path: root/gas/doc/as.texinfo

\input texinfo @c -*-texinfo-*-
@tex
\special{twoside}
@end tex
@setfilename as
@settitle as
@titlepage
@center @titlefont{as}
@sp 1
@center The GNU Assembler
@sp 2
@center Dean Elsner, Jay Fenlason & friends
@sp 13
The Free Software Foundation Inc.  thanks The Nice Computer
Company of Australia for loaning Dean Elsner to write the
first (Vax) version of @code{as} for Project GNU.
The proprietors, management and staff of TNCCA thank FSF for
distracting the boss while they got some work
done.
@sp 3

Copyright @copyright{} 1986,1987 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

@ignore
Permission is granted to process this file through Tex and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).

@end ignore
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the same conditions as for modified versions.

@end titlepage
@node top, Syntax, top, top
@chapter Overview, Usage
@menu
* Syntax::           The (machine independent) syntax that assembly language
                files must follow.  The machine dependent syntax
                can be found in the machine dependent section of
                the manual for the machine that you are using.
* Segments::         How to use segments and subsegments, and how the
                assembler and linker will relocate things.
* Symbols::          How to set up and manipulate symbols.
* Expressions::      And how the assembler deals with them.
* PseudoOps::        The assorted machine directives that tell the
                assembler exactly what to do with its input.
* MachineDependent:: Information specific to each machine.
* Maintenance::      Keeping the assembler running.
* Retargeting::      Teaching the assembler about new machines.
@end menu
		
This document describes the GNU assembler @code{as}.  This document
does @emph{not} describe what an assembler does, or how it works.
This document also does @emph{not} describe the opcodes, registers
or addressing modes that @code{as} uses on any paticular computer
that @code{as} runs on.  Consult a good book on assemblers or the
machine's architecture if you need that information.

This document describes the directives that @code{as} understands,
and their syntax.  This document also describes some of the
machine-dependent features of various flavors of the assembler.
This document also describes how the assembler works internally, and
provides some information that may be useful to people attempting to
port the assembler to another machine.


Throughout this document, we assume that you are running @dfn{GNU},
the portable operating system from the @dfn{Free Software
Foundation, Inc.}.  This restricts our attention to certain kinds of
computer (in paticular, the kinds of computers that GNU can run on);
once this assumption is granted examples and definitions need less
qualification.

Readers should already comprehend:
@itemize @bullet
@item
Central processing unit
@item
registers
@item
memory address
@item
contents of memory address
@item
bit
@item
8-bit byte
@item
2's complement arithmetic
@end itemize

@code{as} is part of a team of programs that turn a high-level
human-readable series of instructions into a low-level
computer-readable series of instructions.  Different versions of
@code{as} are used for different kinds of computer.  In paticular,
at the moment, @code{as} only works for the DEC Vax, the Motorola
680x0, the Intel 80386, the Sparc, and the National Semiconductor
32032/32532.

@section Notation
GNU and @code{as} assume the computer that will run the programs it
assembles will obey these rules.

A (memory) @dfn{address} is 32 bits. The lowest address is zero.

The @dfn{contents} of any memory address is one @dfn{byte} of
exactly 8 bits.

A @dfn{word} is 16 bits stored in two bytes of memory. The addresses
of the bytes differ by exactly 1.  Notice that the interpretation of
the bits in a word and of how to address a word depends on which
particular computer you are assembling for.

A @dfn{long word}, or @dfn{long}, is 32 bits composed of four bytes.
It is stored in 4 bytes of memory; these bytes have contiguous
addresses.  Again the interpretation and addressing of those bits is
machine dependent.  National Semiconductor 32x32 computers say
@i{double word} where we say @i{long}.

Numeric quantities are usually @i{unsigned} or @i{2's complement}.
Bytes, words and longs may store numbers.  @code{as} manipulates
integer expressions as 32-bit numbers in 2's complement format.
When asked to store an integer in a byte or word, the lowest order
bits are stored.  The order of bytes in a word or long in memory is
determined by what kind of computer will run the assembled program.
We won't mention this important @i{caveat} again.

The meaning of these terms has changed over time.  Although @i{byte}
used to mean any length of contiguous bits, @i{byte} now pervasively
means exactly 8 contiguous bits.  A @i{word} of 16 bits made sense
for 16-bit computers.  Even on 32-bit computers, a @i{word} still
means 16 bits (to machine language programmers).  To many other
programmers of GNU a @i{word} means 32 bits, so beware.  Similarly
@i{long} means 32 bits: from ``long word''.  National Semiconductor
32x32 machine language calls a 32-bit number a ``double word''.

@example

       Names for integers of different sizes: some conventions


length   as          vax        32x32        680x0        GNU C
(bits)

  8    byte         byte         byte         byte         char
 16    word         word         word         word  short (int)
 32    long  long(-word)  double-word  long(-word)   long (int)
 64    quad  quad(-word)
128    octa    octa-word

@end example

@section as, the GNU Assembler
@dfn{As} is an assembler; it is one of the team of programs that
`compile' your programs into the binary numbers that a computer uses
to `run' your program.  Often @code{as} reads a @i{source} program
written by a compiler and writes an @dfn{object} program for the
linker (sometimes referred to as a @dfn{loader}) @code{ld} to read.

The source program consists of @dfn{statements} and comments.  Each
statement might @dfn{assemble} to one (and only one) machine
language instruction or to one very simple datum.

Mostly you don't have to think about the assembler because the
compiler invokes it as needed; in that sense the assembler is just
another part of the compiler.  If you write your own assembly
language program, then you must run the assembler yourself to get an
object file suitable for linking.  You can read below how to do this.

@code{as} is only intended to assemble the output of the C compiler
@code{cc} for use by the linker @code{ld}.  @code{as} tries to
assemble correctly everything that the standard assembler would
assemble, with a few exceptions (described in the machine-dependent
chapters.)  Note that this doesn't mean @code{as} will use the same
syntax as the standard assembler.  For example, we know of several
incompatable syntaxes for the 680x0.

Each version of the assembler knows about just one kind of machine
language, but much is common between the versions, including object
file formats, (most) assembler directives (often called
@dfn{pseudo-ops)} and assembler syntax.

Unlike older assemblers, @code{as} tries to assemble a source program
in one pass of the source file.  This subtly changes the meaning of
the @kbd{.org} directive (@xref{Org}.).

If you want to write assembly language programs, you must tell
@code{as} what numbers should be in a computer's memory, and which
addresses should contain them, so that the program may be executed
by the computer.  Using symbols will prevent many bookkeeping
mistakes that can occur if you use raw numbers.

@section Command Line Synopsis
@example
as [ options @dots{} ] [ file1 @dots{} ]
@end example

After the program name @code{as}, the command line may contain
options and file names.  Options may be in any order, and may be
before, after, or between file names.  The order of file names is
significant.

@subsection Options

Except for @samp{--} any command line argument that begins with a
hyphen (@samp{-}) is an option.  Each option changes the behavior of
@code{as}.  No option changes the way another option works.  An
option is a @samp{-} followed by one ore more letters; the case of
the letter is important.  No option (letter) should be used twice on
the same command line.  (Nobody has decided what two copies of the
same option should mean.)  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:

@example
as -o my-object-file.o mumble
as -omy-object-file.o mumble
@end example

Always, @file{--} (that's two hyphens, not one) by itself names the
standard input file.

@section Input File(s)

We use the words @dfn{source program}, abbreviated @dfn{source}, to
describe the program input to one run of @code{as}.  The program may
be in one or more files; how the source is partitioned into files
doesn't change the meaning of the source.

The source text is a catenation of the text in each file.

Each time you run @code{as} it assembles exactly one source
program.  A source program text is made of one or more files.
(The standard input is also a file.)

You give @code{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 @code{as} is given no file
names it attempts to read one input file from @code{as}'s standard
input.

Use @file{--} if you need to explicitly name the standard input file
in your command line.

It is OK to assemble an empty source.  @code{as} will produce a
small, empty object file.

If you try to assemble no files then @code{as} will try to read
standard input, which is normally your terminal.  You may have to
type @key{ctl-D} to tell @code{as} there is no more program to
assemble.

@subsection Input Filenames and Line-numbers
A line is text up to and including the next newline.  The first line
of a file is numbered @b{1}, the next @b{2} and so on.

There are two ways of locating a line in the input file(s) and both
are used in reporting error messages.  One way refers to a line
number in a physical file; the other refers to a line number in a
logical file.

@dfn{Physical files} are those files named in the command line given
to @code{as}.

@dfn{Logical files} are ``pretend'' files which bear no relation to
physical files.  Logical file names help error messages reflect the
proper source file.  Often they are used when @code{as}' source is
itself synthesized from other files.

@section Output (Object) File
Every time you run @code{as} it produces an output file, which is
your assembly language program translated into numbers.  This file
is the object file; named @code{a.out} unless you tell @code{as} to
give it another name by using the @code{-o} option.  Conventionally,
object file names end with @file{.o}.  The default name of
@file{a.out} is used for historical reasons.  Older assemblers were
capable of assembling self-contained programs directly into a
runnable program.  This may still work, but hasn't been tested.

The object file is for input to the linker @code{ld}.  It contains
assembled program code, information to help @code{ld} to integrate
the assembled program into a runnable file and (optionally) symbolic
information for the debugger.  The precise format of object files is
described elsewhere.

@comment link above to some info file(s) like the description of a.out.
@comment don't forget to describe GNU info as well as Unix lossage.

@section Error and Warning Messages

@code{as} may write warnings and error messages to the standard
error file (usually your terminal).  This should not happen when
@code{as} is run automatically by a compiler.  Error messages are
useful for those (few) people who still write in assembly language.

Warnings report an assumption made so that @code{as} could keep
assembling a flawed program.

Errors report a grave problem that stops the assembly.

Warning messages have the format
@example
file_name:line_number:Warning Message Text
@end example
If a logical file name has been given (@xref{File}.) it is used for
the filename, otherwise the name of the current input file is used.
If a logical line number was given (@xref{Line}.) then it is used to
calculate the number printed, otherwise the actual line in the
current source file is printed.  The message text is intended to be
self explanatory (In the grand Unix tradition).

Error messages have the format
@example
file_name:line_number:FATAL:Error Message Text
@end example
The file name and line number are derived the same as for warning
messages.  The actual message text may be rather less explanatory
because many of them aren't supposed to happen.

@section Options
@subsection -f Works Faster
@samp{-f} should only be used when assembling programs written by a
(trusted) compiler.  @samp{-f} causes the assembler to not bother
pre-processing the input file(s) before assembling them.  Needless
to say, if the files actually need to be pre-processed (if the
contain comments, for example), @code{as} will not work correctly if
@samp{-f} is used.

@subsection -L Includes Local Labels
For historical reasons, labels beginning with @samp{L} (upper case
only) are called @dfn{local labels}.  Normally you don't see such
labels because they are intended for the use of programs (like
compilers) that compose assembler programs, not for your notice.
Normally both @code{as} and @code{ld} discard such labels, so you
don't normally debug with them.

This option tells @code{as} to retain those @samp{L@dots{}} symbols
in the object file.  Usually if you do this you also tell the linker
@code{ld} to preserve symbols whose names begin with @samp{L}.

@subsection -o Names the Object File
There is always one object file output when you run @code{as}.  By
default it has the name @file{a.out}.  You use this option (which
takes exactly one filename) to give the object file a different name.

Whatever the object file is called, @code{as} will overwrite any
existing file of the same name.

@subsection -R Folds Data Segment into Text Segment
@code{-R} tells @code{as} to write the object file as if all
data-segment data lives in the text segment.  This is only done at
the very last moment:  your binary data are the same, but data
segment parts are relocated differently.  The data segment part of
your object file is zero bytes long because all it bytes are
appended to the text segment.  (@xref{Segments}.)

When you use @code{-R} it would be nice to generate shorter address
displacements (possible because we don't have to cross segments)
between text and data segment.  We don't do this simply for
compatibility with older versions of @code{as}.  @code{-R} may work
this way in future.

@subsection -W Represses Warnings
@code{as} should never give a warning or error message when
assembling compiler output.  But programs written by people often
cause @code{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, any warning is repressed.  This option only
affects warning messages: it cannot change any detail of how
@code{as} assembles your file.  Errors, which stop the assembly, are
still reported.

@section Special Features to support Compilers

In order to assemble compiler output into something that will work,
@code{as} will occasionlly do strange things to @samp{.word}
directives.  In particular, when @code{gas} assembles a directive of
the form @samp{.word sym1-sym2}, and the difference between
@code{sym1} and @code{sym2} does not fit in 16 bits, @code{as} will
create a @dfn{secondary jump table}, immediately before the next
label.  This @var{secondary jump table} will be preceeded by a
short-jump to the first byte after the table.  The short-jump
prevents the flow-of-control from accidentally falling into the
table.  Inside the table will be a long-jump to @code{sym2}.  The
original @samp{.word} will contain @code{sym1} minus (the address of
the long-jump to sym2) If there were several @samp{.word sym1-sym2}
before the secondary jump table, all of them will be adjusted.  If
ther was a @samp{.word sym3-sym4}, that also did not fit in sixteen
bits, a long-jump to @code{sym4} will be included in the secondary
jump table, and the @code{.word}(s), will be adjusted to contain
@code{sym3} minus (the address of the long-jump to sym4), etc.

@emph{This feature may be disabled by compiling @code{as} with the
@samp{-DWORKING_DOT_WORD} option.} This feature is likely to confuse
assembly language programmers.

@node Syntax, Segments, top, top
@chapter Syntax
This chapter informally defines the machine-independent syntax
allowed in a source file.  @code{as} has ordinary syntax; it tries
to be upward compatible from BSD 4.2 assembler except @code{as} does
not assemble Vax bit-fields.

@section The Pre-processor
The preprocess phase handles several aspects of the syntax.  The
pre-processor will be disabled by the @samp{-f} option, or if the
first line of the source file is @code{#NO_APP}.  The option to
disable the pre-processor was designed to make compiler output
assemble as fast as possible.

The pre-processor adjusts and removes extra whitespace.  It leaves
one space or tab before the keywords on a line, and turns any other
whitespace on the line into a single space.

The pre-processor removes all comments, replacing them with a single
space (for /* @dots{} */ comments), or an appropriate number of
newlines.

