[cc65] / doc / ld65.sgml

<!doctype linuxdoc system>

<article>
<title>ld65 Users Guide
<author>Ullrich von Bassewitz, <htmlurl url="mailto:uz@cc65.org" name="uz@cc65.org">
<date>02.12.2000, 02.10.2001

<abstract>
The ld65 linker combines object files into an executable file. ld65 is highly
configurable and uses configuration files for high flexibility.
</abstract>

<!-- Table of contents -->
<toc>

<!-- Begin the document -->

<sect>Overview<p>

The ld65 linker combines several object modules created by the ca65
assembler, producing an executable file. The object modules may be read
from a library created by the ar65 archiver (this is somewhat faster and
more convenient). The linker was designed to be as flexible as possible.
It complements the features that are built into the ca65 macroassembler:

<itemize>

<item>  Accept any number of segments to form an executable module.

<item>  Resolve arbitrary expressions stored in the object files.

<item>  In case of errors, use the meta information stored in the object files
        to produce helpful error messages. In case of undefined symbols,
        expression range errors, or symbol type mismatches, ld65 is able to
        tell you the exact location in the original assembler source, where
        the symbol was referenced.

<item>  Flexible output. The output of ld65 is highly configurable by a config
        file. More common platforms are supported by builtin configurations
        that may be activated by naming the target system. The output
        generation was designed with different output formats in mind, so
        adding other formats shouldn't be a great problem.

</itemize>


<sect>Usage<p>


<sect1>Command line option overview<p>

The linker is called as follows:

<tscreen><verb>
---------------------------------------------------------------------------
Usage: ld65 [options] module ...
Short options:
  -(                    Start a library group
  -)                    End a library group
  -C name               Use linker config file
  -D sym=val            Define a symbol
  -L path               Specify a library search path
  -Ln name              Create a VICE label file
  -S addr               Set the default start address
  -V                    Print the linker version
  -h                    Help (this text)
  -m name               Create a map file
  -o name               Name the default output file
  -t sys                Set the target system
  -u sym                Force an import of symbol `sym'
  -v                    Verbose mode
  -vm                   Verbose map file

Long options:
  --cfg-path path       Specify a config file search path
  --config name         Use linker config file
  --dbgfile name        Generate debug information
  --define sym=val      Define a symbol
  --dump-config name    Dump a builtin configuration
  --end-group           End a library group
  --force-import sym    Force an import of symbol `sym'
  --help                Help (this text)
  --lib file            Link this library
  --lib-path path       Specify a library search path
  --mapfile name        Create a map file
  --module-id id        Specify a module id
  --obj file            Link this object file
  --obj-path path       Specify an object file search path
  --start-addr addr     Set the default start address
  --start-group         Start a library group
  --target sys          Set the target system
  --version             Print the linker version
---------------------------------------------------------------------------
</verb></tscreen>


<sect1>Command line options in detail<p>

Here is a description of all the command line options:

<descrip>

  <label id="option--start-group">
  <tag><tt>-(, --start-group</tt></tag>

  Start a library group. The libraries specified within a group are searched
  multiple times to resolve crossreferences within the libraries. Normally,
  crossreferences are only resolved within a library, that is the library is
  searched multiple times. Libraries specified later on the command line
  cannot reference otherwise unreferenced symbols in libraries specified
  earlier, because the linker has already handled them. Library groups are
  a solution for this problem, because the linker will search repeatedly
  through all libraries specified in the group, until all possible open
  symbol references have been satisfied.


  <tag><tt>-), --end-group</tt></tag>

  End a library group. See the explanation of the <tt><ref
  id="option--start-group" name="--start-group"></tt> option.


  <tag><tt>-h, --help</tt></tag>

  Print the short option summary shown above.


  <label id="option-m">
  <tag><tt>-m name, --mapfile name</tt></tag>

  This option (which needs an argument that will used as a filename for
  the generated map file) will cause the linker to generate a map file.
  The map file does contain a detailed overview over the modules used, the
  sizes for the different segments, and a table containing exported
  symbols.


  <label id="option-o">
  <tag><tt>-o name</tt></tag>

  The -o switch is used to give the name of the default output file.
  Depending on your output configuration, this name may NOT be used as
  name for the output file. However, for the builtin configurations, this
  name is used for the output file name.


  <label id="option-t">
  <tag><tt>-t sys, --target sys</tt></tag>

  The argument for the -t switch is the name of the target system. Since this
  switch will activate a builtin configuration, it may not be used together
  with the <tt><ref id="option-C" name="-C"></tt> option. The following target
  systems are currently supported:

  <itemize>
  <item>none
  <item>apple2
  <item>apple2enh
  <item>atari
  <item>atmos
  <item>c16 (works also for the c116 with memory up to 32K)
  <item>c64
  <item>c128
  <item>plus4
  <item>cbm510 (CBM-II series with 40 column video)
  <item>cbm610 (all CBM series-II computers with 80 column video)
  <item>pet (all CBM PET systems except the 2001)
  <item>geos
  <item>lunix
  <item>atmos
  <item>nes
  <item>supervision
  </itemize>

  There are a few more targets defined but neither of them is actually
  supported.


