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.
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.'
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.
The as
internal preprocessor:
-
Adjusts and removes extra whitespace. It leaves one space or tab before
the keywords on a line, and turns any other whitespace on the line into
a single space.
-
Removes all comments, replacing them with a single space, or an
appropriate number of newlines.
-
Converts character constants into the appropriate numeric values.
It does not do macro processing, include file handling, or
anything else you may get from 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 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.
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.
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).
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, ...
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.
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.
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.
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.
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.
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).
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.
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)
-
The digit
'0'
.
-
A letter, to tell
as
the rest of the number is a flonum.
'e'
is recommended. Case is not important.
-
An optional sign: either
'+'
or '-'
.
-
An optional integer part: zero or more decimal digits.
-
An optional fractional part:
'.'
followed by zero
or more decimal digits.
-
An optional exponent, consisting of:
-
An
'E'
or 'e'
.
-
Optional sign: either
'+'
or '-'
.
-
One or more decimal digits.
At least one of the integer part or the fractional part must be
present. The floating point number has the usual base-10 value.
as
does all processing using integers. Flonums are computed
independently of any floating point hardware in the computer running
as
.
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 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'
.
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.
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-Op | BYTE | WORD | 68020 LONG | 68000/10 LONG | non-PC relative |
jbsr | bsr.s | bsr | bsr.l | jsr | jsr |
jra | bra.s | bra | bra.l | jmp | jmp |
(*) jXX | bXX.s | bXX | bXX.l | bNX; jmp.l | bNX; jmp |
(*) dbXX | dbXX | dbXX | dbXX; bra; jmp.l | dbXX; bra; jmp.l | dbXX; bra; jmp.l |
(*) fjXX | fbXX.w | fbXX.w | fbXX.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'
.
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.
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:
-
Where in the object file is the beginning of this reference to
an address?
-
How long (in bytes) is this reference?
-
Which section does the address refer to? What is the numeric value of
(address) - (start-address of section)
?
-
Is the reference to an address "Program-Counter relative"?
In fact, every address as
ever uses is expressed as
(section) + (offset into section)
Further, 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.
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.
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.
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 name, expression'
.
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.
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.
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.
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.
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 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 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'
.
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.
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.
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.
The symbol name is set with .def
; the value and type,
respectively, with .val
and .type
.
The as
directives .dim
, .line
, .scl
,
.size
, and .tag
can generate auxiliary symbol table
information for COFF.
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.
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.
An integer expression is one or more arguments delimited
by operators.
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 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.
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 take two arguments, one on either side. Operators
have precedence, but operations with equal precedence are performed left
to right. Apart from '+'
or '-'
, both arguments must be
absolute, and the result is absolute.
-
Highest Precedence
+ |
Multiplication.
|
/ |
Division. Truncation is the same as the C operator '/' .
|
% |
Remainder.
|
< (or << ) |
Shift Left. Same as the C operator '<<' .
|
> (or >> ) |
Shift Right. Same as the C operator '>>' .
|
-
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.
|
-
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.
All assembler directives have names that begin with a period ('.'
).
The rest of the name is letters, usually in lower case.
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.
When producing COFF output, as
accepts this directive as a
synonym for '.abort'
.
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
(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
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
is just like .ascii
, but each string is followed by
a zero byte. The "z" in '.asciz'
stands for "zero".
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
expects zero or more expressions, separated by commas.
Each expression is assembled into the next byte.
.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
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
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.
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.
This directive is generated by compilers to include auxiliary debugging
information in the symbol table. It is only permitted inside
.def
/.endef
pairs.
.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.
Force a page break at this point, when generating assembly listings.
.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.
This directive flags the end of a symbol definition begun with
.def
.
.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.
Mark the end of a macro definition. See .macro.
This directive sets the value of symbol to expression.
It is synonymous with .set directive.
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
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.
Exit early from the current macro definition. See .macro.
.extern
is accepted in the source program (for compatibility
with other assemblers) but it is ignored. as
treats
all undefined symbols as external.
.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.
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.
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
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.
Spelling '.globl'
is accepted as an alternative for '.global'
, for
compatibility with other assemblers.
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.
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
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'
.
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.
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).
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)
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)
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.
as
accepts this directive, for compatibility with other
assemblers, but ignores it.
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.
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'
is a synonym for '.line'
.
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.
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
is the same as .int.
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.
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.
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.
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.
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.
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
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.
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
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.
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.
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:
b | bss section (uninitialized data) |
n | section is not loaded |
w | writable section |
d | data section |
r | read-only section |
x | executable 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 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
is normally the same as .word.
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.
This directive is generated by compilers to include auxiliary debugging
information in the symbol table. It is only permitted inside
.def
/.endef
pairs.
sleb128 stands for "signed little endian base 128." This is a
compact, variable length representation of numbers used by the DWARF
symbolic debugging format.
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'
.
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'
.
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.
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.
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.
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.
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.
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.
This directive, permitted only within .def
/.endef
pairs,
records the integer int as the type attribute of a symbol table entry.
uleb128 stands for "unsigned little endian base 128." This is a
compact, variable length representation of numbers used by the DWARF
symbolic debugging format.
This directive, permitted only within .def
/.endef
pairs,
records the address addr as the value attribute of a symbol table
entry.
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).