The GNU Assember


  
The GNU assembler 'as' is primarily intended to assemble the output of the GNU C compiler for use by the linker, so it may be regarded as an internal part of TIGCC package. Although, it may be called as a standalone program, and GNU team tried to make as assemble correctly everything that other assemblers for the same machine would assemble. Any exceptions are documented explicitly. This doesn't mean as always uses the same syntax as another assembler for the same architecture; for example, there exist several incompatible versions of MC 68000 assembly language syntax, so the syntax used in GNU assembler is not exactly the same as in some other assemblers (so, it is incompatible with the A68k Assembler, which is the most used assembler for the TI-89 and TI-92+, and which is also included in the TIGCC package as a standalone program).

This documentation will cover 'as' features which are applicable to the TIGCC. The most frequent usage of 'as' would probably be as an Inline Assembler, which allows mixing assembly statements with C code using asm keyword.

This documentation is not intended as an introduction to programming in assembly language. In a similar vein, you will not find here details about machine architecture: here you can not expect detailed description of the instruction set, standard mnemonics, registers or addressing modes. You may want to consult the Motorola manufacturer's machine architecture manual for such information.

Command-Line Options

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 ctrl-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.

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.

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
If you are invoking as via the GNU C compiler (which is usually the case), you can use the '-Wa' option to pass arguments through to the assembler. The assembler arguments must be separated from each other (and the '-Wa') by commas. For example:
gcc -c -g -O -Wa,-alh,-L file.c
emits a listing to standard output with high-level and assembly source.

Usually you do not need to use this '-Wa' mechanism, since many compiler command-line options are automatically passed to the assembler by the compiler (you can call the GNU compiler driver with the '-v' option to see precisely what options it passes to each compilation pass, including the assembler).
-a[cdhlmns]

These options enable listing output from the assembler. By itself, '-a' requests high-level, assembly, and symbols listing. You can use other letters to select specific options for the list: '-ah' requests a high-level language listing, '-al' requests an output-program assembly listing '-as' requests a symbol table listing, and '-am' requests including macro expansions. High-level listings require that a compiler debugging option like '-g' be used, and that assembly listings ('-al') be requested also.

Use the '-ac' option to omit false conditionals from a listing. Any lines which are not assembled because of a false .if (or .ifdef, or any other conditional), or a true .if followed by an .else, will be omitted from the listing.

Use the '-ad' option to omit debugging directives from the listing.

Once you have specified one of these options, you can further control listing output and its appearance using the directives .list, .nolist, .psize, .eject, .title, and .sbttl.

The '-an' option turns off all forms processing. If you do not request listing output with one of the '-a' options, the listing-control directives have no effect.

The letters after '-a' may be combined into one option, e.g. '-aln' for assembly listing without forms processing. You can also use '=file' to set the name of the listing file. If this option is used, it must be the last one. By itself, '-a' defaults to '-ahls'.

--base-size-default-16
--base-size-default-32

This is a MC680x0-specific option. If you use an addressing mode with a base register without specifying the size, as will normally use the full 32 bit value. For example, the addressing mode '%a0@(%d0)' is equivalent to '%a0@(%d0:l)'. You may use the '--base-size-default-16' option to tell as to default to using the 16 bit value. In this case, '%a0@(%d0)' is equivalent to '%a0@(%d0:w)'. You may use the '--base-size-default-32' option to restore the default behaviour.

--bitwise-or

This is a MC680x0-specific option. Normally the character '|' is treated as a comment character, which means that it can not be used in expressions. The '--bitwise-or' option turns '|' into a normal character. In this mode, you must either use C style comments, or start comments with a '#' character at the beginning of a line.

-D

This option has no effect whatsoever, but it is accepted to make it more likely that scripts written for other assemblers also work with as.

--defsym sym=value

Define the symbol sym to be value before assembling the input file. value must be an integer constant. As in C, a leading '0x' indicates a hexadecimal value, and a leading '0' indicates an octal value.

--disp-size-default-16
--disp-size-default-32

This is a MC680x0-specific option. If you use an addressing mode with a displacement, and the value of the displacement is not known, as will normally assume that the value is 32 bits. For example, if the symbol 'disp' has not been defined, as will assemble the addressing mode '%a0@(disp,%d0)' as though 'disp' is a 32 bit value. You may use the '--disp-size-default-16' option to tell as to instead assume that the displacement is 16 bits. In this case, as will assemble '%a0@(disp,%d0)' as though 'disp' is a 16 bit value. You may use the '--disp-size-default-32' option to restore the default behaviour.

-f

'-f' should only be used when assembling programs written by a (trusted) compiler. '-f' stops the assembler from doing whitespace and comment preprocessing on the input file(s) before assembling them. This would speed up assembling. See Preprocessing.

Warning: if you use '-f' when the files actually need to be preprocessed (if they contain comments, for example), as does not work correctly.

--gstabs

Generate stabs debugging information for each assembler line. This may help debugging assembler code, if the debugger can handle it.

--help

Prints a summary of the command line options and exits.

-I path

Use this option to add a path to the list of directories as searches for files specified in .include directives (see .include directive). 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.

-J

Don't warn about signed overflow.

-K

as sometimes alters the code emitted for directives of the form '.word sym1-sym2'; see .word directive. You can use the '-K' option if you want a warning issued when this is done.

--keep-locals
-L

Labels beginning with 'L' (upper case only) are called local labels (see section Symbol Names). Normally you do not see such labels when debugging, because they are intended for the use of programs (like compilers) that compose assembler programs, not for your notice. Normally both as and the linker discard such labels, so you do not 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 to preserve symbols whose names begin with 'L'. Anyway, this is useless with TIGCC, because there is still no debuger for it.

-l

This is a MC680x0-specific option. You can use the '-l' option to shorten the size of references to undefined symbols. If you do not use the '-l' option, references to undefined symbols are wide enough for a full long (32 bits). Since as cannot know where these symbols end up, as can only allocate space for the linker to fill in later. Since as does not know how far away these symbols are, it allocates as much space as it can. If you use this option, the references are only 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 are always less than 17 bits away.

-M
--mri

The '-M' or '--mri' option selects MRI compatibility mode. This changes the syntax and pseudo-op handling of as to make it compatible with the ASM68K assembler from Microtec Research. The exact nature of the MRI syntax will not be documented here; in particular, the handling of macros and macro arguments is somewhat different. The purpose of this option is to permit assembling existing MRI assembler code using as. As this option is completely irrelevant with TIGCC, it will not be described in details.