The pre-processor converts character constants into the appropriate
numeric values.

This means that excess whitespace, comments, and character constants
cannot be used in the portions of the input text that are not
pre-processed.

If the first line of an input file is @code{#NO_APP} or the
@samp{-f} option is given, the input file will not be
pre-processed.  Within such an input file, parts of the file can be
pre-processed by putting a line that says @code{#APP} before the
text that should be pre-processed, and putting a line that says
@code{#NO_APP} after them.  This feature is mainly intend to support
asm statements in compilers whose output normally does not need to
be pre-processed.

@section Whitespace
@dfn{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
(@xref{Characters}.), any whitespace means the same as exactly one
space.

@section Comments
There are two ways of rendering comments to @code{as}.  In both
cases the comment is equivalent to one space.

Anything from @samp{/*} through the next @samp{*/} is a comment.

@example
/*
  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. */
@end example

Anything from the @dfn{line comment} character to the next newline
considered a comment and is ignored.  The line comment character is
@samp{#} on the Vax, and @samp{|} on the 680x0.
@xref{MachineDependent}.  On some machines there are two different
line comment characters.  One will only begin a comment if it is the
first non-whitespace character on a line, while the other will
always begin a comment.

To be compatible with past assemblers a special interpretation is
given to lines that begin with @samp{#}.  Following the @samp{#} an
absolute expression (@pxref{Expressions}) is expected:  this will be
the logical line number of the @b{next} line.  Then a string
(@xref{Strings}.) is allowed: if present it is a new logical file
name.  The rest of the line, if any, should be whitespace.

If the first non-whitespace characters on the line are not numeric,
the line is ignored.  (Just like a comment.)
@example
                          # This is an ordinary comment.
# 42-6 "new_file_name"    # New logical file name
                          # This is logical line # 36.
@end example
This feature is deprecated, and may disappear from future versions
of @code{as}.

@section Symbols
A @dfn{symbol} is one or more characters chosen from the set of all
letters (both upper and lower case), digits and the three characters
@samp{_.$}.  No symbol may begin with a digit.  Case is
significant.  There is no length limit: all characters are
significant.  Symbols are delimited by characters not in that set,
or by begin/end-of-file.  (@xref{Symbols}.)

@section Statements
A @dfn{statement} ends at a newline character (@samp{\n}) or at a
semicolon (@samp{;}).  The newline or semicolon is considered part
of the preceding statement.  Newlines and semicolons within
character constants are an exception:  they don't end statements.
It is an error to end any statement with end-of-file:  the last
character of any input file should be a newline.

You may write a statement on more than one line if you put a
backslash (@kbd{\}) immediately in front of any newlines within the
statement.  When @code{as} reads a backslashed newline both
characters are ignored.  You can even put backslashed newlines in
the middle of symbol names without changing the meaning of your
source program.

An empty statement is OK, and may include whitespace.  It is ignored.

Statements begin with zero or more labels, followed by a @dfn{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 (@t{.}) then the statement is an assembler
directive: typically valid for any computer.  If the symbol begins
with a letter the statement is an assembly language
@dfn{instruction}: it will assemble into a machine language
instruction.  Different versions of @code{as} for different
computers will recognize different instructions.  In fact, the same
symbol may represent a different instruction in a different
computer's assembly language.

A label is usually a symbol immediately followed by a colon
(@code{:}).  Whitespace before a label or after a colon is OK.  You
may not have whitespace between a label's symbol and its colon.
Labels are explained below.
@xref{Labels}.

@example
label:     .directive    followed by something
another$label:           # This is an empty statement.
           instruction   operand_1, operand_2, @dots{}
@end example

@section Constants
A constant is a number, written so that its value is known by
inspection, without knowing any context.  Like this:
@example
.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.
@end example

@node Characters, Strings, , Syntax
@subsection Character Constants
There are two kinds of character constants.  @dfn{Characters} stand
for one character in one byte and their values may be used in
numeric expressions.  String constants (properly called string
@i{literals}) are potentially many bytes and their values may not be
used in arithmetic expressions.

@node Strings, , Characters, Syntax
@subsubsection Strings
A @dfn{string} is written between double-quotes.  It may contain
double-quotes or null characters.  The way to get weird characters
into a string is to @dfn{escape} these characters: precede them with
a backslash (@code{\}) character.  For example @samp{\\} represents
one backslash:  the first @code{\} is an escape which tells
@code{as} to interpret the second character literally as a backslash
(which prevents @code{as} from recognizing the second @code{\} as an
escape character).  The complete list of escapes follows.

@table @kbd
@item \EOF
A @kbd{\} followed by end-of-file erroneous.  It is treated just
like an end-of-file without a preceding backslash.
@c	@item \a
@c	Mnemonic for ACKnowledge; for ASCII this is octal code 007.
@item \b
Mnemonic for backspace; for ASCII this is octal code 010.
@c	@item \e
@c	Mnemonic for EOText; for ASCII this is octal code 004.
@item \f
Mnemonic for FormFeed; for ASCII this is octal code 014.
@item \n
Mnemonic for newline; for ASCII this is octal code 012.
@c	@item \p
@c	Mnemonic for prefix; for ASCII this is octal code 033, usually known as @code{escape}.
@item \r
Mnemonic for carriage-Return; for ASCII this is octal code 015.
@c	@item \s
@c	Mnemonic for space; for ASCII this is octal code 040.  Included for compliance with
@c	other assemblers.
@item \t
Mnemonic for horizontal Tab; for ASCII this is octal code 011.
@c	@item \v
@c	Mnemonic for Vertical tab; for ASCII this is octal code 013.
@c	@item \x @var{digit} @var{digit} @var{digit}
@c	A hexadecimal character code.  The numeric code is 3 hexadecimal digits.
@item \ @var{digit} @var{digit} @var{digit}
An octal character code.  The numeric code is 3 octal digits.
For compatibility with other Unix systems, 8 and 9 are legal digits
with values 010 and 011 respectively.
@item \\
Represents one @samp{\} character.
@c	@item \'
@c	Represents one @samp{'} (accent acute) character.
@c	This is needed in single character literals
@c      (@xref{Characters}.) to represent
@c	a @samp{'}.
@item \"
Represents one @samp{"} character.  Needed in strings to represent
this character, because an unescaped @samp{"} would end the string.
@item \ @var{anything-else}
Any other character when escaped by @kbd{\} will give a warning, but
assemble as if the @samp{\} 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 @code{as} has no
other interpretation, so @code{as} knows it is giving you the wrong
code and warns you of the fact.
@end table

Which characters are escapable, and what those escapes represent,
varies widely among assemblers.  The current set is what we think
BSD 4.2 @code{as} recognizes, and is a subset of what most C
compilers recognize.  If you are in doubt, don't use an escape
sequence.

@subsubsection 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 @kbd{'\\} where the first @code{\} escapes the second
@code{\}.  As you can see, the quote is an accent acute, not an
accent grave.  A newline (or semicolon (@samp{;})) immediately
following an accent acute is taken as a literal character and does
not count as the end of a statement.  The value of a character
constant in a numeric expression is the machine's byte-wide code for
that character.  @code{as} assumes your character code is ASCII: @kbd{'A}
means 65, @kbd{'B} means 66, and so on.

@subsection Number Constants
@code{as} distinguishes 3 flavors of numbers according to how they
are stored in the target machine.  @i{Integers} are numbers that
would fit into an @code{int} in the C language.  @i{Bignums} are
integers, but they are stored in a more than 32 bits.  @i{Flonums}
are floating point numbers, described below.

@subsubsection Integers
An octal integer is @samp{0} followed by zero or more of the octal
digits (@samp{01234567}).

A decimal integer starts with a non-zero digit followed by zero or
more digits (@samp{0123456789}).

A hexadecimal integer is @samp{0x} or @samp{0X} followed by one or
more hexadecimal digits chosen from @samp{0123456789abcdefABCDEF}.

Integers have the obvious values.  To denote a negative integer, use
the unary operator @samp{-} discussed under expressions
(@xref{Unops}.).

@subsubsection Bignums
A @dfn{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.

@subsubsection Flonums
A @dfn{flonum} represents a floating point number.  The translation
is complex: a decimal floating point number from the text is
converted by @code{as} to a generic binary floating point number of
more than sufficient precision.  This generic floating point number
is converted to the particular computer's floating point format(s)
by a portion of @code{as} specialized to that computer.

A flonum is written by writing (in order)
@itemize @bullet
@item
The digit @samp{0}.
@item
A letter, to tell @code{as} the rest of the number is a flonum.
@kbd{e}
is recommended.  Case is not important.
(Any otherwise illegal letter will work here,
but that might be changed.  Vax BSD 4.2 assembler
seems to allow any of @samp{defghDEFGH}.)
@item
An optional sign: either @samp{+} or @samp{-}.
@item
An optional integer part: zero or more decimal digits.
@item
An optional fraction part: @samp{.} followed by zero
or more decimal digits.
@item
An optional exponent, consisting of:
@itemize @bullet
@item
A letter; the exact significance varies according to
the computer that executes the program.  @code{as}
accepts any letter for now.  Case is not important.
@item
Optional sign: either @samp{+} or @samp{-}.
@item
One or more decimal digits.
@end itemize
@end itemize

At least one of @var{integer part} or @var{fraction part} must be
present.  The floating point number has the obvious value.

The computer running @code{as} needs no floating point hardware.
@code{as} does all processing using integers.

@node Segments, Symbols, Syntax, top
@chapter (Sub)Segments & Relocation
Roughly, a @dfn{segment} is a range of addresses, with no gaps, with
all data ``in'' those addresses being treated the same.  For example
there may be a ``read only'' segment.

The linker @code{ld} reads many object files (partial programs) and
combines their contents to form a runnable program.  When @code{as}
emits an object file, the partial program is assumed to start at
address 0.  @code{ld} will assign the final addresses the partial
program occupies, so that different partial programs don't overlap.
That explanation is too simple, but it will suffice to explain how
@code{as} works.

@code{ld} moves blocks of bytes of your program to their run-time
addresses.  These blocks slide to their run-time addresses as rigid
units; their length does not change and neither does the order of
bytes within them.  Such a rigid unit is called a @i{segment}.
Assigning run-time addresses to segments is called
@dfn{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 @code{as} has three segments, any of which
may be empty.  These are named @i{text}, @i{data} and @i{bss}
segments.  Within the object file, the text segment starts at
address 0, the data segment follows, and the bss segment follows the
data segment.

To let @code{ld} know which data will change when the segments are
relocated, and how to change that data, @code{as} also writes to the
object file details of the relocation needed.  To perform relocation
@code{ld} must know for each mention of an address in the object
file:
@itemize @bullet
@item
At what address in the object file does this mention of
an address begin?
@item
How long (in bytes) is this mention?
@item
Which segment does the address refer to?
What is the numeric value of (@var{address} @t{-}
@var{start-address of segment})?
@item
Is the mention of an address ``Program counter relative''?
@end itemize

In fact, every address @code{as} ever thinks about is expressed as
(@var{segment} @t{+} @var{offset into segment}).  Further, every
expression @code{as} computes is of this segmented nature.  So
@dfn{absolute expression} means an expression with segment
``absolute'' (@xref{LdSegs}.).  A @dfn{pass1 expression} means an
expression with segment ``pass1'' (@xref{MythSegs}.).  In this
document ``(segment, offset)'' will be written as @{ segment-name
(offset into segment) @}.

Apart from text, data and bss segments you need to know about the
@dfn{absolute} segment.  When @code{ld} mixes partial programs,
addresses in the absolute segment remain unchanged.  That is,
address @{absolute 0@} is ``relocated'' to run-time address 0 by
@code{ld}.  Although two partial programs' data segments will not
overlap addresses after linking, @b{by definition} their absolute
segments will overlap.  Address @{absolute 239@} in one partial
program will always be the same address when the program is running
as address @{absolute 239@} in any other partial program.

The idea of segments is extended to the @dfn{undefined} segment.
Any address whose segment is unknown at assembly time is by
definition rendered @{undefined (something, unknown yet)@}.  Since
numbers are always defined, the only way to generate an undefined
address is to mention an undefined symbol.  A reference to a named
common block would be such a symbol: its value is unknown at assembly
time so it has segment @i{undefined}.

By analogy the word @i{segment} is to describe groups of segments in
the linked program.  @code{ld} puts all partial program's text
segments in contiguous addresses in the linked program.  It is
customary to refer to the @i{text segment} of a program, meaning all
the addresses of all partial program's text segments.  Likewise for
data and bss segments.

@section Segments
Some segments are manipulated by @code{ld}; others are invented for
use of @code{as} and have no meaning except during assembly.

@node LdSegs, , ,
@subsection ld segments
@code{ld} deals with just 5 kinds of segments, summarized below.
@table @b
@item text segment
@itemx data segment
These segments hold your program bytes.  @code{as} and @code{ld}
treat them as separate but equal segments.  Anything you can say of
one segment is true of the other.  When the program is running
however it is customary for the text segment to be unalterable:  it
will contain instructions, constants and the like.  The data segment
of a running program is usually alterable: for example, C variables
would be stored in the data segment.
@item bss segment
This segment contains zeroed bytes when your program begins
running.  It is used to hold unitialized variables or common
storage.  The length of each partial program's bss segment is
important, but because it starts out containing zeroed bytes there
is no need to store explicit zero bytes in the object file.  The Bss
segment was invented to eliminate those explicit zeros from object
files.
@item absolute segment
Address 0 of this segment is always ``relocated'' to runtime address
0.  This is useful if you want to refer to an address that @code{ld}
must not change when relocating.  In this sense we speak of absolute
addresses being ``unrelocatable'': they don't change during
relocation.
@item undefined segment
This ``segment'' is a catch-all for address references to objects
not in the preceding segments.  See the description of @file{a.out}
for details.
@end table
An idealized example of the 3 relocatable segments follows.  Memory
addresses are on the horizontal axis.

@example
                      +-----+----+--+
partial program # 1:  |ttttt|dddd|00|
                      +-----+----+--+

                      text   data bss
                      seg.   seg. seg.

