3' minus (the address of the long-jump to sym4), etc. *This feature may be disabled by compiling `as' with the `-DWORKING_DOT_WORD' option.* This feature is likely to confuse assembly language programmers.  File: as Node: Syntax, Prev: top, Up: top, Next: Segments Syntax ****** This chapter informally defines the machine-independent syntax allowed in a source file. `as' has ordinary syntax; it tries to be upward compatible from BSD 4.2 assembler except `as' does not assemble Vax bit-fields. The Pre-processor ================= The preprocess phase handles several aspects of the syntax. The pre-processor will be disabled by the `-f' option, or if the first line of the source file is `#NO_APP'. The option to disable the pre-processor was designed to make compiler output assemble as fast as possible. 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. The pre-processor removes all comments, replacing them with a single space (for /* ... */ comments), or an appropriate number of newlines. The pre-processor converts character constants into the appropriate numeric values. This means that 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. 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 Characters::.), any whitespace means the same as exactly one space. Comments ======== There are two ways of rendering comments to `as'. In both cases the comment is equivalent to one space. Anything from `/*' to the next `*/' inclusive is a comment. /* 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 considered a comment and is ignored. The line comment character is `#' on the Vax, and `|' on the 68020. *Note MachineDependent::. 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'. 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 `_.$'. No symbol may begin with a digit. Case is significant. There is no length limit: all characters are significant. Symbols are delimited by characters not in that set, or by begin/end-of-file. (*Note Symbols::.) Statements ========== A "statement" ends at a newline character (`\n') or at a semicolon (`;'). The newline or semicolon is considered part of the preceding statement. Newlines and semicolons within character constants are an exception: they don't end statements. It is an error to end any statement with end-of-file: the last character of any input file should be a newline. You may write a statement on more than one line if you put a backslash (`\') immediately in front of any newlines within the statement. When `as' reads a backslashed newline both characters are ignored. You can even put backslashed newlines in the middle of symbol names without changing the meaning of your source program. An empty statement is OK, and may include whitespace. It is ignored. Statements begin with zero or more labels, followed by a "key symbol" which determines what kind of statement it is. The key symbol determines the syntax of the rest of the statement. If the symbol begins with a dot (.) then the statement is an assembler directive: typically valid for any computer. If the symbol begins with a letter the statement is an assembly language "instruction": it will assemble into a machine language instruction. Different versions of `as' for different computers will recognize different instructions. In fact, the same symbol may represent a different instruction in a different computer's assembly language. A label is usually a symbol immediately followed by a colon (`:'). Whitespace before a label or after a colon is OK. You may not have whitespace between a label's symbol and its colon. Labels are explained below. *Note Labels::. label: .directive followed by something another$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.  File: as Node: Characters, Up: Syntax, Next: Strings Character Constants ------------------- There are two kinds of character constants. "Characters" stand for one character in one byte and their values 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.  File: as Node: Strings, Prev: Characters, Up: Syntax Strings ....... A "string" is written between double-quotes. It may contain double-quotes or null characters. The way to get weird 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. `\EOF' A `\' followed by end-of-file erroneous. It is treated just like an end-of-file without a preceding backslash. `\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 Un*x systems, 8 and 9 are legal digits with values 010 and 011 respectively. `\\' 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 BSD 4.2 `as' recognizes, and is a subset of what most C compilers recognize. If you are in doubt, don't 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 accent acute, not an accent grave. A newline (or semicolon (`;')) immediately following an accent acute 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. GNU assumes your character code is ASCII: `'A' means 65, `'B' means 66, and so on. Number Constants ---------------- `as' distinguishes 3 flavors 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 a more than 32 bits. Flonums are floating point numbers, described below. Integers ........ 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 obvious values. To denote a negative integer, use the unary operator `-' discussed under expressions (*Note Unops::.). 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 complex: 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 the particular computer's floating point format(s) 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. `e' is recommended. Case is not important. (Any otherwise illegal letter will work here, but that might be changed. VAX BSD 4.2 assembler seems to allow any of `defghDEFGH'.) * An optional sign: either `+' or `-'. * An optional integer part: zero or more decimal digits. * An optional fraction part: `.' followed by zero or more decimal digits. * An optional exponent, consisting of: * A letter; the exact significance varies according to the computer that executes the program. `as' accepts any letter for now. Case is not important. * Optional sign: either `+' or `-'. * One or more decimal digits. At least one of INTEGER PART or FRACTION PART must be present. The floating point number has the obvious value. The computer running `as' needs no floating point hardware. `as' does all processing using integers.  