--MD

as can generate a dependency file for the file it creates. This file consists of a single rule suitable for make describing the dependencies of the main source file. The rule is written to the file named in its argument. This feature is used in the automatic updating of makefiles. Not particulary useful with TIGCC.

-m CPU_ID

as can assemble code for several different members of the Motorola 680x0 family. The default depends upon how as was configured when it was built; normally, the default is to assemble code for the 68020 microprocessor. Note that it is true with the as which is the part of TIGCC package, so you by default can use even instructions which will be rejected by TI-89 and TI-92+ (be aware of this). '-m' options may be used to change the default. These options control which instructions and addressing modes are permitted. The members of the 680x0 family are very similar (for detailed information about the differences, see the Motorola manuals). There is a lot of various '-m' suboptions, but only '-m68000' is meaningful with TIGCC, because the processor which is built into TI-89 and TI-92+ is compatible only with ordinary MC 68000. That's why other '-m' variants will not be described here.

-o objfile

There is always one object file output when you run as. By default it has the name 'a.out'. You may use this option (which takes exactly one filename) to give the object file a different name. Whatever the object file is called, as overwrites any existing file of the same name.

-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 its bytes are appended to the text section (see section Sections and Relocation). In TIGCC, as is configured for COFF output, so this option is only useful if you use sections named '.text' and '.data'.

--register-prefix-optional

This is a MC680x0-specific option. In this configuration of as (which does not prepend an underscore to the names of user variables), the assembler requires a '%' before any use of a register name. This is intended to let the assembler distinguish between C variables and functions named 'a0' through 'a7', and so on. The '--register-prefix-optional' option may be used to permit omitting the '%'. If this is done, it will generally be impossible to refer to C variables and functions with the same names as register names.

--statistics

Use '--statistics' to display two statistics about the resources used by as: the maximum amount of space allocated during the assembly (in bytes), and the total execution time taken for the assembly (in CPU seconds).

--strip-local-absolute

Remove local absolute symbols from the outgoing symbol table.

--traditional-format

For some targets, the output of as is different in some ways from the output of some existing assembler. This switch requests as to use the traditional format instead. For example, it disables the exception frame optimizations which as normally does by default on gcc output.

-v
-version

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.

--version

Like '-v', but prints the version and exits immediately.

-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.

-Z

After an error message, as normally produces no output. If for some reason you are interested in object file output even after as gives an error message on your program, use the '-Z' option. If there are any errors, as continues anyways, and writes an object file after a final warning message of the form

'n errors, m warnings, generating bad object file.'

GNU Assembler Syntax

The general machine-independent syntax which as allows in a source file is similar to what many other assemblers use; it is inspired by the BSD 4.2 assembler. Motorola-specific features are explained at the end of this chapter.

Preprocessing

The as internal preprocessor: It does not do macro processing, include file handling, or anything else you may get from C compiler's preprocessor. You can do include file processing with the .include directive. You can use the GNU C compiler driver to get other "CPP" style preprocessing, by giving the input file a '.S' suffix.

Excess whitespace, comments, and character constants cannot be used in the portions of the input text that are not preprocessed.

If the first line of an input file is #NO_APP or if you use the '-f' option, whitespace and comments are not removed from the input file. Within an input file, you can ask for whitespace and comment removal in specific portions of the by putting a line that says #APP before the text that may contain whitespace or comments, and putting a line that says #NO_APP after this text. This feature is mainly intend to support asm statements in compilers whose output is otherwise free of comments and whitespace.

Whitespace

Whitespace is one or more blanks or tabs, in any order. Whitespace is used to separate symbols, and to make programs neater for people to read. Unless within character constants (see section Character Constants), 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 '/*' 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 MC 680x0; family of processors.

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).

Statements

A statement ends at a newline character ('\n') or line separator character (';' on MC 68000). The newline or separator character is considered part of the preceding statement. Newlines and separators within character constants are an exception: they do not end statements.

It is an error to end any statement with end-of-file: the last character of any input file should be a newline.

You may write a statement on more than one line if you put a backslash ('\') immediately in front of any newlines within the statement. When 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. If the symbol begins with a letter the statement is an assembly language instruction: it assembles into a machine language instruction.

A label is a symbol immediately followed by a colon (:). Whitespace before a label or after a colon is permitted, but you may not have whitespace between a label's symbol and its colon.
label:     .directive    followed by something
another_label:           /* This is an empty statement */
           instruction   operand_1, operand_2, ...

Constants

A constant is a number, written so that its value is known by inspection, without knowing any context. Like this:
.byte  74, 0112, 092, 0x4A, 0X4a, 'J, '\J    | All the same value.
.ascii "Ring the bell\7"                     | A string constant.
.octa  0x123456789abcdef0123456789ABCDEF0    | A bignum.
.float 0f-314159265358979323846264338327\
  95028841971.693993751E-40                  | - pi, a flonum.

Character Constants

There are two kinds of character constants. A character stands for one character in one byte and its value may be used in numeric expressions. String constants (properly called string literals) are potentially many bytes and their values may not be used in arithmetic expressions.
Strings
A string is written between double-quotes. It may contain double-quotes or null characters. The way to get special characters into a string is to escape these characters: precede them with a backslash '\' character. For example '\\' represents one backslash: the first '\' is an escape which tells 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.
\x hex_digits... A hex character code. All trailing hex digits are combined. Either upper or lower case x works.
\\ 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 '\' gives a warning, but assembles as if the '\' was not present. The idea is that if you used an escape sequence you clearly didn't want the literal interpretation of the following character. However 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, do not use an escape sequence.
Characters
A single character may be written as a single quote immediately followed by that character. The same escapes apply to characters as to strings. So if you want to write the character backslash, you must write '\\ where the first '\' escapes the second '\'. As you can see, the quote is an acute accent, not a grave accent. A newline (or semicolon ';') immediately following an acute accent is taken as a literal character and does not count as the end of a statement. The value of a character constant in a numeric expression is the byte-wide code for that character. as assumes your character code is ASCII: 'A means 65, and so on.

Number Constants

as distinguishes three kinds of numbers according to how they are stored in the target machine. Strong are numbers that would fit into an int in the C language. Bignums are integers, but they are stored in more than 32 bits. Flonums are floating point numbers, described below.
Integers
A binary integer is '0b' or '0B' followed by zero or more of the binary digits '01'.

An octal integer is '0' followed by zero or more of the octal digits ('01234567').

A decimal integer starts with a non-zero digit followed by zero or more digits ('0123456789').

A hexadecimal integer is '0x' or '0X' followed by one or more hexadecimal digits chosen from '0123456789abcdefABCDEF'.