                      +---+---+---+
partial program # 2:  |TTT|DDD|000|
                      +---+---+---+

                      +--+---+-----+--+----+---+-----+~~
linked program:       |  |TTT|ttttt|  |dddd|DDD|00000|
                      +--+---+-----+--+----+---+-----+~~

    addresses:        0 @dots{}
@end example

@node MythSegs, , ,
@subsection Mythical Segments
These segments are invented for the internal use of @code{as}.  They
have no meaning at run-time.  You don't need to know about these
segments except that they might be mentioned in @code{as}' warning
messages.  These segments are invented to permit the value of every
expression in your assembly language program to be a segmented
address.

@table @b
@item absent segment
An expression was expected and none was found.
@item goof segment
An internal assembler logic error has been found.  This means there
is a bug in the assembler.
@item grand segment
A @dfn{grand number} is a bignum or a flonum, but not an integer.
If a number can't be written as a C @code{int} constant, it is a
grand number.  @code{as} has to remember that a flonum or a bignum
does not fit into 32 bits, and cannot be a primary (@xref{Primary}.)
in an expression: this is done by making a flonum or bignum be of
type ``grand''.  This is purely for internal @code{as} convenience;
grand segment behaves similarly to absolute segment.
@item pass1 segment
The expression was impossible to evaluate in the first pass.  The
assembler will attempt a second pass (second reading of the source)
to evaluate the expression.  Your expression mentioned an undefined
symbol in a way that defies the one-pass (segment + offset in
segment) assembly process.  No compiler need emit such an expression.
@item difference segment
As an assist to the C compiler, expressions of the forms
@itemize @bullet
@item
(undefined symbol) @t{-} (expression)
@item
(something) @t{-} (undefined symbol)
@item
(undefined symbol) @t{-} (undefined symbol)
@end itemize
are permitted to belong to the ``difference'' segment.  @code{as}
re-evaluates such expressions after the source file has been read
and the symbol table built.  If by that time there are no undefined
symbols in the expression then the expression assumes a new segment.
The intention is to permit statements like @samp{.word label -
base_of_table} to be assembled in one pass where both @code{label}
and @code{base_of_table} are undefined.  This is useful for
compiling C and Algol switch statements, Pascal case statements,
FORTRAN computed goto statements and the like.
@end table

@section Sub-Segments
Assembled bytes fall into two segments: text and data.  Because you
may have groups of text or data that you want to end up near to each
other in the object file, @code{as}, allows you to use
@dfn{subsegments}.  Within each segment, there can be numbered
subsegments with values from 0 to 8192.  Objects assembled into the
same subsegment will be grouped with other objects in the same
subsegment when they are all put into the object file.  For example,
a compiler might want to store constants in the text segment, but
might not want to have them intersperced with the program being
assembled.  In this case, the compiler could issue a @code{text 0}
before each section of code being output, and a @code{text 1} before
each group of constants being output.

Subsegments are optional.  If you don't used subsegments, everything
will be stored in subsegment number zero.

Each subsegment is zero-padded up to a multiple of four bytes.
(Subsegments may be padded a different amount on different flavors
of @code{as}.)  Subsegments appear in your object file in numeric
order, lowest numbered to highest.  (All this to be compatible with
other people's assemblers.)  The object file, @code{ld} @i{etc.}
have no concept of subsegments.  They just see all your text
subsegments as a text segment, and all your data subsegments as a
data segment.

To specify which subsegment you want subsequent statements assembled
into, use a @samp{.text @var{expression}} or a @samp{.data
@var{expression}} statement.  @var{Expression} should be an absolute
expression.  (@xref{Expressions}.)  If you just say @samp{.text}
then @samp{.text 0} is assumed.  Likewise @samp{.data} means
@samp{.data 0}.  Assembly begins in @code{text 0}.
For instance:
@example
.text 0     # The default subsegment is text 0 anyway.
.ascii "This lives in the first text subsegment. *"
.text 1
.ascii "But this lives in the second text subsegment."
.data 0
.ascii "This lives in the data segment,"
.ascii "in the first data subsegment."
.text 0
.ascii "This lives in the first text segment,"
.ascii "immediately following the asterisk (*)."
@end example

Each segment has a @dfn{location counter} incremented by one for
every byte assembled into that segment.  Because subsegments are
merely a convenience restricted to @code{as} there is no concept of
a subsegment location counter.  There is no way to directly
manipulate a location counter.  The location counter of the segment
that statements are being assembled into is said to be the
@dfn{active} location counter.

@section Bss Segment
The @code{bss} segment is used for local common variable storage.
You may allocate address space in the @code{bss} segment, but you may
not dictate data to load into it before your program executes.  When
your program starts running, all the contents of the @code{bss}
segment are zeroed bytes.

Addresses in the bss segment are allocated with a special statement;
you may not assemble anything directly into the bss segment.  Hence
there are no bss subsegments.

@node Symbols, Expressions, Segments, top
@chapter Symbols
Because the linker uses symbols to link, the debugger uses symbols
to debug and the programmer uses symbols to name things, symbols are
a central concept.  Symbols do not appear in the object file in the
order they are declared.  This may break some debuggers.

@node Labels, , , Symbols
@section Labels
A @dfn{label} is written as a symbol immediately followed by a colon
(@samp{:}).  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.

@section Giving Symbols Other Values
A symbol can be given an arbitrary value by writing a symbol followed
by an equals sign (@samp{=}) followed by an expression
(@pxref{Expressions}).  This is equivalent to using the @code{.set}
directive.  (@xref{Set}.)

@section Symbol Names
Symbol names begin with a letter or with one of @samp{$._}.  That
character may be followed by any string of digits, letters,
underscores and dollar signs.  Case of letters is significant:
@code{foo} is a different symbol name than @code{Foo}.

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

@subsection Local Symbol Names

Local symbols help compilers and programmers use names temporarily.
There are ten @dfn{local} symbol names, which are re-used throughout
the program.  Their names are @samp{0} @samp{1} @dots{} @samp{9}.
To define a local symbol, write a label of the form
@var{digit}@t{:}.  To refer to the most recent previous definition
of that symbol write @var{digit}@t{b}, using the same digit as when
you defined the label.  To refer to the next definition of a local
label, write @var{digit}@t{f} where @var{digit} gives you a choice
of 10 forward references.  The @samp{b} stands for ``backwards'' and
the @samp{f} stands for ``forwards''.

Local symbols are not used by the current C compiler.

There is no restriction on how you can use these labels, but
remember that at any point in the assembly you can refer to at most
10 prior local labels and to at most 10 forward local labels.

Local symbol names are only a notation device. They are immediately
transformed into more conventional symbol names before the assembler
thinks about them.  The symbol names stored in the symbol table,
appearing in error messages and optionally emitted to the object
file have these parts:
@table @kbd
@item L
All local labels begin with @samp{L}. Normally both @code{as} and
@code{ld} forget symbols that start with @samp{L}. These labels are
used for symbols you are never intended to see.  If you give the
@samp{-L} option then @code{as} will retain these symbols in the
object file. By instructing @code{ld} to also retain these symbols,
you may use them in debugging.
@item @i{a digit}
If the label is written @samp{0:} then the digit is @samp{0}.
If the label is written @samp{1:} then the digit is @samp{1}.
And so on up through @samp{9:}.
@item @i{control}-A
This unusual character is included so you don't accidentally invent
a symbol of the same name.  The character has ASCII value
@samp{\001}.
@item @i{an ordinal number}
This is like a serial number to keep the labels distinct.  The first
@samp{0:} gets the number @samp{1}; The 15th @samp{0:} gets the
number @samp{15}; @i{etc.}.  Likewise for the other labels @samp{1:}
through @samp{9:}.
@end table
For instance, the
first @code{1:} is named @code{L1^A1}, the 44th @code{3:} is named @code{L3^A44}.

@section The Special Dot Symbol

The special symbol @code{.} refers to the current address that
@code{as} is assembling into.  Thus, the expression @samp{melvin:
.long .} will cause @var{melvin} to contain its own address.
Assigning a value to @code{.} is treated the same as a @code{.org}
directive.  Thus, the expression @samp{.=.+4} is the same as saying
@samp{.space 4}.

@section Symbol Attributes
Every symbol has the attributes discussed below.  The detailed
definitions are in <a.out.h>.

If you use a symbol without defining it, @code{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.

@subsection Value
The value of a symbol is (usually) 32 bits, the size of one C
@code{int}.  For a symbol which labels a location in the
@code{text}, @code{data}, @code{bss} or @code{Absolute} segments the
value is the number of addresses from the start of that segment to
the label.  Naturally for @code{text} @code{data} and @code{bss}
segments the value of a symbol changes as @code{ld} changes segment
base addresses during linking.  @code{absolute} symbols' values do
not change during linking: that is why they are called absolute.

The value of an undefined symbol is treated in a special way.  If it
is 0 then the symbol is not defined in this assembler source
program, and @code{ld} will try to determine its value from other
programs it is linked with.  You make this kind of symbol simply by
mentioning a symbol name without defining it.  A non-zero value
represents a @code{.comm} common declaration.  The value is how much
common storage to reserve, in bytes (@i{i.e.} addresses).  The
symbol refers to the first address of the allocated storage.

@subsection Type
The type attribute of a symbol is 8 bits encoded in a devious way.
We kept this coding standard for compatibility with older operating
systems.

@example

        7     6     5     4     3     2     1     0     bit numbers
     +-----+-----+-----+-----+-----+-----+-----+-----+
     |                 |                       |     |
     |   N_STAB bits   |      N_TYPE bits      |N_EXT|
     |                 |                       | bit |
     +-----+-----+-----+-----+-----+-----+-----+-----+

                     n_type byte
@end example

@subsubsection N_EXT bit
This bit is set if @code{ld} might need to use the symbol's value
and type bits.  If this bit is re-set then @code{ld} can ignore the
symbol while linking.  It is set in two cases.  If the symbol is
undefined, then @code{ld} is expected to find the symbol's value
elsewhere in another program module.  Otherwise the symbol has the
value given, but this symbol name and value are revealed to any other
programs linked in the same executable program.  This second use of
the @code{N_EXT} bit is most often done by a @code{.globl} statement.

@subsubsection N_TYPE bits
These establish the symbol's ``type'', which is mainly a relocation
concept.  Common values are detailed in the manual describing the
executable file format.

@subsubsection N_STAB bits
Common values for these bits are described in the manual on the
executable file format.

@subsection Desc(riptor)
This is an arbitrary 16-bit value.  You may establish a symbol's
descriptor value by using a @code{.desc} statement (@xref{Desc}.).
A descriptor value means nothing to @code{as}.

@subsection Other
This is an arbitrary 8-bit value.  It means nothing to @code{as}.

@node Expressions, PseudoOps, Symbols, top
@chapter Expressions
An @dfn{expression} specifies an address or numeric value.
Whitespace may precede and/or follow an expression.

@section Empty Expressions
An empty expression has no operands: it is just whitespace or null.
Wherever an absolute expression is required, you may omit the
expression and @code{as} will assume a value of (absolute) 0.  This
is compatible with other assemblers.

@section Integer Expressions
An @dfn{integer expression} is one or more @i{primaries} delimited
by @i{operators}.

@node Primary, Unops, , Expressions
@subsection Primaries
@dfn{Primaries} are symbols, numbers or subexpressions.  Other
languages might call primaries ``arithmetic operands'' but we don't
want them confused with ``instruction operands'' of the machine
language so we give them a different name.

Symbols are evaluated to yield @{@var{segment} @var{value}@} where
@var{segment} is one of @b{text}, @b{data}, @b{bss}, @b{absolute},
or @b{undefined}.  @var{value} is a signed 2's complement 32 bit
integer.

Numbers are usually integers.

A number can be a flonum or bignum.  In this case, you are warned
that only the low order 32 bits are used, and @code{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 (@t{(}) followed by an integer
expression followed by a right parenthesis (@t{)}), or a unary
operator followed by an primary.

@subsection Operators
@dfn{Operators} are arithmetic marks, like @t{+} or @t{%}.  Unary
operators are followed by an primary.  Binary operators appear
between primaries.  Operators may be preceded and/or followed by
whitespace.

@subsection Unary Operators
@node Unops, , Primary, Expressions
@code{as} has the following @dfn{unary operators}.  They each take
one primary, which must be absolute.
@table @t
@item -
Hyphen.  @dfn{Negation}.  Two's complement negation.
@item ~
Tilde.  @dfn{Complementation}.  Bitwise not.
@end table

@subsection Binary Operators
@dfn{Binary operators} are infix.  Operators are prioritized, but
equal priority operators are performed left to right.  Apart from
@samp{+} or @samp{-}, both primaries must be absolute, and the
result is absolute, else one primary can be either undefined or
pass1 and the result is pass1.
@enumerate
@item
Highest Priority
@table @code
@item *
@dfn{Multiplication}.
@item /
@dfn{Division}.  Truncation is the same as the C operator @samp{/}
of the compiler that compiled @code{as}.
@item %
@dfn{Remainder}.
@item <
@itemx <<
@dfn{Shift Left}.  Same as the C operator @samp{<<} of
the compiler that compiled @code{as}.
@item >
@itemx >>
@dfn{Shift Right}.  Same as the C operator @samp{>>} of
the compiler that compiled @code{as}.
@end table
@item
Intermediate priority
@table @t
@item |
@dfn{Bitwise Inclusive Or}.
@item &
@dfn{Bitwise And}.
@item ^
@dfn{Bitwise Exclusive Or}.
@item !
@dfn{Bitwise Or Not}.
@end table
@item
Lowest Priority
@table @t
@item +
@dfn{Addition}.  If either primary is absolute, the result
has the segment of the other primary.
If either primary is pass1 or undefined, result is pass1.
Otherwise @t{+} is illegal.
@item -
@dfn{Subtraction}.  If the right primary is absolute, the
result has the segment of the left primary.
If either primary is pass1 the result is pass1.
If either primary is undefined the result is difference segment.
If both primaries are in the same segment, the result is absolute; provided
that segment is one of text, data or bss.
Otherwise @t{-} is illegal.
@end table
@end enumerate

The sense of the rules is that you can't add or subtract quantities
from two different segments.  If both primaries are in one of these
segments, they must be in the same segment:  @b{text}, @b{data} or
@b{bss}, and the operator must be @samp{-}.