File: as Node: Segments, Prev: Syntax, Up: top, Next: Symbols (Sub)Segments & Relocation ************************** Roughly, a "segment" is a range of addresses, with no gaps, with all data "in" those addresses being treated the same. For example there may be a "read only" segment. 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. That explanation is too simple, but it will suffice to explain how `as' works. `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 segment. Assigning run-time addresses to segments 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 three segments, any of which may be empty. These are named text, data and bss segments. Within the object file, the text segment starts at address 0, the data segment follows, and the bss segment follows the data segment. To let `ld' know which data will change when the segments 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 for each mention of an address in the object file: * At what address in the object file does this mention of an address begin? * How long (in bytes) is this mention? * Which segment does the address refer to? What is the numeric value of (ADDRESS - START-ADDRESS OF SEGMENT)? * Is the mention of an address "Program counter relative"? In fact, every address `as' ever thinks about is expressed as (SEGMENT + OFFSET INTO SEGMENT). Further, every expression `as' computes is of this segmented nature. So "absolute expression" means an expression with segment "absolute" (*Note LdSegs::.). A "pass1 expression" means an expression with segment "pass1" (*Note MythSegs::.). In this document "(segment, offset)" will be written as { segment-name (offset into segment) }. Apart from text, data and bss segments you need to know about the "absolute" segment. When `ld' mixes partial programs, addresses in the absolute segment remain unchanged. That is, address {absolute 0} is "relocated" to run-time address 0 by `ld'. Although two partial programs' data segments will not overlap addresses after linking, by definition their absolute segments 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 segments is extended to the "undefined" segment. Any address whose segment is unknown at assembly time is by definition rendered {undefined (something, unknown yet)}. 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 segment undefined. By analogy the word segment is to describe groups of segments in the linked program. `ld' puts all partial program's text segments in contiguous addresses in the linked program. It is customary to refer to the text segment of a program, meaning all the addresses of all partial program's text segments. Likewise for data and bss segments. Segments ======== Some segments are manipulated by `ld'; others are invented for use of `as' and have no meaning except during assembly.  File: as Node: LdSegs ld segments ----------- `ld' deals with just 5 kinds of segments, summarized below. text segment data segment These segments hold your program bytes. `as' and `ld' treat them as separate but equal segments. Anything you can say of one segment is true of the other. When the program is running however it is customary for the text segment to be unalterable: it will contain instructions, constants and the like. The data segment of a running program is usually alterable: for example, C variables would be stored in the data segment. bss segment This segment 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 segment 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 segment was invented to eliminate those explicit zeros from object files. absolute segment Address 0 of this segment 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 segment This "segment" is a catch-all for address references to objects not in the preceding segments. See the description of `a.out' for details. An idealized example of the 3 relocatable segments 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 Node: MythSegs Mythical Segments ----------------- These segments are invented for the internal use of `as'. They have no meaning at run-time. You don't need to know about these segments except that they might be mentioned in `as'' warning messages. These segments are invented to permit the value of every expression in your assembly language program to be a segmented address. absent segment An expression was expected and none was found. goof segment An internal assembler logic error has been found. This means there is a bug in the assembler. grand segment A "grand number" is a bignum or a flonum, but not an integer. If a number can't be written as a C `int' constant, it is a grand number. `as' has to remember that a flonum or a bignum does not fit into 32 bits, and cannot be a primary (*Note Primary::.) in an expression: this is done by making a flonum or bignum be of type "grand". This is purely for internal `as' convenience; grand segment behaves similarly to absolute segment. pass1 segment 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 (segment + offset in segment) assembly process. No compiler need emit such an expression. difference segment As an assist to the C compiler, expressions of the forms * (undefined symbol) - (expression) * (something) - (undefined symbol) * (undefined symbol) - (undefined symbol) are permitted to belong to the "difference" segment. `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 segment. 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. Sub-Segments ============ Assembled bytes fall into two segments: text and data. Because you may have groups of text or data that you want to end up near to each other in the object file, `as', allows you to use "subsegments". Within each segment, there can be numbered subsegments with values from 0 to 8192. Objects assembled into the same subsegment will be grouped with other objects in the same subsegment when they are all put into the object file. For example, a compiler might want to store constants in the text segment, but might not want to have them intersperced 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. Subsegments are optional. If you don't used subsegments, everything will be stored in subsegment number zero. Each subsegment is zero-padded up to a multiple of four bytes. (Subsegments may be padded a different amount on different flavors of `as'.) Subsegments 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, `ld' etc. have no concept of subsegments. They just see all your text subsegments as a text segment, and all your data subsegments as