Integers have the usual values. To denote a negative integer, use the prefix operator '-' discussed under expressions (see section Prefix Operator).
Bignums
A bignum has the same syntax and semantics as an integer except that the number (or its negative) takes more than 32 bits to represent in binary. The distinction is made because in some places integers are permitted while bignums are not.
Flonums
A flonum represents a floating point number. The translation is indirect: a decimal floating point number from the text is converted by 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. For the moment, this feature is not properly implemented under GNU Assembler used in TIGCC: it generates IEEE floats, instead of special SMAP II BCD floats used in TIOS Flonums will be explained here anyway, because the proper interpretation of them would be implemented in the future.

A flonum is written by writing (in order) 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.

Motorola 680x0 Dependent Features

In this configuration of as (which does not prepend an underscore to the names of user variables), the assembler requires a '%' before any use of a register name. This is intended to let the assembler distinguish between C variables and functions named 'a0' through 'a7', and so on.

There exists two quite different syntaxes for the Motorola 680x0. The first one was developed at MIT. The second one is the standard Motorola syntax for this chip, and it differs from the MIT syntax. as can accept Motorola syntax for operands, even if MIT syntax is used for other operands in the same instruction. The two kinds of syntax are fully compatible.

MIT Syntax

MIT Syntax uses instructions names and syntax compatible with the Sun assembler. Intervening periods are ignored; for example, 'movl' is equivalent to 'mov.l'.

In the following table apc stands for any of the address registers ('%a0' through '%a7'), the program counter ('%pc'), the zero-address relative to the program counter ('%zpc'), a suppressed address register ('%za0' through '%za7'), or it may be omitted entirely. The use of size means one of 'w' or 'l', and it may be omitted, along with the leading colon, unless a scale is also specified. The use of scale means one of '1', '2', '4', or '8', and it may always be omitted along with the leading colon.

The following addressing modes are understood (note that some of them are valid only on 68020 or later processors, not on ordinary 68000):
Immediate

'#number'.

Data Register

'%d0' through '%d7'.

Address Register

'%a0' through '%a7'. '%a7' is also known as '%sp', i.e. the Stack Pointer. %a6 is also known as '%fp', the Frame Pointer.

Address Register Indirect

'%a0@' through '%a7@'.

Address Register Postincrement

'%a0@+' through '%a7@+'.

Address Register Predecrement

'%a0@-' through '%a7@-'.

Indirect Plus Offset

'apc@(number)'.

Index

'apc@(number,register:size:scale)'. The number may be omitted.

Postindex

'apc@(number)@(onumber,register:size:scale)'. The onumber or the register, but not both, may be omitted.

Preindex

'apc@(number,register:size:scale)@(onumber)'. The number may be omitted. Omitting the register produces the Postindex addressing mode.

Absolute

'symbol', or 'digits', optionally followed by ':b', ':w', or ':l'.

Motorola Syntax

In the following table apc stands for any of the address registers ('%a0' through '%a7'), the program counter ('%pc'), the zero-address relative to the program counter ('%zpc'), or a suppressed address register ('%za0' through '%za7'). The use of size means one of 'w' or 'l', and it may always be omitted along with the leading dot. The use of scale means one of '1', '2', '4', or '8', and it may always be omitted along with the leading asterisk.

The following addressing modes are understood (note that some of them are valid only on 68020 or later processors, not on ordinary 68000):
Address Register Indirect

'(%a0)' through '(%a7)'. '%a7' is also known as '%sp', i.e. the Stack Pointer. %a6 is also known as '%fp', the Frame Pointer.

Address Register Postincrement

'(%a0)+' through '(%a7)+'.

Address Register Predecrement

'-(%a0)' through '-(%a7)'.

Indirect Plus Offset

'number(%a0)' through 'number(%a7)', or 'number(%pc)'. The number may also appear within the parentheses, as in '(number,%a0)'. When used with the 'pc', the number may be omitted (with an address register, omitting the number produces Address Register Indirect mode).

Index

'number(apc,register.size*scale)'. The number may be omitted, or it may appear within the parentheses. The apc may be omitted. The register and the apc may appear in either order. If both apc and register are address registers, and the size and scale are omitted, then the first register is taken as the base register, and the second as the index register.

Postindex

'([number,apc],register.size*scale,onumber)'. The onumber, or the register, or both, may be omitted. Either the number or the apc may be omitted, but not both.

Preindex

'([number,apc,register.size*scale],onumber)'. The number, or the apc, or the register, or any two of them, may be omitted. The onumber may be omitted. The register and the apc may appear in either order. If both apc and register are address registers, and the size and scale are omitted, then the first register is taken as the base register, and the second as the index register.

Branch Improvements

Certain pseudo opcodes are permitted for branch instructions. They expand to the shortest branch instruction that reach the target. Generally these mnemonics are made by substituting 'j' for 'b' at the start of a Motorola mnemonic.

The following table summarizes the pseudo-operations. A (*) flags cases that are more fully described after the table:

 Displacement
Pseudo-OpBYTEWORD68020 LONG68000/10 LONGnon-PC relative
jbsrbsr.sbsrbsr.ljsrjsr
jrabra.sbrabra.ljmpjmp
(*) jXXbXX.sbXXbXX.lbNX; jmp.lbNX; jmp
(*) dbXXdbXXdbXXdbXX; bra; jmp.ldbXX; bra; jmp.ldbXX; bra; jmp.l
(*) fjXXfbXX.wfbXX.wfbXX.l-fbNX.w; jmp

XX: condition
NX: negative of condition XX
jbsr
jra

These are the simplest jump pseudo-operations; they always map to one particular machine instruction, depending on the displacement to the branch target.

jXX

Here, 'jXX' stands for an entire family of pseudo-operations, where XX is a conditional branch or condition-code test. The full list of pseudo-ops in this family is:
 jhi   jls   jcc   jcs   jne   jeq   jvc
 jvs   jpl   jmi   jge   jlt   jgt   jle
For the cases of non-PC relative displacements and long displacements on the 68000 or 68010, as issues a longer code fragment in terms of NX, the opposite condition to XX. For example, for the non-PC relative case:
     jXX foo
gives
     bNXs oof
      jmp foo
 oof:

dbXX

The full family of pseudo-operations covered here is
 dbhi   dbls   dbcc   dbcs   dbne   dbeq   dbvc
 dbvs   dbpl   dbmi   dbge   dblt   dbgt   dble
 dbf    dbra   dbt
Other than for word and byte displacements, when the source reads 'dbXX foo', as emits
      dbXX oo1
      bra oo2
 oo1: jmpl foo
 oo2:

fjXX

This family includes
 fjne   fjeq   fjge   fjlt   fjgt   fjle   fjf
 fjt    fjgl   fjgle  fjnge  fjngl  fjngle fjngt
 fjnle  fjnlt  fjoge  fjogl  fjogt  fjole  fjolt
 fjor   fjseq  fjsf   fjsne  fjst   fjueq  fjuge
 fjugt  fjule  fjult  fjun
For branch targets that are not PC relative, as emits
      fbNX oof
      jmp foo
 oof:
when it encounters 'fjXX foo'.