@node PseudoOps, MachineDependent, Expressions, top
@chapter Assembler Directives
@menu
* Abort::	The Abort directive causes as to abort
* Align::	Pad the location counter to a power of 2
* Ascii::	Fill memory with bytes of ASCII characters
* Asciz::	Fill memory with bytes of ASCII characters followed
                by a null.
* Byte::	Fill memory with 8-bit integers
* Comm::	Reserve public space in the BSS segment
* Data::	Change to the data segment
* Desc::	Set the n_desc of a symbol
* Double::	Fill memory with double-precision floating-point numbers
* File::	Set the logical file name
* Fill::	Fill memory with repeated values
* Float::	Fill memory with single-precision floating-point numbers
* Global::	Make a symbol visible to the linker
* Int::		Fill memory with 32-bit integers
* Lcomm::	Reserve private space in the BSS segment
* Line::	Set the logical line number
* Long::	Fill memory with 32-bit integers
* Lsym::	Create a local symbol
* Octa::	Fill memory with 128-bit integers
* Org::		Change the location counter
* Quad::	Fill memory with 64-bit integers
* Set::		Set the value of a symbol
* Short::	Fill memory with 16-bit integers
* Space::	Fill memory with a repeated value
* Stab::	Store debugging information
* Text::	Change to the text segment
* Word::	Fill memory with 16-bit integers
@end menu

All assembler directives begin with a symbol that begins with a
period (@samp{.}).  The rest of the symbol is letters: their case
does not matter.

@node Abort, Align, PseudoOps, PseudoOps
@section .abort
This directive stops the assembly immediately.  It is for
compatibility with other assemblers.  The original idea was that the
assembler program would be piped into the assembler.  If the source
of program wanted to quit, then this directive tells @code{as} to
quit also.  One day @code{.abort} will not be supported.

@node Align, Ascii, Abort, PseudoOps
@section .align @var{absolute-expression} , @var{absolute-expression}
Pad the location counter (in the current subsegment) to a word,
longword or whatever boundary.  The first expression is the number
of low-order zero bits the location counter will have after
advancement.  For example @samp{.align 3} will advance 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 gives the value to be stored in the padding
bytes.  It (and the comma) may be omitted.  If it is omitted, the
padding bytes are zeroed.

@node Ascii, Asciz, Align, PseudoOps
@section .ascii @var{strings}
This expects zero or more string literals (@xref{Strings}.)
separated by commas.  It assembles each string (with no automatic
trailing zero byte) into consecutive addresses.

@node Asciz, Byte, Ascii, PseudoOps
@section .asciz @var{strings}
This is just like .ascii, but each string is followed by a zero byte.
The `z' in `.asciz' stands for `zero'.

@node Byte, Comm, Asciz, PseudoOps
@section .byte @var{expressions}

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

@node Comm, Data, Byte, PseudoOps
@section .comm @var{symbol} , @var{length}
This declares a named common area in the bss segment.  Normally
@code{ld} reserves memory addresses for it during linking, so no
partial program defines the location of the symbol.  Tell @code{ld}
that it must be at least @var{length} bytes long.  @code{ld} will
allocate space that is at least as long as the longest @code{.comm}
request in any of the partial programs linked.  @var{length} is an
absolute expression.

@node Data, Desc, Comm, PseudoOps
@section .data @var{subsegment}
This tells @code{as} to assemble the following statements onto the
end of the data subsegment numbered @var{subsegment} (which is an
absolute expression).  If @var{subsegment} is omitted, it defaults
to zero.

@node Desc, Double, Data, PseudoOps
@section .desc @var{symbol}, @var{absolute-expression}
This sets @code{n_desc} of the symbol to the low 16 bits of
@var{absolute-expression}.

@node Double, File, Desc, PseudoOps
@section .double @var{flonums}
This expects zero or more flonums, separated by commas.  It assembles
floating point numbers.  The exact kind of floating point numbers
emitted depends on what computer @code{as} is assembling for.  See
the machine-specific part of the manual for the machine the
assembler is running on for more information.

@node File, Fill, Double, PseudoOps
@section .file @var{string}
This tells @code{as} that we are about to start a new logical
file.  @var{String} is the new file name.  An empty file name
is OK, but you must still give the quotes: @code{""}.  This
statement may go away in future: it is only recognized to
be compatible with old @code{as} programs.

@node Fill, Float, File, PseudoOps
@section .fill @var{repeat} , @var{size} , @var{value}
@var{result}, @var{size} and @var{value} are absolute expressions.
This emits @var{repeat} copies of @var{size} bytes.  @var{Repeat}
may be zero or more.  @var{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 @var{repeat} bytes
is taken from an 8-byte number.  The highest order 4 bytes are
zero.  The lowest order 4 bytes are @var{value} rendered in the
byte-order of an integer on the computer @code{as} is assembling for.
Each @var{size} bytes in a repetition is taken from the lowest order
@var{size} bytes of this number.  Again, this bizarre behavior is
compatible with other people's assemblers.

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

@node Float, Global, Fill, PseudoOps
@section .float @var{flonums}
This directive assembles zero or more flonums, separated by commas.
The exact kind of floating point numbers emitted depends on what
computer @code{as} is assembling for.  See the machine-specific part
of the manual for the machine the assembler is running on for more
information.

@node Global, Int, Float, PseudoOps
@section .global @var{symbol}
This makes the symbol visible to @code{ld}.  If you define
@var{symbol} in your partial program, its value is made available to
other partial programs that are linked with it.  Otherwise,
@var{symbol} will take its attributes from a symbol of the same name
from another partial program it is linked with.

This is done by setting the @code{N_EXT} bit
of that symbol's @code{n_type} to 1.

@node Int, Lcomm, Global, PseudoOps
@section .int @var{expressions}
Expect zero or more @var{expressions}, of any segment, separated by
commas.  For each expression, emit a 32-bit number that will, at run
time, be the value of that expression.  The byte order of the
expression depends on what kind of computer will run the program.

@node Lcomm, Line, Int, PseudoOps
@section .lcomm @var{symbol} , @var{length}
Reserve @var{length} (an absolute expression) bytes for a local
common and denoted by @var{symbol}, whose segment and value are
those of the new local common.  The addresses are allocated in the
@code{bss} segment, so at run-time the bytes will start off zeroed.
@var{Symbol} is not declared global (@xref{Global}.), so is normally
not visible to @code{ld}.

@node Line, Long, Lcomm, PseudoOps
@section .line @var{logical line number}
This tells @code{as} to change the logical line number.
@var{logical line number} is an absolute expression.  The next line
will have that logical line number.  So any other statements on the
current line (after a @code{;}) will be reported as on logical line
number @var{logical line number} - 1.  One day this directive will
be unsupported: it is used only for compatibility with existing
assembler programs.

@node Long, Lsym, Line, PseudoOps
@section .long @var{expressions}
This is the same as @samp{.int}, @pxref{Int}.

@node Lsym, Octa, Long, PseudoOps
@section .lsym @var{symbol}, @var{expression}
This creates a new symbol named @var{symbol}, but do not put it in
the hash table, ensuring it cannot be referenced by name during the
rest of the assembly.  This sets the attributes of the symbol to be
the same as the expression value.  @code{n_other} = @code{n_desc} =
0.  @code{n_type} = (whatever segment the expression has); the
@code{N_EXT} bit of @code{n_type} is zero.  @code{n_value} =
(expression's value).

@node Octa, Org, Lsym, PseudoOps
@section .octa @var{bignums}
This expects zero or more bignums, separated by commas.  For each
bignum, it emits an 16-byte (@b{octa}-word) integer.

@node Org, Quad, Octa, PseudoOps
@section .org @var{new-lc} , @var{fill}
This will advance the location counter of the current segment to
@var{new-lc}.  @var{new-lc} is either an absolute expression or an
expression with the same segment as the current subsegment.  That
is, you can't use @code{.org} to cross segments.  Because @code{as}
tries to assemble programs in one pass @var{new-lc} must be defined.
If you really detest this restriction we eagerly await a chance to
share your improved assembler.  To be compatible with former
assemblers, if the segment of @var{new-lc} is absolute then we
pretend the segment of @var{new-lc} is the same as the current
subsegment.

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

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

@node Quad, Set, Org, PseudoOps
@section .quad @var{bignums}
This expects zero or more bignums, separated by commas.  For each
bignum, it emits an 8-byte (@b{quad}-word) integer.  If the bignum
won't fit in a quad-word, it prints a warning message; and just
takes the lowest order 8 bytes of the bignum.

@node Set, Short, Quad, PseudoOps
@section .set @var{symbol}, @var{expression}

This sets the value of @var{symbol} to expression.  This will change
@code{n_value} and @code{n_type} to conform to the @var{expression}.
if @code{n_ext} is set, it remains set.

It is OK to @code{.set} a symbol many times in the same assembly.
If the expression's segment is unknowable during pass 1, a second
pass over the source program will be forced.  The second pass is
currently not implemented.  @code{as} will abort with an error
message if one is required.

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

@node Short, Space, Set, PseudoOps
@section .short @var{expressions}
Except on the Sparc this is the same as @samp{.word}.  @xref{Word}.
On the sparc, this expects zero or more @var{expressions}, and emits
a 16 bit number for each.

@node Space, Stab, Short, PseudoOps
@section .space @var{size} , @var{fill}
This emits @var{size} bytes, each of value @var{fill}.  Both
@var{size} and @var{fill} are absolute expressions.  If the comma
and @var{fill} are omitted, @var{fill} is assumed to be zero.

@node Stab, Text, Space, PseudoOps
@section .stabd, .stabn, .stabs
There are three directives that begin @code{.stab@dots{}}.
All emit symbols, for use by symbolic debuggers.
The symbols are not entered in @code{as}' hash table: they
cannot be referenced elsewhere in the source file.
Up to five fields are required:
@table @var
@item string
This is the symbol's name.  It may contain any character except @samp{\000},
so is more general than ordinary symbol names.  Some debuggers used to
code arbitrarily complex structures into symbol names using this technique.
@item type
An absolute expression.  The symbol's @code{n_type} is set to the low 8
bits of this expression.
Any bit pattern is permitted, but @code{ld} and debuggers will choke on
silly bit patterns.
@item other
An absolute expression.
The symbol's @code{n_other} is set to the low 8 bits of this expression.
@item desc
An absolute expression.
The symbol's @code{n_desc} is set to the low 16 bits of this expression.
@item value
An absolute expression which becomes the symbol's @code{n_value}.
@end table

If a warning is detected while reading the @code{.stab@dots{}}
statement the symbol has probably already been created and you will
get a half-formed symbol in your object file.  This is compatible
with earlier assemblers (!)

.stabd @var{type} , @var{other} , @var{desc}

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 @code{n_value} is set to the location counter,
relocatably.  When your program is linked, the value of this symbol
will be where the location counter was when the @code{.stabd} was
assembled.

.stabn @var{type} , @var{other} , @var{desc} , @var{value}

The name of the symbol is set to the empty string @code{""}.

.stabs @var{string} ,  @var{type} , @var{other} , @var{desc} , @var{value}

@node Text, Word, Stab, PseudoOps
@section .text @var{subsegment}
Tells @code{as} to assemble the following statements onto the end of
the text subsegment numbered @var{subsegment}, which is an absolute
expression.  If @var{subsegment} is omitted, subsegment number zero
is used.

@node Word, , Text, PseudoOps
@section .word @var{expressions}
On the Sparc, this produces 32-bit numbers instead of 16-bit ones.
This expect zero or more @var{expressions}, of any segment,
separated by commas.  For each expression, emit a 16-bit number that
will, at run time, be the value of that expression.  The byte order
of the expression depends on what kind of computer will run the
program.

@section Deprecated Directives
One day these directives won't work.
They are included for compatibility with older assemblers.
@table @t
@item .abort
@item .file
@item .line
@end table

@node MachineDependent, Maintenance, PseudoOps, top
@chapter Machine Dependent Features
@section Vax
@subsection Options

The Vax version of @code{as} accepts any of the following options,
gives a warning message that the option was ignored and proceeds.
These options are for compatibility with scripts designed for other
people's assemblers.

@table @asis
@item @kbd{-D} (Debug)
@itemx @kbd{-S} (Symbol Table)
@itemx @kbd{-T} (Token Trace)
These are obsolete options used to debug old assemblers.

@item @kbd{-d} (Displacement size for JUMPs)
This option expects a number following the @kbd{-d}.  Like options
that expect filenames, the number may immediately follow the
@kbd{-d} (old standard) or constitute the whole of the command line
argument that follows @kbd{-d} (GNU standard).

@item @kbd{-V} (Virtualize Interpass Temporary File)
Some other assemblers use a temporary file.  This option
commanded them to keep the information in active memory rather
than in a disk file.  @code{as} always does this, so this
option is redundant.

@item @kbd{-J} (JUMPify Longer Branches)
Many 32-bit computers permit a variety of branch instructions
to do the same job.  Some of these instructions are short (and
fast) but have a limited range; others are long (and slow) but
can branch anywhere in virtual memory.  Often there are 3
flavors of branch: short, medium and long.  Some other
assemblers would emit short and medium branches, unless told by
this option to emit short and long branches.

@item @kbd{-t} (Temporary File Directory)
Some other assemblers may use a temporary file, and this option
takes a filename being the directory to site the temporary
file.  @code{as} does not use a temporary disk file, so this
option makes no difference.  @kbd{-t} needs exactly one
filename.
@end table

The Vax version of the assembler accepts two options when
compiled for VMS.  They are @kbd{-h}, and @kbd{-+}.  The
@kbd{-h} option prevents @code{as} from modifying the
symbol-table entries for symbols that contain lowercase
characters (I think).  The @kbd{-+} option causes @code{as} to
print warning messages if the FILENAME part of the object file,
or any symbol name is larger than 31 characters.  The @kbd{-+}
option also insertes some code following the @samp{_main}
symbol so that the object file will be compatable with Vax-11
"C".

@subsection Floating Point
Conversion of flonums to floating point is correct, and
compatible with previous assemblers.  Rounding is
towards zero if the remainder is exactly half the least significant bit.

@code{D}, @code{F}, @code{G} and @code{H} floating point formats
are understood.

Immediate floating literals (@i{e.g.} @samp{S`$6.9})
are rendered correctly.  Again, rounding is towards zero in the
boundary case.

The @code{.float} directive produces @code{f} format numbers.
The @code{.double} directive produces @code{d} format numbers.