a data segment. To specify which subsegment you want subsequent statements assembled into, use 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 subsegment is text 0 anyway. .ascii "This lives in the first text subsegment. *" .text 1 .ascii "But this lives in the second text subsegment." .data 0 .ascii "This lives in the data segment," .ascii "in the first data subsegment." .text 0 .ascii "This lives in the first text segment," .ascii "immediately following the asterisk (*)." Each segment has a "location counter" incremented by one for every byte assembled into that segment. Because subsegments are merely a convenience restricted to `as' there is no concept of a subsegment location counter. There is no way to directly manipulate a location counter. The location counter of the segment that statements are being assembled into is said to be the "active" location counter. Bss Segment =========== The `bss' segment is used for local common variable storage. You may allocate address space in the `bss' segment, 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' segment are zeroed bytes. Addresses in the bss segment are allocated with a special statement; you may not assemble anything directly into the bss segment. Hence there are no bss subsegments.  File: as Node: Symbols, Prev: Segments, Up: top, Next: Expressions Symbols ******* Because the linker uses symbols to link, the debugger uses symbols to debug and the programmer uses symbols to name things, symbols are a central concept. Symbols do not appear in the object file in the order they are declared. This may break some debuggers.  File: as Node: Labels, 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. 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::.) 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 program refers to exactly one symbol. You may use that symbol name any number of times in an assembly 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. Their names are `0' `1' ... `9'. To define a local symbol, write a label of the form DIGIT:. To refer to the most recent previous definition of that symbol write DIGITb, using the same digit as when you defined the label. To refer to the next definition of a local label, write DIGITf where DIGIT gives you a choice of 10 forward references. The `b' stands for "backwards" and the `f' stands for "forwards". Local symbols are not used by the current 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 thinks about 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' switch then `as' will retain these symbols in the object file. By instructing `ld' to also retain these symbols, you may use them in debugging. `a 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:'. `control-A' This unusual character is included so you don't accidentally invent a symbol of the same name. The character has ASCII value `\001'. `an ordinal number' This is like 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 `L1^A1', the 44th `3:' is named `L3^A44'. Symbol Attributes ================= Every symbol has the attributes discussed below. The detailed definitions are in . 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. Value ----- The value of a symbol is (usually) 32 bits, the size of one C `int'. For a symbol which labels a location in the `text', `data', `bss' or `Absolute' segments the value is the number of addresses from the start of that segment to the label. Naturally for `text' `data' and `bss' segments the value of a symbol changes as `ld' changes segment 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 (i.e. addresses). The symbol refers to the first address of the allocated storage. Type ---- The type attribute of a symbol is 8 bits encoded in a devious way. We kept this coding standard for compatibility with older operating systems. 7 6 5 4 3 2 1 0 bit numbers +-----+-----+-----+-----+-----+-----+-----+-----+ | | | | | N_STAB bits | N_TYPE bits |N_EXT| | | | bit | +-----+-----+-----+-----+-----+-----+-----+-----+ n_type byte N_EXT bit ......... This bit is set if `ld' might need to use the symbol's value and type bits. If this bit is re-set then `ld' can ignore the symbol while linking. It is set in two cases. If the symbol is undefined, then `ld' is expected to find the symbol's value elsewhere in another program module. Otherwise the symbol has the value given, but this symbol name and value are revealed to any other programs linked in the same executable program. This second use of the `N_EXT' bit is most often done by a `.globl' statement. N_TYPE bits ........... These establish the symbol's "type", which is mainly a relocation concept. Common values are detailed in the manual describing the executable file format. N_STAB bits ........... Common values for these bits are described in the manual on the executable file format.. Desc(riptor) ------------ This is an arbitrary 16-bit value. You may establish a symbol's descriptor value by using a `.desc' statement (*Note Desc::.). A descriptor value means nothing to `as'. Other ----- This is an arbitrary 8-bit value. It means nothing to `as'. 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' pseudo-op. Thus, the expression `.=.+4' is the same as saying `.space 4'.  File: as Node: Expressions, Prev: Symbols, Up: top, Next: PseudoOps Expressions *********** An "expression" specifies an address or numeric value. Whitespace may precede and/or follow an expression. Empty Expressions ================= An empty expression has no operands: 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. Integer Expressions =================== An "integer expression" is one or more primaries delimited by operators.  File: as Node: Primary, Up: Expressions, Next: Unops Primaries --------- "Primaries" are symbols, numbers or subexpressions. Other languages might call primaries "arithmetic operands" but we don't want them confused with "instruction operands" of the machine language so we give them a different name. Symbols are evaluated to yield {SEGMENT VALUE} where SEGMENT is one of text, data, bss, absolute, or undefined. VALUE 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 unary operator followed by an primary. Operators --------- "Operators" are arithmetic marks, like + or %. Unary operators are followed by an primary. Binary operators appear between primaries. Operators may be preceded and/or followed by whitespace. Unary Operators ---------------