Special Characters

The immediate character is '#' for Sun compatibility. The line-comment character is '|' (unless the '--bitwise-or' option is used). If a '#' appears at the beginning of a line, it is treated as a comment unless it looks like '# line file', in which case it is treated normally.


Sections and Relocation

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 reads many object files (partial programs) and combines their contents to form a runnable program (note that the TIGCC linker is still buggy in linking multiple object files). When as emits an object file, the partial program is assumed to start at address 0. The linker assigns the final addresses for the partial program, so that different partial programs do not overlap. This is actually an oversimplification, but it suffices to explain how as uses sections.

The linker 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.

When it generates COFF output (which is the case with the TIGCC), as can also generate whatever other named sections you specify using the .section directive. If you do not use any directives that place output in the '.text' or '.data' sections, these sections still exist, but are empty.

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 the linker know which data changes 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 the linker must know, each time an address in the object file is mentioned:
In fact, every address as ever uses is expressed as
(section) + (offset into section)
Further, most expressions as computes have this section-relative nature. Here, the notation {secname N} to mean "offset N into section secname" will be used.

Apart from text, data and bss sections you need to know about the absolute section (not supported in TIGCC linker yet). When the linker mixes partial programs, addresses in the absolute section should remain unchanged. For example, address {absolute 0} should be "relocated" to run-time address 0 by the linker. Although the linker never arranges two partial programs data sections with overlapping addresses after linking, by definition their absolute sections must overlap. Address {absolute 239} in one part of a program is always the same address when the program is running as address {absolute 239} in any other part of the program.

The idea of sections is extended to the undefined section. Any address whose section is unknown at assembly time is by definition rendered {undefined U} - where U is filled in later. Since numbers are always defined, the only way to generate an undefined address is to mention an undefined symbol. A reference to a named common block would be such a symbol: its value is unknown at assembly time so it has section undefined.

By analogy the word 'section' is used to describe groups of sections in the linked program. The linker should put all partial programs text sections in contiguous addresses in the linked program. It is customary to refer to the text section of a program, meaning all the addresses of all partial programs text sections. Likewise for data and bss sections.

Some sections are manipulated by the linker; others are invented for use of as and have no meaning except during assembly.

Linker Sections

The linker deals with just four kinds of sections, summarized below.
named sections
text section
data section

These sections hold your program. as and the linker 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 contains 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.

NOTE: As TIOS does not support bss sections, but various kernels like DoorsOS, Universal OS etc. does, bss sections are avaliable only if you compile your program in "Doors" mode. That's why all global variables must to be initialized if you compile the program in "nostub" mode.

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 the linker must not change when relocating. In this sense we speak of absolute addresses being "unrelocatable": they do not change during relocation. TIGCC linker does not support this section yet.

undefined section

This "section" is a catch-all for address references to objects not in the preceding sections.

Assembler Internal Sections

These sections are meant only for the internal use of as. They have no meaning at run-time. You do not really need to know about these sections for most purposes; but they can be mentioned in 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.
ASSEMBLER-INTERNAL-LOGIC-ERROR!

An internal assembler logic error has been found. This means there is a bug in the assembler.

expr section

The assembler stores complex expression internally as combinations of symbols. When it needs to represent an expression as a symbol, it puts it in the expr section.

Sub-Sections

Assembled bytes conventionally fall into two sections: text and data. You may have separate groups of data in named sections 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 go into the object file together with other objects in the same subsection. For example, a compiler might want to store constants in the text section, but might not want to have them interspersed with the program being assembled. In this case, the compiler could issue a '.text 0' before each section of code being output, and a '.text 1' before each group of constants being output.

Subsections are optional. If you do not use subsections, everything goes in subsection number zero.

Each subsection is zero-padded up to a multiple of four bytes. (Subsections may be padded a different amount on different flavors of as.)

Subsections appear in your object file in numeric order, lowest numbered to highest. The object file contains no representation of subsections; the linker and other programs that manipulate object files see no trace of them. They just see all your text subsections as a text section, and all your data subsections as a data section.

To specify which subsection you want subsequent statements assembled into, use a numeric argument to specify it, in a '.text expression' or a '.data expression' statement. When generating COFF output (which is the case with TIGCC), you can also use an extra subsection argument with arbitrary named sections: '.section nameexpression'. Expression should be an absolute expression (see section Expressions). If you just say '.code' 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 changes it, and any label definition captures its current value. The location counter of the section where statements are being assembled is said to be the active location counter.

bss Section

The bss section is used for local common variable storage. You may allocate address space in the bss section, but you may not dictate data to load into it before your program executes. When your program starts running, all the contents of the bss section are zeroed bytes. As TIOS does not support bss sections, but various kernels like DoorsOS, Universal OS etc. does, bss sections are avaliable only if you compile your program in "Doors" mode. That's why all global variables must to be initialized if you compile the program in "nostub" mode.

The .lcomm pseudo-op defines a symbol in the bss section. The .comm pseudo-op may be used to declare a common symbol, which is another form of uninitialized symbol.

When assembling for a target which supports multiple sections, such as COFF (used in TIGCC), you may switch into the .bss section and define symbols as usual (see .section directive). You may only assemble zero values into the section. Typically the section will only contain symbol definitions and .skip directives.


Assembler 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 (the TIGCC does not include the debugger yet). Note that as does not place symbols in the object file in the same order they were declared.

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 (see section Expressions). This is equivalent to using the .set directive.

Symbol Names

Symbol names begin with a letter or with one of '._$'. That character may be followed by any string of digits, letters, dollar signs, and underscores.

Case of letters is significant: foo is a different symbol name than Foo.

Each symbol has exactly one name. Each name in an assembly language program refers to exactly one symbol. You may use that symbol name any number of times in a program.

Local Symbol Names

Local symbols help compilers and programmers use names temporarily. There are ten local symbol names, which are re-used throughout the program. You may refer to them using the names '0' '1' ... '9'. To define a local symbol, write a label of the form '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' 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 (except if you refer them explicitely in 'asm' statements).