@subsection Machine Directives
The Vax version of the assembler supports four directives for
generating Vax floating point constants.  They are described in the
table below.

@table @code
@item .dfloat
This expects zero or more flonums, separated by commas, and
assembles Vax @code{d} format 64-bit floating point constants.

@item .ffloat
This expects zero or more flonums, separated by commas, and
assembles Vax @code{f} format 32-bit floating point constants.

@item .gfloat
This expects zero or more flonums, separated by commas, and
assembles Vax @code{g} format 64-bit floating point constants.

@item .hfloat
This expects zero or more flonums, separated by commas, and
assembles Vax @code{h} format 128-bit floating point constants.

@end table

@subsection Opcodes
All DEC mnemonics are supported.  Beware that @code{case@dots{}}
instructions have exactly 3 operands.  The dispatch table that
follows the @code{case@dots{}} instruction should be made with
@code{.word} statements.  This is compatible with all unix
assemblers we know of.

@subsection Branch Improvement
Certain pseudo opcodes are permitted.  They are for branch
instructions.  They expand to the shortest branch instruction that
will reach the target.  Generally these mnemonics are made by
substituting @samp{j} for @samp{b} at the start of a DEC mnemonic.
This feature is included both for compatibility and to help
compilers.  If you don't need this feature, don't use these
opcodes.  Here are the mnemonics, and the code they can expand into.

@table @code
@item jbsb
@samp{Jsb} is already an instruction mnemonic, so we chose @samp{jbsb}.
@table @asis
@item (byte displacement)
@kbd{bsbb @dots{}}
@item (word displacement)
@kbd{bsbw @dots{}}
@item (long displacement)
@kbd{jsb @dots{}}
@end table
@item jbr
@itemx jr
Unconditional branch.
@table @asis
@item (byte displacement)
@kbd{brb @dots{}}
@item (word displacement)
@kbd{brw @dots{}}
@item (long displacement)
@kbd{jmp @dots{}}
@end table
@item j@var{COND}
@var{COND} may be any one of the conditional branches
@code{neq nequ eql eqlu gtr geq lss gtru lequ vc vs gequ cc lssu cs}.
@var{COND} may also be one of the bit tests
@code{bs bc bss bcs bsc bcc bssi bcci lbs lbc}.
@var{NOTCOND} is the opposite condition to @var{COND}.
@table @asis
@item (byte displacement)
@kbd{b@var{COND} @dots{}}
@item (word displacement)
@kbd{b@var{UNCOND} foo ; brw @dots{} ; foo:}
@item (long displacement)
@kbd{b@var{UNCOND} foo ; jmp @dots{} ; foo:}
@end table
@item jacb@var{X}
@var{X} may be one of @code{b d f g h l w}.
@table @asis
@item (word displacement)
@kbd{@var{OPCODE} @dots{}}
@item (long displacement)
@kbd{@var{OPCODE} @dots{}, foo ; brb bar ; foo: jmp @dots{} ; bar:}
@end table
@item jaob@var{YYY}
@var{YYY} may be one of @code{lss leq}.
@item jsob@var{ZZZ}
@var{ZZZ} may be one of @code{geq gtr}.
@table @asis
@item (byte displacement)
@kbd{@var{OPCODE} @dots{}}
@item (word displacement)
@kbd{@var{OPCODE} @dots{}, foo ; brb bar ; foo: brw @var{destination} ; bar:}
@item (long displacement)
@kbd{@var{OPCODE} @dots{}, foo ; brb bar ; foo: jmp @var{destination} ; bar: }
@end table
@item aobleq
@itemx aoblss
@itemx sobgeq
@itemx sobgtr
@table @asis
@item (byte displacement)
@kbd{@var{OPCODE} @dots{}}
@item (word displacement)
@kbd{@var{OPCODE} @dots{}, foo ; brb bar ; foo: brw @var{destination} ; bar:}
@item (long displacement)
@kbd{@var{OPCODE} @dots{}, foo ; brb bar ; foo: jmp @var{destination} ; bar:}
@end table
@end table

@subsection operands
The immediate character is @samp{$} for Unix compatibility, not
@samp{#} as DEC writes it.

The indirect character is @samp{*} for Unix compatibility, not
@samp{@@} as DEC writes it.

The displacement sizing character is @samp{`} (an accent grave) for
Unix compatibility, not @samp{^} as DEC writes it.  The letter
preceding @samp{`} may have either case.  @samp{G} is not
understood, but all other letters (@code{b i l s w}) are understood.

Register names understood are @code{r0 r1 r2 @dots{} r15 ap fp sp
pc}.  Any case of letters will do.

For instance
@example
tstb *w`$4(r5)
@end example

Any expression is permitted in an operand.  Operands are comma
separated.

@c There is some bug to do with recognizing expressions
@c in operands, but I forget what it is.  It is
@c a syntax clash because () is used as an address mode
@c and to encapsulate sub-expressions.
@subsection Not Supported
Vax bit fields can not be assembled with @code{as}.  Someone
can add the required code if they really need it.

@section 680x0
@subsection Options
The 680x0 version of @code{as} has two machine dependent options.
One shortens undefined references from 32 to 16 bits, while the
other is used to tell @code{as} what kind of machine it is
assembling for.

You can use the @kbd{-l} option to shorten the size of references to
undefined symbols.  If the @kbd{-l} option is not given, references
to undefined symbols will be a full long (32 bits) wide.  (Since
@code{as} cannot know where these symbols will end up being,
@code{as} can only allocate space for the linker to fill in later.
Since @code{as} doesn't know how far away these symbols will be, it
allocates as much space as it can.) If this option is given, the
references will only be one word wide (16 bits).  This may be useful
if you want the object file to be as small as possible, and you know
that the relevant symbols will be less than 17 bits away.

The 680x0 version of @code{as} is usually used to assemble programs
for the Motorola MC68020 microprocessor.  Occasionally it is used to
assemble programs for the mostly-similar-but-slightly-different
MC68000 or MC68010 microprocessors.  You can give @code{as} the
options @samp{-m68000}, @samp{-mc68000}, @samp{-m68010},
@samp{-mc68010}, @samp{-m68020}, and @samp{-mc68020} to tell it what
processor it should be assembling for.  Unfortunately, these options
are almost entirely unused and untried.  They make work, but nobody
has tested them much.

@subsection Syntax

The 680x0 version of @code{as} uses syntax similar to the Sun
assembler.  Size modifieres are appended directly to the end of the
opcode without an intervening period.  Thus, @samp{move.l} is
written @samp{movl}, etc.

@c   This is no longer true
@c Explicit size modifiers for branch instructions are ignored; @code{as}
@c automatically picks the smallest size that will reach the
destination.

If @code{as} is compiled with SUN_ASM_SYNTAX defined, it will also
allow Sun-style local labels of the form @samp{1$} through @samp{$9}.

In the following table @dfn{apc} stands for any of the address
registers (@samp{a0} through @samp{a7}), nothing, (@samp{}), the
Program Counter (@samp{pc}), or the zero-address relative to the
program counter (@samp{zpc}).

The following addressing modes are understood:
@table @dfn
@item Immediate
@samp{#@var{digits}}

@item Data Register
@samp{d0} through @samp{d7}

@item Address Register
@samp{a0} through @samp{a7}

@item Address Register Indirect
@samp{a0@@} through @samp{a7@@}

@item Address Register Postincrement
@samp{a0@@+} through @samp{a7@@+}

@item Address Register Predecrement
@samp{a0@@-} through @samp{a7@@-}

@item Indirect Plus Offset
@samp{@var{apc}@@(@var{digits})}

@item Index
@samp{@var{apc}@@(@var{digits},@var{register}:@var{size}:@var{scale})}
or @samp{@var{apc}@@(@var{register}:@var{size}:@var{scale})}

@item Postindex
@samp{@var{apc}@@(@var{digits})@@(@var{digits},@var{register}:@var{size}:@var{scale})}
or @samp{@var{apc}@@(@var{digits})@@(@var{register}:@var{size}:@var{scale})}

@item Preindex
@samp{@var{apc}@@(@var{digits},@var{register}:@var{size}:@var{scale})@@(@var{digits})}
or @samp{@var{apc}@@(@var{register}:@var{size}:@var{scale})@@(@var{digits})}

@item Memory Indirect
@samp{@var{apc}@@(@var{digits})@@(@var{digits})}

@item Absolute
@samp{@var{symbol}}, or @samp{@var{digits}}, or either of the above followed
by @samp{:b}, @samp{:w}, or @samp{:l}.
@end table

@subsection Floating Point
The floating point code is not too well tested, and may have
subtle bugs in it.

Packed decimal (P) format floating literals are not supported.
Feel free to add the code yourself.

The floating point formats generated by directives are these.
@table @code
@item .float
@code{Single} precision floating point constants.
@item .double
@code{Double} precision floating point constants.
@end table

There is no directive to produce regions of memory holding
extended precision numbers, however they can be used as
immediate operands to floating-point instructions.  Adding a
directive to create extended precision numbers would not be
hard.  Nobody has felt any burning need to do it.

@subsection Machine Directives
In order to be compatible with the Sun assembler the 680x0 assembler
understands the following directives.
@table @code
@item .data1
This directive is identical to a @code{.data 1} directive.
@item .data2
This directive is identical to a @code{.data 2} directive.
@item .even
This directive is identical to a @code{.align 1} directive.
@c Is this true?  does it work???
@item .skip
This directive is identical to a @code{.space} directive.
@end table

@subsection Opcodes
Danger:  Several bugs have been found in the opcode table (and
fixed).  More bugs may exist.  Be careful when using obscure
instructions.

The assembler automatically chooses the proper size for branch
instructions.  However, most attempts to force a short displacement
will be honored.  Branches that are forced to use a short
displacement will not be adjusted if the target is out of range.
Let The User Beware.

The immediate character is @samp{#} for Sun compatibility.  The
line-comment character is @samp{|}.  If a @samp{#} appears at the
beginning of a line, it is treated as a comment unless it looks like
@samp{# line file}, in which case it is treated normally.

@section 32x32
@subsection Options
The 32x32 version of @code{as} accepts a @kbd{-m32032} option to
specify thiat it is compiling for a 32032 processor, or a
@kbd{-m32532} to specify that it is compiling for a 32532 option.
The default (if neither is specified) is chosen when the assembler
is compiled.

@subsection Syntax
I don't know anything about the 32x32 syntax assembled by
@code{as}.  Someone who undersands the processor (I've never seen
one) and the possible syntaxes should write this section.

@subsection Floating Point
The 32x32 uses IEEE floating point numbers, but @code{as} will only
create single or double precision values.  I don't know if the 32x32
understands extended precision numbers.

@subsection Machine Directives
The 32x32 has no machine dependent directives.

@section Sparc
@subsection Options
The sparc has no machine dependent options.

@subsection syntax
I don't know anything about Sparc syntax.  Someone who does
will have to write this section.

@subsection Floating Point
The Sparc uses ieee floating-point numbers.

@subsection Machine Directives
The Sparc version of @code{as} supports the following additional
machine directives:

@table @code
@item .common
This must be followed by a symbol name, a positive number, and
@code{"bss"}.  This behaves somewhat like @code{.comm}, but the
syntax is different.

@item .global
This is functionally identical to @code{.globl}.

@item .half
This is functionally identical to @code{.short}.

@item .proc
This directive is ignored.  Any text following it on the same
line is also ignored.

@item .reserve
This must be followed by a symbol name, a positive number, and
@code{"bss"}.  This behaves somewhat like @code{.lcomm}, but the
syntax is different.

@item .seg
This must be followed by @code{"text"}, @code{"data"}, or
@code{"data1"}.  It behaves like @code{.text}, @code{.data}, or
@code{.data 1}.

@item .skip
This is functionally identical to the .space directive.

@item .word
On the Sparc, the .word directive produces 32 bit values,
instead of the 16 bit values it produces on every other machine.

@end table

@section Intel 80386
@subsection Options
The 80386 has no machine dependent options.

@subsection AT&T Syntax versus Intel Syntax
In order to maintain compatibility with the output of @code{GCC},
@code{as} supports AT&T System V/386 assembler syntax.  This is quite
different from Intel syntax.  We mention these differences because
almost all 80386 documents used only Intel syntax.  Notable differences
between the two syntaxes are:
@itemize @bullet
@item
AT&T immediate operands are preceded by @samp{$}; Intel immediate
operands are undelimited (Intel @samp{push 4} is AT&T @samp{pushl $4}).
AT&T register operands are preceded by @samp{%}; Intel register operands
are undelimited.  AT&T absolute (as opposed to PC relative) jump/call
operands are prefixed by @samp{*}; they are undelimited in Intel syntax.

@item
AT&T and Intel syntax use the opposite order for source and destination
operands.  Intel @samp{add eax, 4} is @samp{addl $4, %eax}.  The
@samp{source, dest} convention is maintained for compatibility with
previous Unix assemblers.

@item
In AT&T syntax the size of memory operands is determined from the last
character of the opcode name.  Opcode suffixes of @samp{b}, @samp{w},
and @samp{l} specify byte (8-bit), word (16-bit), and long (32-bit)
memory references.  Intel syntax accomplishes this by prefixes memory
operands (@emph{not} the opcodes themselves) with @samp{byte ptr},
@samp{word ptr}, and @samp{dword ptr}.  Thus, Intel @samp{mov al, byte
ptr @var{foo}} is @samp{movb @var{foo}, %al} in AT&T syntax.

@item
Immediate form long jumps and calls are
@samp{lcall/ljmp $@var{segment}, $@var{offset}} in AT&T syntax; the
Intel syntax is
@samp{call/jmp far @var{segment}:@var{offset}}.  Also, the far return
instruction 
is @samp{lret $@var{stack-adjust}} in AT&T syntax; Intel syntax is
@samp{ret far @var{stack-adjust}}.

@item
The AT&T assembler does not provide support for multiple segment
programs.  Unix style systems expect all programs to be single segments.
@end itemize

@subsection Opcode Naming
Opcode names are suffixed with one character modifiers which specify the
size of operands.  The letters @samp{b}, @samp{w}, and @samp{l} specify
byte, word, and long operands.  If no suffix is specified by an
instruction and it contains no memory operands then @code{as} tries to
fill in the missing suffix based on the destination register operand
(the last one by convention).  Thus, @samp{mov %ax, %bx} is equivalent
to @samp{movw %ax, %bx}; also, @samp{mov $1, %bx} is equivalent to
@samp{movw $1, %bx}.  Note that this is incompatible with the AT&T Unix
assembler which assumes that a missing opcode suffix implies long
operand size.  (This incompatibility does not affect compiler output
since compilers always explicitly specify the opcode suffix.)

Almost all opcodes have the same names in AT&T and Intel format.  There
are a few exceptions.  The sign extend and zero extend instructions need
two sizes to specify them.  They need a size to sign/zero extend
@emph{from} and a size to zero extend @emph{to}.  This is accomplished
by using two opcode suffixes in AT&T syntax.  Base names for sign extend
and zero extend are @samp{movs@dots{}} and @samp{movz@dots{}} in AT&T
syntax (@samp{movsx} and @samp{movzx} in Intel syntax).  The opcode
suffixes are tacked on to this base name, the @emph{from} suffix before
the @emph{to} suffix.  Thus, @samp{movsbl %al, %edx} is AT&T syntax for
``move sign extend @emph{from} %al @emph{to} %edx.''  Possible suffixes,
thus, are @samp{bl} (from byte to long), @samp{bw} (from byte to word),
and @samp{wl} (from word to long).

The Intel syntax conversion instructions
@itemize @bullet
@item
@samp{cbw} --- sign-extend byte in @samp{%al} to word in @samp{%ax},
@item
@samp{cwde} --- sign-extend word in @samp{%ax} to long in @samp{%eax},
@item
@samp{cwd} --- sign-extend word in @samp{%ax} to long in @samp{%dx:%ax},
@item
@samp{cdq} --- sign-extend dword in @samp{%eax} to quad in @samp{%edx:%eax},
@end itemize
are called @samp{cbtw}, @samp{cwtl}, @samp{cwtd}, and @samp{cltd} in
AT&T naming.  @code{as} accepts either naming for these instructions.

Far call/jump instructions are @samp{lcall} and @samp{ljmp} in
AT&T syntax, but are @samp{call far} and @samp{jump far} in Intel
convention.  

@subsection Register Naming
Register operands are always prefixes with @samp{%}.  The 80386 registers
consist of
@itemize @bullet
@item
the 8 32-bit registers @samp{%eax} (the accumulator), @samp{%ebx},
@samp{%ecx}, @samp{%edx}, @samp{%edi}, @samp{%esi}, @samp{%ebp} (the
frame pointer), and @samp{%esp} (the stack pointer).

@item
the 8 16-bit low-ends of these: @samp{%ax}, @samp{%bx}, @samp{%cx},
@samp{%dx}, @samp{%di}, @samp{%si}, @samp{%bp}, and @samp{%sp}.

@item
the 8 8-bit registers: @samp{%ah}, @samp{%al}, @samp{%bh},
@samp{%bl}, @samp{%ch}, @samp{%cl}, @samp{%dh}, and @samp{%dl} (These
are the high-bytes and low-bytes of @samp{%ax}, @samp{%bx},
@samp{%cx}, and @samp{%dx})

@item
the 6 segment registers @samp{%cs} (code segment), @samp{%ds}
(data segment), @samp{%ss} (stack segment), @samp{%es}, @samp{%fs},
and @samp{%gs}.

@item
the 3 processor control registers @samp{%cr0}, @samp{%cr2}, and
@samp{%cr3}.

@item
the 6 debug registers @samp{%db0}, @samp{%db1}, @samp{%db2},
@samp{%db3}, @samp{%db6}, and @samp{%db7}.

@item
the 2 test registers @samp{%tr6} and @samp{%tr7}.

@item
the 8 floating point register stack @samp{%st} or equivalently
@samp{%st(0)}, @samp{%st(1)}, @samp{%st(2)}, @samp{%st(3)},
@samp{%st(4)}, @samp{%st(5)}, @samp{%st(6)}, and @samp{%st(7)}.
@end itemize

@subsection Opcode Prefixes
Opcode prefixes are used to modify the following opcode.  They are used
to repeat string instructions, to provide segment overrides, to perform
bus lock operations, and to give operand and address size (16-bit
operands are specified in an instruction by prefixing what would
normally be 32-bit operands with a ``operand size'' opcode prefix).
Opcode prefixes are usually given as single-line instructions with no
operands, and must directly precede the instruction they act upon.  For
example, the @samp{scas} (scan string) instruction is repeated with:
@example
	repne
	scas
@end example

Here is a list of opcode prefixes:
@itemize @bullet
@item
Segment override prefixes @samp{cs}, @samp{ds}, @samp{ss}, @samp{es},
@samp{fs}, @samp{gs}.  These are automatically added by specifying
using the @var{segment}:@var{memory-operand} form for memory references.

@item
Operand/Address size prefixes @samp{data16} and @samp{addr16}
change 32-bit operands/addresses into 16-bit operands/addresses.  Note
that 16-bit addressing modes (i.e. 8086 and 80286 addressing modes)
are not supported (yet).

@item
The bus lock prefix @samp{lock} inhibits interrupts during
execution of the instruction it precedes.  (This is only valid with
certain instructions; see a 80386 manual for details).

@item
The wait for coprocessor prefix @samp{wait} waits for the
coprocessor to complete the current instruction.  This should never be
needed for the 80386/80387 combination.

@item
The @samp{rep}, @samp{repe}, and @samp{repne} prefixes are added
to string instructions to make them repeat @samp{%ecx} times.
@end itemize

@subsection Memory References
An Intel syntax indirect memory reference of the form
@example
@var{segment}:[@var{base} + @var{index}*@var{scale} + @var{disp}]
@end example
is translated into the AT&T syntax
@example
@var{segment}:@var{disp}(@var{base}, @var{index}, @var{scale})
@end example
where @var{base} and @var{index} are the optional 32-bit base and
index registers, @var{disp} is the optional displacement, and
@var{scale}, taking the values 1, 2, 4, and 8, multiplies @var{index}
to calculate the address of the operand.  If no @var{scale} is
specified, @var{scale} is taken to be 1.  @var{segment} specifies the
optional segment register for the memory operand, and may override the
default segment register (see a 80386 manual for segment register
defaults). Note that segment overrides in AT&T syntax @emph{must} have
be preceded by a @samp{%}.  If you specify a segment override which
coincides with the default segment register, @code{as} will @emph{not}
output any segment register override prefixes to assemble the given
instruction.  Thus, segment overrides can be specified to emphasize which
segment register is used for a given memory operand.

Here are some examples of Intel and AT&T style memory references:
@table @asis

@item AT&T: @samp{-4(%ebp)}, Intel:  @samp{[ebp - 4]}
@var{base} is @samp{%ebp}; @var{disp} is @samp{-4}. @var{segment} is
missing, and the default segment is used (@samp{%ss} for addressing with
@samp{%ebp} as the base register).  @var{index}, @var{scale} are both missing.

@item AT&T: @samp{foo(,%eax,4)}, Intel: @samp{[foo + eax*4]}
@var{index} is @samp{%eax} (scaled by a @var{scale} 4); @var{disp} is
@samp{foo}.  All other fields are missing.  The segment register here
defaults to @samp{%ds}.

@item AT&T: @samp{foo(,1)}; Intel @samp{[foo]}
This uses the value pointed to by @samp{foo} as a memory operand.
Note that @var{base} and @var{index} are both missing, but there is only
@emph{one} @samp{,}.  This is a syntactic exception.

@item AT&T: @samp{%gs:foo}; Intel @samp{gs:foo}
This selects the contents of the variable @samp{foo} with segment
register @var{segment} being @samp{%gs}.
	
@end table

Absolute (as opposed to PC relative) call and jump operands must be
prefixed with @samp{*}.  If no @samp{*} is specified, @code{as} will
always choose PC relative addressing for jump/call labels.  

Any instruction that has a memory operand @emph{must} specify its size (byte,
word, or long) with an opcode suffix (@samp{b}, @samp{w}, or @samp{l},
respectively).

@subsection Handling of Jump Instructions
Jump instructions are always optimized to use the smallest possible
displacements.  This is accomplished by using byte (8-bit) displacement
jumps whenever the target is sufficiently close.  If a byte displacement
is insufficient a long (32-bit) displacement is used.  We do not support
word (16-bit) displacement jumps (i.e. prefixing the jump instruction
with the @samp{addr16} opcode prefix), since the 80386 insists upon masking
@samp{%eip} to 16 bits after the word displacement is added.

Note that the @samp{jcxz}, @samp{jecxz}, @samp{loop}, @samp{loopz},
@samp{loope}, @samp{loopnz} and @samp{loopne} instructions only come in
byte displacements, so that it is possible that use of these
instructions (@code{GCC} does not use them) will cause the assembler to
print an error message (and generate incorrect code).  The AT&T 80386
assembler tries to get around this problem by expanding @samp{jcxz foo} to
@example
         jcxz cx_zero
         jmp cx_nonzero
cx_zero: jmp foo
cx_nonzero:
@end example

@subsection Floating Point
All 80387 floating point types except packed BCD are supported.
(BCD support may be added without much difficulty).  These data
types are 16-, 32-, and 64- bit integers, and single (32-bit),
double (64-bit), and extended (80-bit) precision floating point.
Each supported type has an opcode suffix and a constructor
associated with it.  Opcode suffixes specify operand's data
types.  Constructors build these data types into memory.

@itemize @bullet
@item
Floating point constructors are @samp{.float} or @samp{.single},
@samp{.double}, and @samp{.tfloat} for 32-, 64-, and 80-bit formats.
These correspond to opcode suffixes @samp{s}, @samp{l}, and @samp{t}.
@samp{t} stands for temporary real, and that the 80387 only supports
this format via the @samp{fldt} (load temporary real to stack top) and
@samp{fstpt} (store temporary real and pop stack) instructions.

@item
Integer constructors are @samp{.word}, @samp{.long} or @samp{.int}, and
@samp{.quad} for the 16-, 32-, and 64-bit integer formats.  The corresponding
opcode suffixes are @samp{s} (single), @samp{l} (long), and @samp{q}
(quad).  As with the temporary real format the 64-bit @samp{q} format is
only present in the @samp{fildq} (load quad integer to stack top) and
@samp{fistpq} (store quad integer and pop stack) instructions.
@end itemize

Register to register operations do not require opcode suffixes,
so that @samp{fst %st, %st(1)} is equivalent to @samp{fstl %st, %st(1)}.

Since the 80387 automatically synchronizes with the 80386 @samp{fwait}
instructions are almost never needed (this is not the case for the
80286/80287 and 8086/8087 combinations).  Therefore, @code{as} supresses
the @samp{fwait} instruction whenever it is implicitly selected by one
of the @samp{fn@dots{}} instructions.  For example, @samp{fsave} and
@samp{fnsave} are treated identically.  In general, all the @samp{fn@dots{}}
instructions are made equivalent to @samp{f@dots{}} instructions.  If
@samp{fwait} is desired it must be explicitly coded.

@subsection Notes
There is some trickery concerning the @samp{mul} and @samp{imul}
instructions that deserves mention.  The 16-, 32-, and 64-bit expanding
multiplies (base opcode @samp{0xf6}; extension 4 for @samp{mul} and 5
for @samp{imul}) can be output only in the one operand form.  Thus,
@samp{imul %ebx, %eax} does @emph{not} select the expanding multiply;
the expanding multiply would clobber the @samp{%edx} register, and this
would confuse @code{GCC} output.  Use @samp{imul %ebx} to get the
64-bit product in @samp{%edx:%eax}.

We have added a two operand form of @samp{imul} when the first operand
is an immediate mode expression and the second operand is a register.
This is just a shorthand, so that, multiplying @samp{%eax} by 69, for
example, can be done with @samp{imul $69, %eax} rather than @samp{imul
$69, %eax, %eax}.

@node Maintenance, Retargeting, MachineDependent, top
@chapter Maintaining the Assembler
[[this chapter is still being built]]

@section Design
We had these goals, in descending priority:
@table @b
@item Accuracy.
For every program composed by a compiler, @code{as} should emit
``correct'' code.  This leaves some latitude in choosing addressing
modes, order of @code{relocation_info} structures in the object
file, @i{etc}.

@item Speed, for usual case.
By far the most common use of @code{as} will be assembling compiler
emissions.

@item Upward compatibility for existing assembler code.
Well @dots{} we don't support Vax bit fields but everything else
seems to be upward compatible.

@item Readability.
The code should be maintainable with few surprises.  (JF: ha!)

@end table

We assumed that disk I/O was slow and expensive while memory was
fast and access to memory was cheap.  We expect the in-memory data
structures to be less than 10 times the size of the emitted object
file.  (Contrast this with the C compiler where in-memory structures
might be 100 times object file size!)
This suggests:
@itemize @bullet
@item
Try to read the source file from disk only one time.  For other
reasons, we keep large chunks of the source file in memory during
assembly so this is not a problem.  Also the assembly algorithm
should only scan the source text once if the compiler composed the
text according to a few simple rules.
@item
Emit the object code bytes only once.  Don't store values and then
backpatch later.
@item
Build the object file in memory and do direct writes to disk of
large buffers.
@end itemize

RMS suggested a one-pass algorithm which seems to work well.  By not
parsing text during a second pass considerable time is saved on
large programs (@i{e.g.} the sort of C program @code{yacc} would
emit).

It happened that the data structures needed to emit relocation
information to the object file were neatly subsumed into the data
structures that do backpatching of addresses after pass 1.

Many of the functions began life as re-usable modules, loosely
connected.  RMS changed this to gain speed.  For example, input
parsing routines which used to work on pre-sanitized strings now
must parse raw data.  Hence they have to import knowledge of the
assemblers' comment conventions @i{etc}.

@section Deprecated Feature(?)s
We have stopped supporting some features:
@itemize @bullet
@item
@code{.org} statements must have @b{defined} expressions.
@item
Vax Bit fields (@kbd{:} operator) are entirely unsupported.
@end itemize

It might be a good idea to not support these features in a future release:
@itemize @bullet
@item
@kbd{#} should begin a comment, even in column 1.
@item
Why support the logical line & file concept any more?
@item
Subsegments are a good candidate for flushing.
Depends on which compilers need them I guess.
@end itemize

@section Bugs, Ideas, Further Work
Clearly the major improvement is DON'T USE A TEXT-READING
ASSEMBLER for the back end of a compiler.  It is much faster to
interpret binary gobbledygook from a compiler's tables than to
ask the compiler to write out human-readable code just so the
assembler can parse it back to binary.