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 the linker forget symbols that start with 'L'. These labels are used for symbols you are never intended to see. If you use the '-L' option then as retains these symbols in the object file.
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:'.
C-A ("smiling face")
This unusual character is included so you do not accidentally invent a symbol of the same name. The character has ASCII value '\001'.
ordinal number
This is a serial number to keep the labels distinct. The first '0:' gets the number '1'; The 15th '0:' gets the number '15' etc. Likewise for the other labels '1:' through '9:'.
For instance, the first 1: is named L1C-A1, the 44th 3: is named L3C-A44.

The Special Dot Symbol

The special symbol '.' refers to the current address that as is assembling into. Thus, the expression 'melvin: .long .' defines melvin to contain its own address. Assigning a value to '.' is treated the same as a .org directive. Thus, the expression '.=.+4' is the same as saying '.space 4'.

Symbol Attributes

Every symbol has, as well as its name, the attributes "Value" and "Type". Depending on output format, symbols can also have auxiliary attributes.

If you use a symbol without defining it, 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 Attribute

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 the linker changes section base addresses during linking. Absolute symbols values do not change during linking: that is why they are called absolute.

The value of an undefined symbol is treated in a special way. If it is 0 then the symbol is not defined in this assembler source file, and the linker should try to determine its value from other files linked into the same program. You make this kind of symbol simply by mentioning a symbol name without defining it. A non-zero value represents a .comm common declaration. The value is how much common storage to reserve, in bytes (addresses). The symbol refers to the first address of the allocated storage.

Type Attribute

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. Here symbol attributes for COFF (used in TIGCC) will be explained.

The COFF format supports a multitude of auxiliary symbol attributes; like the primary symbol attributes, they are set between .def and .endef directives.
Primary Attributes
The symbol name is set with .def; the value and type, respectively, with .val and .type.
Auxiliary Attributes
The as directives .dim, .line, .scl, .size, and .tag can generate auxiliary symbol table information for COFF.


Assembler Expressions

An expression specifies an address or numeric value. Whitespace may precede and/or follow an expression.

The result of an expression must be an absolute number, or else an offset into a particular section. If an expression is not absolute, and there is not enough information when as sees the expression to know its section, a second pass over the source program might be necessary to interpret the expression--but the second pass is currently not implemented. as aborts with an error message in this situation.

Empty Expressions

An empty expression has no value: it is just whitespace or null. Wherever an absolute expression is required, you may omit the expression, and as assumes a value of (absolute) 0. This is compatible with other assemblers.

Integer Expressions

An integer expression is one or more arguments delimited by operators.

Arguments

Arguments are symbols, numbers or subexpressions. In other contexts arguments are sometimes called "arithmetic operands". In this documentation, 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. Principally, 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.

Operators

Operators are arithmetic functions, like '+' or '%'. Prefix operators are followed by an argument. Infix operators appear between their arguments. Operators may be preceded and/or followed by whitespace.

Prefix Operators

as has the following prefix operators. They each take one argument, which must be absolute.

- Negation. Two's complement negation.
~ Complementation. Bitwise not.

Infix Operators

Infix operators take two arguments, one on either side. Operators have precedence, but operations with equal precedence are performed left to right. Apart from '+' or '-', both arguments must be absolute, and the result is absolute.
  1. Highest Precedence

    + Multiplication.
    / Division. Truncation is the same as the C operator '/'.
    % Remainder.
    < (or <<) Shift Left. Same as the C operator '<<'.
    > (or >>) Shift Right. Same as the C operator '>>'.

  2. Intermediate precedence

    | Bitwise Inclusive Or (it seems that it does not work correctly on MC 68000, because it causes the colision with the line comment character '|').
    & Bitwise And.
    ^ Bitwise Exclusive Or.
    ! Bitwise Or Not.

  3. Lowest Precedence

    + Addition. If either argument is absolute, the result has the section of the other argument. You may not add together arguments from different sections.
    - Subtraction. If the right argument is absolute, the result has the section of the left argument. If both arguments are in the same section, the result is absolute. You may not subtract arguments from different sections.
In short, it's only meaningful to add or subtract the offsets in an address; you can only have a defined section in one of the two arguments.


Assembler Directives

All assembler directives have names that begin with a period ('.'). The rest of the name is letters, usually in lower case.

.abort

This directive stops the assembly immediately. It is for compatibility with other assemblers. The original idea was that the assembly language source would be piped into the assembler. If the sender of the source quit, it could use this directive tells as to quit also. One day .abort will not be supported.

.ABORT

When producing COFF output, as accepts this directive as a synonym for '.abort'.

.align abs-expr, abs-expr, abs-expr

Pad the location counter (in the current subsection) to a particular storage boundary. The first expression (which must be absolute) is the alignment required, as described below.

The second expression (also absolute) gives the fill value to be stored in the padding bytes. It (and the comma) may be omitted. If it is omitted, the padding bytes are normally zero. However, on some systems, if the section is marked as containing code and the fill value is omitted, the space is filled with no-op instructions (I didn't checked what is the case with TIGCC).

The third expression is also absolute, and is also optional. If it is present, it is the maximum number of bytes that should be skipped by this alignment directive. If doing the alignment would require skipping more bytes than the specified maximum, then the alignment is not done at all. You can omit the fill value (the second argument) entirely by simply using two commas after the required alignment; this can be useful if you want the alignment to be filled with no-op instructions when appropriate.

The way the required alignment is specified varies from system to system. For the MC 68000, the first expression is the alignment request in bytes. For example '.align 8' advances the location counter until it is a multiple of 8. If the location counter is already a multiple of 8, no change is needed. GAS also provides .balign and .p2align directives, described later, which have a consistent behavior across all architectures.

.app-file string

.app-file (which may also be spelled '.file') tells as that we are about to start a new logical file. string is the new file name. In general, the filename is recognized whether or not it is surrounded by quotes '"'; but if you wish to specify an empty file name is permitted, you must give the quotes (""). This statement may go away in future: it is only recognized to be compatible with old as programs.

.ascii string, string...

.ascii expects zero or more string literals (see section Strings) separated by commas. It assembles each string (with no automatic trailing zero byte) into consecutive addresses.

.asciz string, string...

.asciz is just like .ascii, but each string is followed by a zero byte. The "z" in '.asciz' stands for "zero".

.balign[wl] abs-expr, abs-expr, abs-expr

Pad the location counter (in the current subsection) to a particular storage boundary. The first expression (which must be absolute) is the alignment request in bytes. For example '.balign 8' advances the location counter until it is a multiple of 8. If the location counter is already a multiple of 8, no change is needed.