Assuming you use @code{as} for human written programs: here are
some ideas:
@itemize @bullet
@item
Document (here) @code{APP}.
@item
Take advantage of knowing no spaces except after opcode
to speed up @code{as}.  (Modify @code{app.c} to flush useless spaces:
only keep space/tabs at begin of line or between 2
symbols.)
@item
Put pointers in this documentation to @file{a.out} documentation.
@item
Split the assembler into parts so it can gobble direct binary
from @i{e.g.} @code{cc}.  It is silly for@code{cc} to compose text
just so @code{as} can parse it back to binary.
@item
Rewrite hash functions: I want a more modular, faster library.
@item
Clean up LOTS of code.
@item
Include all the non-@file{.c} files in the maintenance chapter.
@item
Document flonums.
@item
Implement flonum short literals.
@item
Change all talk of expression operands to expression quantities,
or perhaps to expression primaries.
@item
Implement pass 2.
@item
Whenever a @code{.text} or @code{.data} statement is seen, we close
of the current frag with an imaginary @code{.fill 0}.  This is
because we only have one obstack for frags, and we can't grow new
frags for a new subsegment, then go back to the old subsegment and
append bytes to the old frag.  All this nonsense goes away if we
give each subsegment its own obstack.  It makes code simpler in
about 10 places, but nobody has bothered to do it because C compiler
output rarely changes subsegments (compared to ending frags with
relaxable addresses, which is common).
@end itemize

@section Sources
@c The following files in the @file{as} directory
@c are symbolic links to other files, of
@c the same name, in a different directory.
@c @itemize @bullet
@c @item
@c @file{atof_generic.c}
@c @item
@c @file{atof_vax.c}
@c @item
@c @file{flonum_const.c}
@c @item
@c @file{flonum_copy.c}
@c @item
@c @file{flonum_get.c}
@c @item
@c @file{flonum_multip.c}
@c @item
@c @file{flonum_normal.c}
@c @item
@c @file{flonum_print.c}
@c @end itemize

Here is a list of the source files in the @file{as} directory.

@table @file
@item app.c
This contains the pre-processing phase, which deletes comments,
handles whitespace, etc.  This was recently re-written, since app
used to be a separate program, but RMS wanted it to be inline.

@item append.c
This is a subroutine to append a string to another string returning a
pointer just after the last @code{char} appended.  (JF:  All these
little routines should probably all be put in one file.)

@item as.c
Here you will find the main program of the assembler @code{as}.

@item expr.c
This is a branch office of @file{read.c}.  This understands
expressions, primaries.  Inside @code{as}, primaries are called
(expression) @i{operands}.  This is confusing, because we also talk
(elsewhere) about instruction @i{operands}.  Also, expression
operands are called @i{quantities} explicitly to avoid confusion
with instruction operands.  What a mess.

@item frags.c
This implements the @b{frag} concept.  Without frags, finding the
right size for branch instructions would be a lot harder.

@item hash.c
This contains the symbol table, opcode table @i{etc.} hashing
functions.

@item hex_value.c
This is a table of values of digits, for use in atoi() type
functions.  Could probably be flushed by using calls to strtol(), or
something similar.

@item input-file.c
This contains Operating system dependent source file reading
routines.  Since error messages often say where we are in reading
the source file, they live here too.  Since @code{as} is intended to
run under GNU and Unix only, this might be worth flushing.  Anyway,
almost all C compilers support stdio.

@item input-scrub.c
This deals with calling the pre-processor (if needed) and feeding the
chunks back to the rest of the assembler the right way.

@item messages.c
This contains operating system independent parts of fatal and
warning message reporting.  See @file{append.c} above.

@item output-file.c
This contains operating system dependent functions that write an
object file for @code{as}.  See @file{input-file.c} above.

@item read.c
This implements all the directives of @code{as}.  This also deals
with passing input lines to the machine dependent part of the
assembler.

@item strstr.c
This is a C library function that isn't in most C libraries yet.
See @file{append.c} above.

@item subsegs.c
This implements subsegments.

@item symbols.c
This implements symbols.

@item write.c
This contains the code to perform relaxation, and to write out
the object file.  It is mostly operating system independent, but
different OSes have different object file formats in any case.

@item xmalloc.c
This implements @code{malloc()} or bust.  See @file{append.c} above.

@item xrealloc.c
This implements @code{realloc()} or bust.  See @file{append.c} above.

@item atof-generic.c
The following files were taken from a machine-independent subroutine
library for manipulating floating point numbers and very large
integers.

@file{atof-generic.c} turns a string into a flonum internal format
floating-point number.

@item flonum-const.c
This contains some potentially useful floating point numbers in
flonum format.

@item flonum-copy.c
This copies a flonum.

@item flonum-multip.c
This multiplies two flonums together.

@item bignum-copy.c
This copies a bignum.

@end table

Here is a table of all the machine-specific files (this includes
both source and header files).  Typically, there is a
@var{machine}.c file, a @var{machine}-opcode.h file, and an
atof-@var{machine}.c file.  The @var{machine}-opcode.h file should
be identical to the one used by GDB (which uses it for disassembly.)

@table @file

@item atof-ieee.c
This contains code to turn a flonum into a ieee literal constant.
This is used by tye 680x0, 32x32, sparc, and i386 versions of @code{as}.

@item i386-opcode.h
This is the opcode-table for the i386 version of the assembler.

@item i386.c
This contains all the code for the i386 version of the assembler.

@item i386.h
This defines constants and macros used by the i386 version of the assembler.

@item m-generic.h
generic 68020 header file.  To be linked to m68k.h on a
non-sun3, non-hpux system.

@item m-sun2.h
68010 header file for Sun2 workstations.  Not well tested.  To be linked
to m68k.h on a sun2.  (See also @samp{-DSUN_ASM_SYNTAX} in the
@file{Makefile}.)

@item m-sun3.h
68020 header file for Sun3 workstations.  To be linked to m68k.h before
compiling on a Sun3 system.  (See also @samp{-DSUN_ASM_SYNTAX} in the
@file{Makefile}.)

@item m-hpux.h
68020 header file for a HPUX (system 5?) box.  Which box, which
version of HPUX, etc?  I don't know.

@item m68k.h
A hard- or symbolic- link to one of @file{m-generic.h},
@file{m-hpux.h} or @file{m-sun3.h} depending on which kind of
680x0 you are assembling for.   (See also @samp{-DSUN_ASM_SYNTAX} in the
@file{Makefile}.)

@item m68k-opcode.h
Opcode table for 68020.  This is now a link to the opcode table
in the @code{GDB} source directory.

@item m68k.c
All the mc680x0 code, in one huge, slow-to-compile file.

@item ns32k.c
This contains the code for the ns32032/ns32532 version of the
assembler.

@item ns32k-opcode.h
This contains the opcode table for the ns32032/ns32532 version
of the assembler.

@item vax-inst.h
Vax specific file for describing Vax operands and other Vax-ish things.

@item vax-opcode.h
Vax opcode table.

@item vax.c
Vax specific parts of @code{as}.  Also includes the former files
@file{vax-ins-parse.c}, @file{vax-reg-parse.c} and @file{vip-op.c}.

@item atof-vax.c
Turns a flonum into a Vax constant.

@item vms.c
This file contains the special code needed to put out a VMS
style object file for the Vax.

@end table

Here is a list of the header files in the source directory.
(Warning:  This section may not be very accurate.  I didn't
write the header files; I just report them.)  Also note that I
think many of these header files could be cleaned up or
eliminated.

@table @file

@item a.out.h
This describes the structures used to create the binary header data
inside the object file.  Perhaps we should use the one in
@file{/usr/include}?

@item as.h
This defines all the globally useful things, and pulls in <stdio.h>
and <assert.h>.

@item bignum.h
This defines macros useful for dealing with bignums.

@item expr.h
Structure and macros for dealing with expression()

@item flonum.h
This defines the structure for dealing with floating point
numbers.  It #includes @file{bignum.h}.

@item frags.h
This contains macro for appending a byte to the current frag.

@item hash.h
Structures and function definitions for the hashing functions.

@item input-file.h
Function headers for the input-file.c functions.

@item md.h
structures and function headers for things defined in the
machine dependent part of the assembler.

@item obstack.h
This is the GNU systemwide include file for manipulating obstacks.
Since nobody is running under real GNU yet, we include this file.

@item read.h
Macros and function headers for reading in source files.

@item struct-symbol.h
Structure definition and macros for dealing with the gas
internal form of a symbol.

@item subsegs.h
structure definition for dealing with the numbered subsegments
of the text and data segments.

@item symbols.h
Macros and function headers for dealing with symbols.

@item write.h
Structure for doing segment fixups.
@end table

@comment ~subsection Test Directory
@comment (Note:  The test directory seems to have disappeared somewhere
@comment along the line.  If you want it, you'll probably have to find a
@comment REALLY OLD dump tape~dots{})
@comment 
@comment The ~file{test/} directory is used for regression testing.
@comment After you modify ~code{as}, you can get a quick go/nogo
@comment confidence test by running the new ~code{as} over the source
@comment files in this directory.  You use a shell script ~file{test/do}.
@comment 
@comment The tests in this suite are evolving.  They are not comprehensive.
@comment They have, however, caught hundreds of bugs early in the debugging
@comment cycle of ~code{as}.  Most test statements in this suite were naturally
@comment selected: they were used to demonstrate actual ~code{as} bugs rather
@comment than being written ~i{a prioi}.
@comment 
@comment Another testing suggestion: over 30 bugs have been found simply by
@comment running examples from this manual through ~code{as}.
@comment Some examples in this manual are selected
@comment to distinguish boundary conditions; they are good for testing ~code{as}.
@comment 
@comment ~subsubsection Regression Testing
@comment Each regression test involves assembling a file and comparing the
@comment actual output of ~code{as} to ``known good'' output files.  Both
@comment the object file and the error/warning message file (stderr) are
@comment inspected.  Optionally ~code{as}' exit status may be checked.
@comment Discrepencies are reported.  Each discrepency means either that
@comment you broke some part of ~code{as} or that the ``known good'' files
@comment are now out of date and should be changed to reflect the new
@comment definition of ``good''.
@comment 
@comment Each regression test lives in its own directory, in a tree
@comment rooted in the directory ~file{test/}.  Each such directory
@comment has a name ending in ~file{.ret}, where `ret' stands for
@comment REgression Test.  The ~file{.ret} ending allows ~code{find
@comment (1)} to find all regression tests in the tree, without
@comment needing to list them explicitly.
@comment 
@comment Any ~file{.ret} directory must contain a file called
@comment ~file{input} which is the source file to assemble.  During
@comment testing an object file ~file{output} is created, as well as
@comment a file ~file{stdouterr} which contains the output to both
@comment stderr and stderr.  If there is a file ~file{output.good} in
@comment the directory, and if ~file{output} contains exactly the
@comment same data as ~file{output.good}, the file ~file{output} is
@comment deleted.  Likewise ~file{stdouterr} is removed if it exactly
@comment matches a file ~file{stdouterr.good}.  If file
@comment ~file{status.good} is present, containing a decimal number
@comment before a newline, the exit status of ~code{as} is compared
@comment to this number.  If the status numbers are not equal, a file
@comment ~file{status} is written to the directory, containing the
@comment actual status as a decimal number followed by newline.
@comment 
@comment Should any of the ~file{*.good} files fail to match their corresponding
@comment actual files, this is noted by a 1-line message on the screen during
@comment the regression test, and you can use ~code{find (1)} to find any
@comment files named ~file{status}, ~file {output} or ~file{stdouterr}.
@comment 
@node Retargeting, , Maintenance, top
@chapter Teaching the Assembler about a New Machine

This chapter describes the steps required in order to make the
assembler work with another machine's assembly language.  This
chapter is not complete, and only describes the steps in the
broadest terms.  You should look at the source for the
currently supported machine in order to discover some of the
details that aren't mentioned here.

You should create a new file called @file{@var{machine}.c}, and
add the appropriate lines to the file @file{Makefile} so that
you can compile your new version of the assembler.  This should
be straighforward; simply add lines similar to the ones there
for the four current versions of the assembler.

If you want to be compatable with GDB, (and the current
machine-dependent versions of the assembler), you should create
a file called @file{@var{machine}-opcode.h} which should
contain all the information about the names of the machine
instructions, their opcodes, and what addressing modes they
support.  If you do this right, the assembler and GDB can share
this file, and you'll only have to write it once.  Note that
while you're writing @code{as}, you may want to use an
independent program (if you have access to one), to make sure
that @code{as} is emitting the correct bytes.  Since @code{as}
and @code{GDB} share the opcode table, an incorrect opcode
table entry may make invalid bytes look OK when you disassemble
them with @code{GDB}.

@section Functions You will Have to Write

Your file @file{@var{machine}.c} should contain definitions for
the following functions and variables.  It will need to include
some header files in order to use some of the structures
defined in the machine-independent part of the assembler.  The
needed header files are mentioned in the descriptions of the
functions that will need them.

@table @code

@item long omagic;
This long integer holds the value to place at the beginning of
the @file{a.out} file.  It is usually @samp{OMAGIC}, except on
machines that store additional information in the magic-number.

@item char comment_chars[];
This character array holds the values of the characters that
start a comment anywhere in a line.  Comments are stripped off
automatically by the machine independent part of the
assembler.  Note that the @samp{/*} will always start a
comment, and that only @samp{*/} will end a comment started by
@samp{*/}.  

@item char line_comment_chars[];
This character array holds the values of the chars that start a
comment only if they are the first (non-whitespace) character
on a line.  If the character @samp{#} does not appear in this
list, you may get unexpected results.  (Various
machine-independent parts of the assembler treat the comments
@samp{#APP} and @samp{#NO_APP} specially, and assume that lines
that start with @samp{#} are comments.)

@item char EXP_CHARS[];
This character array holds the letters that can separate the
mantissa and the exponent of a floating point number.  Typical
values are @samp{e} and @samp{E}.

@item char FLT_CHARS[];
This character array holds the letters that--when they appear
immediately after a leading zero--indicate that a number is a
floating-point number.  (Sort of how 0x indicates that a
hexadecimal number follows.)