The second expression (also absolute) gives the fill value to be stored in the padding bytes. It (and the comma) may be omitted. If it is omitted, the padding bytes are normally zero. However, on some systems, if the section is marked as containing code and the fill value is omitted, the space is filled with no-op instructions (I didn't checked what is the case with TIGCC).

The third expression is also absolute, and is also optional. If it is present, it is the maximum number of bytes that should be skipped by this alignment directive. If doing the alignment would require skipping more bytes than the specified maximum, then the alignment is not done at all. You can omit the fill value (the second argument) entirely by simply using two commas after the required alignment; this can be useful if you want the alignment to be filled with no-op instructions when appropriate.

The .balignw and .balignl directives are variants of the .balign directive. The .balignw directive treats the fill pattern as a two byte word value. The .balignl directives treats the fill pattern as a four byte longword value. For example, '.balignw 4,0x368d' will align to a multiple of 4. If it skips two bytes, they will be filled in with the value 0x368d. If it skips 1 or 3 bytes, the fill value is undefined.

.byte expressions

.byte expects zero or more expressions, separated by commas. Each expression is assembled into the next byte.

.comm symbol, length

.comm declares a common symbol named symbol. When linking, a common symbol in one object file may be merged with a defined or common symbol of the same name in another object file. If the linker does not see a definition for the symbol (just one or more common symbols) then it should allocate length bytes of uninitialized memory. length must be an absolute expression. If the linker sees multiple common symbols with the same name, and they do not all have the same size, it will allocate space using the largest size.

.data subsection

.data tells as to assemble the following statements onto the end of the data subsection numbered subsection (which is an absolute expression). If subsection is omitted, it defaults to zero.

In order to be compatible with the Sun assembler the 680x0 assembler understands directives .data1 and .data2 as alternatives to .data 1 and .data 2

.def name

Begin defining debugging information for a symbol name; the definition extends until the .endef directive is encountered. Anyway, it is useless under TIGCC, because the debugger is not implemented yet.

.desc symbol, abs-expression

This directive is not available when as is configured for COFF output (what is the case with TIGCC). For the sake of compatibility, as accepts it, but produces no output, when configured for COFF.

.dim

This directive is generated by compilers to include auxiliary debugging information in the symbol table. It is only permitted inside .def/.endef pairs.

.double flonums

.double expects zero or more flonums, separated by commas. It assembles floating point numbers. This is not yet particulary useful on GNU Assembler used in TIGCC, because it currently stores IEEE floats, but TIOS uses SMAP II BCD floats.

.eject

Force a page break at this point, when generating assembly listings.

.else

.else is part of the as support for conditional assembly; see .if directive. It marks the beginning of a section of code to be assembled if the condition for the preceding .if was false.

.endef

This directive flags the end of a symbol definition begun with .def.

.endif

.endif is part of the as support for conditional assembly; it marks the end of a block of code that is only assembled conditionally. See .if directive.

.endm

Mark the end of a macro definition. See .macro.

.equ symbol, expression

This directive sets the value of symbol to expression. It is synonymous with .set directive.

.equiv symbol, expression

The .equiv directive is like .equ and .set, except that the assembler will signal an error if symbol is already defined.

Except for the contents of the error message, this is roughly equivalent to
.ifdef SYM
.err
.endif
.equ SYM,VAL

.err

If as assembles a .err directive, it will print an error message and, unless the '-Z' command option was used, it will not generate an object file. This can be used to signal error an conditionally compiled code.

.even

This directive is a special case of the .align directive; it aligns the output to an even byte boundary. It is MC68000-specific directive introduced in order to be compatible with the Sun assembler the 680x0 assembler. understands the following directives.

.exitm

Exit early from the current macro definition. See .macro.

.extern

.extern is accepted in the source program (for compatibility with other assemblers) but it is ignored. as treats all undefined symbols as external.

.file string

.file (which may also be spelled '.app-file') tells as that we are about to start a new logical file. string is the new file name. In general, the filename is recognized whether or not it is surrounded by quotes '"'; but if you wish to specify an empty file name, you must give the quotes (""). This statement may go away in future: it is only recognized to be compatible with old as programs.

.fill repeat , size , value

result, size and value are absolute expressions. This emits repeat copies of size bytes. repeat may be zero or more. size may be zero or more, but if it is more than 8, then it is deemed to have the value 8, compatible with other people's assemblers. The contents of each repeat bytes is taken from an 8-byte number. The highest order 4 bytes are zero. The lowest order 4 bytes are value rendered in the byte-order of an integer on the computer as is assembling for (big endian on MC 68000). Each size bytes in a repetition is taken from the lowest order size bytes of this number. Again, this bizarre behavior is compatible with other people's assemblers.

size and value are optional. If the second comma and value are absent, value is assumed zero. If the first comma and following tokens are absent, size is assumed to be 1.

.float flonums

This directive assembles zero or more flonums, separated by commas. It has the same effect as .single. This is not yet particulary useful on GNU Assembler used in TIGCC, because it currently stores IEEE floats, but TIOS uses SMAP II BCD floats.

.global symbol

.global makes the symbol visible to the linker. If you define symbol in your partial program, its value is made available to other partial programs that are linked with it. Otherwise, symbol takes its attributes from a symbol of the same name from another file linked into the same program.

.globl symbol

Spelling '.globl' is accepted as an alternative for '.global', for compatibility with other assemblers.

.hword expressions

This expects zero or more expressions, and emits a 16 bit number for each.

This directive is a synonym for '.short'; depending on the target architecture, it may also be a synonym for '.word', which is the case on MC 68000.

.ident

This directive is used by some assemblers to place tags in object files. as simply accepts the directive for source-file compatibility with such assemblers, but does not actually emit anything for it.

.if absolute expression

.if marks the beginning of a section of code which is only considered part of the source program being assembled if the argument (which must be an absolute expression) is non-zero. The end of the conditional section of code must be marked by .endif; optionally, you may include code for the alternative condition, flagged by .else.

.ifdef symbol

Assembles the following section of code if the specified symbol has been defined.

.ifndef symbol

Assembles the following section of code if the specified symbol has not been defined.

.ifnotdef symbol

Alternative spelling for '.ifndef'.

.include "file"

This directive provides a way to include supporting files at specified points in your source program. The code from file is assembled as if it followed the point of the '.include'; when the end of the included file is reached, assembly of the original file continues. You can control the search paths used with the '-I' command-line option (see section Command-Line Options). Quotation marks are required around file.

.int expressions

Expect zero or more expressions, of any section, separated by commas. For each expression, emit a number that, at run time, is the value of that expression. The byte order and bit size of the number depends on what kind of target the assembly is for (big endian 32-bit value on MC 68000; be aware that in the TIGCC, C language 'int' variables are 16-bit values by default).

.irp symbol, values...

Evaluate a sequence of statements assigning different values to symbol. The sequence of statements starts at the .irp directive, and is terminated by an .endr directive. For each value, symbol is set to value, and the sequence of statements is assembled. If no value is listed, the sequence of statements is assembled once, with symbol set to the null string. To refer to symbol within the sequence of statements, use \symbol.

For example, assembling
.irp   param,1,2,3
move.l  %d\param,-(%sp)
.endr
is equivalent to assembling
move.l %d1,-(%sp)
move.l %d2,-(%sp)
move.l %d3,-(%sp)

.irpc symbol, value

Evaluate a sequence of statements assigning different values to symbol. The sequence of statements starts at the .irpc directive, and is terminated by an .endr directive. For each character in value, symbol is set to the character, and the sequence of statements is assembled. If no value is listed, the sequence of statements is assembled once, with symbol set to the null string. To refer to symbol within the sequence of statements, use \symbol.

For example, assembling
.irp   param,123
move.l  %d\param,-(%sp)
.endr
is equivalent to assembling
move.l %d1,-(%sp)
move.l %d2,-(%sp)
move.l %d3,-(%sp)

.lcomm symbol, length

Reserve length (an absolute expression) bytes for a local common denoted by symbol. The section and value of symbol are those of the new local common. The addresses are allocated in the bss section, so that at run-time the bytes start off zeroed. Symbol is not declared global (see .global directive), so is normally not visible to the linker.

.lflags

as accepts this directive, for compatibility with other assemblers, but ignores it.

.line line-number

Change the logical line number. line-number must be an absolute expression. The next line has that logical line number. Therefore any other statements on the current line (after a statement separator character) are reported as on logical line number line-number - 1. One day as will no longer support this directive: it is recognized only for compatibility with existing assembler programs.

.linkonce [type]

Mark the current section so that the linker only includes a single copy of it. This directive is not supported with the object file format used with TIGCC, so it will not be described.

.ln line-number

'.ln' is a synonym for '.line'.

.mri val

If val is non-zero, this tells as to enter MRI mode. If val is zero, this tells as to exit MRI mode. This change affects code assembled until the next .mri directive, or until the end of the file. See command switch '-M' for more info.

.list

Control (in conjunction with the .nolist directive) whether or not assembly listings are generated. These two directives maintain an internal counter (which is zero initially). .list increments the counter, and .nolist decrements it. Assembly listings are generated whenever the counter is greater than zero.

By default, listings are disabled. When you enable them (with the '-a' command line option; see section Command-Line Options), the initial value of the listing counter is one.

.long expressions

.long is the same as .int.

.macro macname [macargs...]

The commands .macro and .endm allow you to define macros that generate assembly output. For example, this definition specifies a macro sum that puts a sequence of numbers into memory:
.macro sum from=0, to=5
.long  \from
.if    \to-\from
sum    "(\from+1)",\to
.endif
.endm
With that (recursive) definition, 'SUM 0,5' is equivalent to this assembly input:
.long   0
.long   1
.long   2
.long   3
.long   4
.long   5
.macro begin the definition of a macro called macname. If your macro definition requires arguments, specify their names after the macro name, separated by commas or spaces. You can supply a default value for any macro argument by following the name with '=default'. For example, these are all valid .macro statements:
.macro comm

Begin the definition of a macro called comm, which takes no arguments.

.macro plus1 p, p1
.macro plus1 p p1

Either statement begins the definition of a macro called plus1, which takes two arguments; within the macro definition, write '\p' or '\p1' to evaluate the arguments.

.macro reserve_str p1=0 p2

Begin the definition of a macro called reserve_str, with two arguments. The first argument has a default value, but not the second. After the definition is complete, you can call the macro either as 'reserve_str a,b' (with '\p1' evaluating to a and '\p2' evaluating to b), or as 'reserve_str ,b' (with '\p1' evaluating as the default, in this case '0', and '\p2' evaluating to b).
When you call a macro, you can specify the argument values either by position, or by keyword. For example, 'sum 9,17' is equivalent to 'sum to=17, from=9'.

as maintains a counter of how many macros it has executed in pseudo-variable '\@'; you can copy that number to your output with '\@', but only within a macro definition.

.nolist

Control (in conjunction with the .list directive) whether or not assembly listings are generated. These two directives maintain an internal counter (which is zero initially). .list increments the counter, and .nolist decrements it. Assembly listings are generated whenever the counter is greater than zero.

.octa bignums

This directive expects zero or more bignums, separated by commas. For each bignum, it emits a 16-byte integer.

The term "octa" comes from contexts in which a "word" is two bytes; hence "octa-word" for 16 bytes.

.org new-lc, fill

Advance the location counter of the current section to new-lc. new-lc is either an absolute expression or an expression with the same section as the current subsection. That is, you can't use .org to cross sections: if new-lc has the wrong section, the .org directive is ignored. To be compatible with former assemblers, if the section of new-lc is absolute, as issues a warning, then pretends the section of new-lc is the same as the current subsection.

.org may only increase the location counter, or leave it unchanged; you cannot use .org to move the location counter backwards.

Because as tries to assemble programs in one pass, new-lc may not be undefined. If you really detest this restriction we eagerly await a chance to share your improved assembler.

Beware that the origin is relative to the start of the section, not to the start of the subsection. This is compatible with other people's assemblers.

When the location counter (of the current subsection) is advanced, the intervening bytes are filled with fill which should be an absolute expression. If the comma and fill are omitted, fill defaults to zero.

.p2align[wl] abs-expr, abs-expr, abs-expr

Pad the location counter (in the current subsection) to a particular storage boundary. The first expression (which must be absolute) is the number of low-order zero bits the location counter must have after advancement. For example '.p2align 3' advances the location counter until it a multiple of 8. If the location counter is already a multiple of 8, no change is needed.

The second expression (also absolute) gives the fill value to be stored in the padding bytes. It (and the comma) may be omitted. If it is omitted, the padding bytes are normally zero. However, on some systems, if the section is marked as containing code and the fill value is omitted, the space is filled with no-op instructions (I didn't check what is true with the TIGCC).

The third expression is also absolute, and is also optional. If it is present, it is the maximum number of bytes that should be skipped by this alignment directive. If doing the alignment would require skipping more bytes than the specified maximum, then the alignment is not done at all. You can omit the fill value (the second argument) entirely by simply using two commas after the required alignment; this can be useful if you want the alignment to be filled with no-op instructions when appropriate.