@item pseudo_typeS md_pseudo_table[];
(@var{pseudo_typeS} is defined in @file{md.h})
This array contains a list of the machine_dependent directives
the assembler must support.  It contains the name of each
pseudo op (Without the leading @samp{.}), a pointer to a
function to be called when that directive is encountered, and
an integer argument to be passed to that function.

@item void md_begin(void)
This function is called as part of the assembler's
initialization.  It should do any initialization required by
any of your other routines.

@item int md_parse_option(char **optionPTR, int *argcPTR, char ***argvPTR)
This routine is called once for each option on the command line
that the machine-independent part of @code{as} does not
understand.  This function should return non-zero if the option
pointed to by @var{optionPTR} is a valid option.  If it is not
a valid option, this routine should return zero.  The variables
@var{argcPTR} and @var{argvPTR} are provided in case the option
requires a filename or something similar as an argument.  If
the option is multi-character, @var{optionPTR} should be
advanced past the end of the option, otherwise every letter in
the option will be treated as a separate single-character
option.

@item void md_assemble(char *string)
This routine is called for every machine-dependent
non-directive line in the source file.  It does all the real
work involved in reading the opcode, parsing the operands,
etc.  @var{string} is a pointer to a null-terminated string,
that comprises the input line, with all excess whitespace and
comments removed.

@item void md_number_to_chars(char *outputPTR,long value,int nbytes)
This routine is called to turn a C long int, short int, or char
into the series of bytes that represents that number on the
target machine.  @var{outputPTR} points to an array where the
result should be stored; @var{value} is the value to store; and
@var{nbytes} is the number of bytes in 'value' that should be
stored.

@item void md_number_to_imm(char *outputPTR,long value,int nbytes)
This routine is called to turn a C long int, short int, or char
into the series of bytes that represent an immediate value on
the target machine.  It is identical to the function @code{md_number_to_chars},
except on NS32K machines.@refill

@item void md_number_to_disp(char *outputPTR,long value,int nbytes)
This routine is called to turn a C long int, short int, or char
into the series of bytes that represent an displacement value on
the target machine.  It is identical to the function @code{md_number_to_chars},
except on NS32K machines.@refill

@item void md_number_to_field(char *outputPTR,long value,int nbytes)
This routine is identical to @code{md_number_to_chars},
except on NS32K machines.

@item void md_ri_to_chars(struct relocation_info *riPTR,ri)
(@code{struct relocation_info} is defined in @file{a.out.h})
This routine emits the relocation info in @var{ri}
in the appropriate bit-pattern for the target machine.
The result should be stored in the location pointed
to by @var{riPTR}.  This routine may be a no-op unless you are
attempting to do cross-assembly.

@item char *md_atof(char type,char *outputPTR,int *sizePTR)
This routine turns a series of digits into the appropriate
internal representation for a floating-point number.
@var{type} is a character from @var{FLT_CHARS[]} that describes
what kind of floating point number is wanted; @var{outputPTR}
is a pointer to an array that the result should be stored in;
and @var{sizePTR} is a pointer to an integer where the size (in
bytes) of the result should be stored.  This routine should
return an error message, or an empty string (not (char *)0) for
success.

@item int md_short_jump_size;
This variable holds the (maximum) size in bytes of a short (16
bit or so) jump created by @code{md_create_short_jump()}.  This
variable is used as part of the broken-word feature, and isn't
needed if the assembler is compiled with
@samp{-DWORKING_DOT_WORD}.

@item int md_long_jump_size;
This variable holds the (maximum) size in bytes of a long (32
bit or so) jump created by @code{md_create_long_jump()}.  This
variable is used as part of the broken-word feature, and isn't
needed if the assembler is compiled with
@samp{-DWORKING_DOT_WORD}.

@item void md_create_short_jump(char *resultPTR,long from_addr,
@code{long to_addr,fragS *frag,symbolS *to_symbol)}
This function emits a jump from @var{from_addr} to @var{to_addr} in
the array of bytes pointed to by @var{resultPTR}.  If this creates a
type of jump that must be relocated, this function should call
@code{fix_new()} with @var{frag} and @var{to_symbol}.  The jump
emitted by this function may be smaller than @var{md_short_jump_size},
but it must never create a larger one.
(If it creates a smaller jump, the extra bytes of memory will not be
used.)  This function is used as part of the broken-word feature,
and isn't needed if the assembler is compiled with
@samp{-DWORKING_DOT_WORD}.@refill

@item void md_create_long_jump(char *ptr,long from_addr,
@code{long to_addr,fragS *frag,symbolS *to_symbol)}
This function is similar to the previous function,
@code{md_create_short_jump()}, except that it creates a long
jump instead of a short one.  This function is used as part of
the broken-word feature, and isn't needed if the assembler is
compiled with @samp{-DWORKING_DOT_WORD}.

@item int md_estimate_size_before_relax(fragS *fragPTR,int segment_type)
This function does the initial setting up for relaxation.  This
includes forcing references to still-undefined symbols to the
appropriate addressing modes.

@item relax_typeS md_relax_table[];
(relax_typeS is defined in md.h)
This array describes the various machine dependent states a
frag may be in before relaxation.  You will need one group of
entries for each type of addressing mode you intend to relax.

@item void md_convert_frag(fragS *fragPTR)
(@var{fragS} is defined in @file{as.h})
This routine does the required cleanup after relaxation.
Relaxation has changed the type of the frag to a type that can
reach its destination.  This function should adjust the opcode
of the frag to use the appropriate addressing mode.
@var{fragPTR} points to the frag to clean up.

@item void md_end(void)
This function is called just before the assembler exits.  It
need not free up memory unless the operating system doesn't do
it automatically on exit.  (In which case you'll also have to
track down all the other places where the assembler allocates
space but never frees it.)

@end table

@section External Variables You will Need to Use

You will need to refer to or change the following external variables
from within the machine-dependent part of the assembler.

@table @code
@item extern char flagseen[];
This array holds non-zero values in locations corresponding to
the options that were on the command line.  Thus, if the
assembler was called with @samp{-W}, @var{flagseen['W']} would
be non-zero.

@item extern fragS *frag_now;
This pointer points to the current frag--the frag that bytes
are currently being added to.  If nothing else, you will need
to pass it as an argument to various machine-independent
functions.  It is maintained automatically by the
frag-manipulating functions; you should never have to change it
yourself.

@item extern LITTLENUM_TYPE generic_bignum[];
(@var{LITTLENUM_TYPE} is defined in @file{bignum.h}.
This is where @dfn{bignums}--numbers larger than 32 bits--are
returned when they are encountered in an expression. You will
need to use this if you need to implement directives (or
anything else) that must deal with these large numbers.
@code{Bignums} are of @code{segT} @code{SEG_BIG} (defined in
@file{as.h}, and have a positive @code{X_add_number}.  The
@code{X_add_number} of a @code{bignum} is the number of
@code{LITTLENUMS} in @var{generic_bignum} that the number takes
up.

@item extern FLONUM_TYPE generic_floating_point_number;
(@var{FLONUM_TYPE} is defined in @file{flonum.h}.
The is where @dfn{flonums}--floating-point numbers within
expressions--are returned.  @code{Flonums} are of @code{segT}
@code{SEG_BIG}, and have a negative @code{X_add_number}.
@code{Flonums} are returned in a generic format.  You will have
to write a routine to turn this generic format into the
appropriate floating-point format for your machine.

@item extern int need_pass_2;
If this variable is non-zero, the assembler has encountered an
expression that cannot be assembled in a single pass.  Since
the second pass isn't implemented, this flag means that the
assembler is punting, and is only looking for additional syntax
errors.  (Or something like that.)

@item extern segT now_seg;
This variable holds the value of the segment the assembler is
currently assembling into.

@end table

@section External functions will you need

You will find the following external functions useful (or
indispensable) when you're writing the machine-dependent part
of the assembler.

@table @code

@item char *frag_more(int bytes)
This function allocates @var{bytes} more bytes in the current
frag (or starts a new frag, if it can't expand the current frag
any more.)  for you to store some object-file bytes in.  It
returns a pointer to the bytes, ready for you to store data in.

@item void fix_new(fragS *frag, int where, short size, symbolS *add_symbol, symbolS *sub_symbol, long offset, int pcrel)
This function stores a relocation fixup to be acted on later.
@var{frag} points to the frag the relocation belongs in;
@var{where} is the location within the frag where the relocation begins;
@var{size} is the size of the relocation, and is usually 1 (a single byte),
  2 (sixteen bits), or 4 (a longword).
The value @var{add_symbol} @minus{} @var{sub_symbol} + @var{offset}, is added to the byte(s)
at @var{frag->literal[where]}.  If @var{pcrel} is non-zero, the address of the
location is subtracted from the result.  A relocation entry is also added
to the @file{a.out} file.  @var{add_symbol}, @var{sub_symbol}, and/or
@var{offset} may be NULL.@refill

@item char *frag_var(relax_stateT type, int max_chars, int var,
@code{relax_substateT subtype, symbolS *symbol, char *opcode)}
This function creates a machine-dependent frag of type @var{type}
(usually @code{rs_machine_dependent}).
@var{max_chars} is the maximum size in bytes that the frag may grow by;
@var{var} is the current size of the variable end of the frag;
@var{subtype} is the sub-type of the frag.  The sub-type is used to index into
@var{md_relax_table[]} during @code{relaxation}.
@var{symbol} is the symbol whose value should be used to when relax-ing this frag.
@var{opcode} points into a byte whose value may have to be modified if the
addressing mode used by this frag changes.  It typically points into the
@var{fr_literal[]} of the previous frag, and is used to point to a location
that @code{md_convert_frag()}, may have to change.@refill

@item void frag_wane(fragS *fragPTR)
This function is useful from within @code{md_convert_frag}.  It
changes a frag to type rs_fill, and sets the variable-sized
piece of the frag to zero.  The frag will never change in size
again.

@item segT expression(expressionS *retval)
(@var{segT} is defined in @file{as.h}; @var{expressionS} is defined in @file{expr.h})
This function parses the string pointed to by the external char
pointer @var{input_line_pointer}, and returns the segment-type
of the expression.  It also stores the results in the
@var{expressionS} pointed to by @var{retval}.
@var{input_line_pointer} is advanced to point past the end of
the expression.  (@var{input_line_pointer} is used by other
parts of the assembler.  If you modify it, be sure to restore
it to its original value.)

@item as_warn(char *message,@dots{})
If warning messages are disabled, this function does nothing.
Otherwise, it prints out the current file name, and the current
line number, then uses @code{fprintf} to print the
@var{message} and any arguments it was passed.

@item as_bad(char *message,@dots{})
This function should be called when @code{as} encounters
conditions that are bad enough that @code{as} should not
produce an object file, but should continue reading input and
printing warning and bad error messages.

@item as_fatal(char *message,@dots{})
This function prints out the current file name and line number,
prints the word @samp{FATAL:}, then uses @code{fprintf} to
print the @var{message} and any arguments it was passed.  Then
the assembler exits.  This function should only be used for
serious, unrecoverable errors.

@item void float_const(int float_type)
This function reads floating-point constants from the current
input line, and calls @code{md_atof} to assemble them.  It is
useful as the function to call for the directives
@samp{.single}, @samp{.double}, @samp{.float}, etc.
@var{float_type} must be a character from @var{FLT_CHARS}.

@item void demand_empty_rest_of_line(void);
This function can be used by machine-dependent directives to
make sure the rest of the input line is empty.  It prints a
warning message if there are additional characters on the line.

@item long int get_absolute_expression(void)
This function can be used by machine-dependent directives to
read an absolute number from the current input line.  It
returns the result.  If it isn't given an absolute expression,
it prints a warning message and returns zero.

@end table


@section The concept of Frags

This assembler works to optimize the size of certain addressing
modes.  (e.g. branch instructions) This means the size of many
pieces of object code cannot be determined until after assembly
is finished.  (This means that the addresses of symbols cannot be
determined until assembly is finished.)  In order to do this,
@code{as} stores the output bytes as @dfn{frags}.

Here is the definition of a frag (from @file{as.h})
@example
struct frag
@{
        long int fr_fix;
        long int fr_var;
        relax_stateT fr_type;
        relax_substateT fr_substate;
        unsigned long fr_address;
        long int fr_offset;
        struct symbol *fr_symbol;
        char *fr_opcode;
        struct frag *fr_next;
        char fr_literal[];
@}
@end example

@table @var
@item fr_fix
is the size of the fixed-size piece of the frag.

@item fr_var
is the maximum (?) size of the variable-sized piece of the frag.

@item fr_type
is the type of the frag.
Current types are:
rs_fill
rs_align
rs_org
rs_machine_dependent

@item fr_substate
This stores the type of machine-dependent frag this is.  (what
kind of addressing mode is being used, and what size is being
tried/will fit/etc.

@item fr_address
@var{fr_address} is only valid after relaxation is finished.
Before relaxation, the only way to store an address is (pointer
to frag containing the address) plus (offset into the frag).

@item fr_offset
This contains a number, whose meaning depends on the type of
the frag.
for machine_dependent frags, this contains the offset from
fr_symbol that the frag wants to go to.  Thus, for branch
instructions it is usually zero.  (unless the instruction was
@samp{jba foo+12}  or something like that.)

@item fr_symbol
for machine_dependent frags, this points to the symbol the frag
needs to reach.

@item fr_opcode
This points to the location in the frag (or in a previous frag)
of the opcode for the instruction that caused this to be a frag.
@var{fr_opcode} is needed if the actual opcode must be changed
in order to use a different form of the addressing mode.
(For example, if a conditional branch only comes in size tiny,
a large-size branch could be implemented by reversing the sense
of the test, and turning it into a tiny branch over a large jump.
This would require changing the opcode.)

@var{fr_literal} is a variable-size array that contains the
actual object bytes.  A frag consists of a fixed size piece of
object data, (which may be zero bytes long), followed by a
piece of object data whose size may not have been determined
yet.  Other information includes the type of the frag (which
controls how it is relaxed), 

@item fr_next
This is the next frag in the singly-linked list.  This is
usually only needed by the machine-independent part of
@code{as}.

@end table

@c Is this really a good idea?
@iftex
@center [end of manual]
@end iftex
@summarycontents
@contents
@bye