The .p2alignw and .p2alignl directives are variants of the .p2align directive. The .p2alignw directive treats the fill pattern as a two byte word value. The .p2alignl directives treats the fill pattern as a four byte longword value. For example, '.p2alignw 2,0x368d' will align to a multiple of 4. If it skips two bytes, they will be filled in with the value 0x368d (the exact placement of the bytes depends upon the endianness of the processor). If it skips 1 or 3 bytes, the fill value is undefined.

.psize lines, columns

Use this directive to declare the number of lines (and, optionally, the number of columns) to use for each page, when generating listings.

If you do not use .psize, listings use a default line-count of 60. You may omit the comma and columns specification; the default width is 200 columns.

as generates formfeeds whenever the specified number of lines is exceeded (or whenever you explicitly request one, using .eject).

If you specify lines as 0, no formfeeds are generated save those explicitly specified with .eject.

.quad bignums

.quad expects zero or more bignums, separated by commas. For each bignum, it emits an 8-byte integer. If the bignum won't fit in 8 bytes, it prints a warning message; and just takes the lowest order 8 bytes of the bignum.

The term "quad" comes from contexts in which a "word" is two bytes; hence "quad-word" for 8 bytes.

.rept count

Repeat the sequence of lines between the .rept directive and the next .endr directive count times. For example, assembling
.rept  3
.long  0
.endr
is equivalent to assembling
.long  0
.long  0
.long  0

.sbttl "subheading"

Use subheading as the title (third line, immediately after the title line) when generating assembly listings.

This directive affects subsequent pages, as well as the current page if it appears within ten lines of the top of a page.

.scl class

Set the storage-class value for a symbol. This directive may only be used inside a .def/.endef pair. Storage class may flag whether a symbol is static or external, or it may record further symbolic debugging information.

.section name

Use the .section directive to assemble the following code into a section named name. For COFF targets (as TIGCC is), the .section directive is used in one of the following ways:
.section name [,"flags"]
.section name [,subsegment]
If the optional argument is quoted, it is taken as flags to use for the section. Each flag is a single character. The following flags are recognized:

bbss section (uninitialized data)
nsection is not loaded
wwritable section
ddata section
rread-only section
xexecutable section

If no flags are specified, the default flags depend upon the section name. If the section name is not recognized, the default will be for the section to be loaded and writable.

If the optional argument to the .section directive is not quoted, it is taken as a subsegment number (see section Sub-Sections).

.set symbol, expression

Set the value of symbol to expression. This changes symbol's value and type to conform to expression. If symbol was flagged as external, it remains flagged (see section Symbol Attributes).

You may .set a symbol many times in the same assembly.

If you .set a global symbol, the value stored in the object file is the last value stored into it.

.short expressions

.short is normally the same as .word.

.single flonums

This directive assembles zero or more flonums, separated by commas. It has the same effect as .float. This is not yet particulary useful on TIGCC, because it currently stores IEEE floats, but TIOS uses SMAP II BCD floats.

.size

This directive is generated by compilers to include auxiliary debugging information in the symbol table. It is only permitted inside .def/.endef pairs.

.sleb128 expressions

sleb128 stands for "signed little endian base 128." This is a compact, variable length representation of numbers used by the DWARF symbolic debugging format.

.skip size, fill

This directive emits size bytes, each of value fill. Both size and fill are absolute expressions. If the comma and fill are omitted, fill is assumed to be zero. This is the same as '.space'.

.space size, fill

This directive emits size bytes, each of value fill. Both size and fill are absolute expressions. If the comma and fill are omitted, fill is assumed to be zero. This is the same as '.skip'.

.stabd type, other, desc
.stabn type , other , desc , value
.stabs string , type , other , desc , value

There are three directives that begin '.stab'. All emit symbols (see section Symbols), for use by symbolic debuggers (not yet supported in TIGCC). The symbols are not entered in the as hash table: they cannot be referenced elsewhere in the source file. Up to five fields are required:
string
This is the symbol's name. It may contain any character except '\000', so is more general than ordinary symbol names. Some debuggers used to code arbitrarily complex structures into symbol names using this field.
type
An absolute expression. The symbol's type is set to the low 8 bits of this expression. Any bit pattern is permitted, but linkers and debuggers choke on silly bit patterns.
other
An absolute expression. The symbol's "other" attribute is set to the low 8 bits of this expression.
desc
An absolute expression. The symbol's descriptor is set to the low 16 bits of this expression.
value
An absolute expression which becomes the symbol's value.
If a warning is detected while reading a .stabd, .stabn, or .stabs statement, the symbol has probably already been created; you get a half-formed symbol in your object file. This is compatible with earlier assemblers!

When .stabd is used, the "name" of the symbol generated is not even an empty string. It is a null pointer, for compatibility. Older assemblers used a null pointer so they didn't waste space in object files with empty strings. The symbol's value is set to the location counter, relocatably. When your program is linked, the value of this symbol is the address of the location counter when the .stabd was assembled. When .stabn is used, the name of the symbol is set to the empty string "". When .stabs is used, all five fields are specified.

.string "str"

Copy the characters in str to the object file. You may specify more than one string to copy, separated by commas. The assembler marks the end of each string with a 0 byte. You can use any of the escape sequences described in section Strings.

.symver

Use the .symver directive to bind symbols to specific version nodes within a source file. This is only supported on ELF platforms, so it is not applicable with TIGCC.

.tag structname

This directive is generated by compilers to include auxiliary debugging information in the symbol table. It is only permitted inside .def/.endef pairs. Tags are used to link structure definitions in the symbol table with instances of those structures.

.text subsection

Tells as to assemble the following statements onto the end of the text subsection numbered subsection, which is an absolute expression. If subsection is omitted, subsection number zero is used.

.title "heading"

Use heading as the title (second line, immediately after the source file name and pagenumber) when generating assembly listings.

This directive affects subsequent pages, as well as the current page if it appears within ten lines of the top of a page.

.type int

This directive, permitted only within .def/.endef pairs, records the integer int as the type attribute of a symbol table entry.

.uleb128 expressions

uleb128 stands for "unsigned little endian base 128." This is a compact, variable length representation of numbers used by the DWARF symbolic debugging format.

.val addr

This directive, permitted only within .def/.endef pairs, records the address addr as the value attribute of a symbol table entry.

.word expressions

This directive expects zero or more expressions, of any section, separated by commas. The size of the number emitted, and its byte order, depend on what target computer the assembly is for (16-bit big-endian on MC 68000).

Return to the main index