  <tag><tt>-u sym[:addrsize], --force-import sym[:addrsize]</tt></tag>

  Force an import of a symbol. While object files are always linked to the
  output file, regardless if there are any references, object modules from
  libraries get only linked in if an import can be satisfied by this module.
  The <tt/--fore-import/ option may be used to add a reference to a symbol and
  as a result force linkage of the module that exports the identifier.

  The name of the symbol may optionally be followed by a colon and an address
  size specifier. If no address size is specified, the default address size
  for the target machine is used.

  Please note that the symbol name needs to have the internal representation,
  meaning you have to prepend an underline for C identifiers.


  <label id="option-v">
  <tag><tt>-v, --verbose</tt></tag>

  Using the -v option, you may enable more output that may help you to
  locate problems. If an undefined symbol is encountered, -v causes the
  linker to print a detailed list of the references (that is, source file
  and line) for this symbol.


  <tag><tt>-vm</tt></tag>

  Must be used in conjunction with <tt><ref id="option-m" name="-m"></tt>
  (generate map file). Normally the map file will not include empty segments
  and sections, or unreferenced symbols. Using this option, you can force the
  linker to include all this information into the map file.


  <label id="option-C">
  <tag><tt>-C</tt></tag>

  This gives the name of an output config file to use. See section 4 for more
  information about config files. -C may not be used together with <tt><ref
  id="option-t" name="-t"></tt>.


  <label id="option-D">
  <tag><tt>-D sym=value, --define sym=value</tt></tag>

  This option allows to define an external symbol on the command line. Value
  may start with a '&dollar;' sign or with <tt/0x/ for hexadecimal values,
  otherwise a leading zero denotes octal values. See also the <ref
  id="SYMBOLS" name="SYMBOLS section"> in the configuration file.


  <label id="option--lib-path">
  <tag><tt>-L path, --lib-path path</tt></tag>

  Specify a library search path. This option may be used more than once. It
  adds a directory to the search path for library files. Libraries specified
  without a path are searched in current directory, in the directory given in
  the <tt/LD65_LIB/ environment variable, and in the list of directories
  specified using <tt/--lib-path/.


  <tag><tt>-Ln</tt></tag>

  This option allows you to create a file that contains all global labels and
  may be loaded into VICE emulator using the <tt/ll/ (load label) command. You
  may use this to debug your code with VICE. Note: Older versions had some
  bugs in the label code. If you have problems, please get the latest VICE
  version.


  <label id="option-S">
  <tag><tt>-S addr, --start-addr addr</tt></tag>

  Using -S you may define the default starting address. If and how this
  address is used depends on the config file in use. For the builtin
  configurations, only the "none", "apple2" and "apple2enh" systems honor an
  explicit start address, all other builtin config provide their own.


  <tag><tt>-V, --version</tt></tag>

  This option print the version number of the linker. If you send any
  suggestions or bugfixes, please include this number.


  <label id="option--cfg-path">
  <tag><tt>--cfg-path path</tt></tag>

  Specify a config file search path. This option may be used more than once.
  It adds a directory to the search path for config files. A config file given
  with the <tt><ref id="option-C" name="-C"></tt> option that has no path in
  its name is searched in the current directory, in the directory given in the
  <tt/LD65_CFG/ environment variable, and in the list of directories specified
  using <tt/--cfg-path/.


  <label id="option--dbgfile">
  <tag><tt>--dbgfile name</tt></tag>

  Specify an output file for debug information. Available information will be
  written to this file. Using the <tt/-g/ option for the compiler and assembler
  will increase the amount of information available. Please note that debug
  information generation is currently being developed, so the format of the
  file and it's contents are subject to change without further notice.


  <tag><tt>--lib file</tt></tag>

  Links a library to the output. Use this command line option instead of just
  naming the library file, if the linker is not able to determine the file
  type because of an unusual extension.


  <tag><tt>--obj file</tt></tag>

  Links an object file to the output. Use this command line option instead
  of just naming the object file, if the linker is not able to determine the
  file type because of an unusual extension.


  <label id="option--obj-path">
  <tag><tt>--obj-path path</tt></tag>

  Specify an object file search path. This option may be used more than once.
  It adds a directory to the search path for object files. An object file
  passed to the linker that has no path in its name is searched in current
  directory, in the directory given in the <tt/LD65_OBJ/ environment variable,
  and in the list of directories specified using <tt/--obj-path/.

</descrip>



<sect>Search paths<p>

Starting with version 2.10 there are now several search paths for files needed
by the linker: One for libraries, one for object files and one for config
files.


<sect1>Library search path<p>

The library search path contains in this order:

<enum>
<item>The current directory.
<item>A compiled in library path which is often <tt>/usr/lib/cc65/lib</tt> on
      Linux systems.
<item>The value of the environment variable <tt/LD65_LIB/ if it is defined.
<item>The value of the environment variable <tt/CC65_LIB/ if it is defined.
      Please note that use of this environment variable is obsolete and may
      get removed in future versions.
<item>Any directory added with the <tt><ref id="option--lib-path"
      name="--lib-path"></tt> option on the command line.
</enum>


<sect1>Object file search path<p>

The object file search path contains in this order:

<enum>
<item>The current directory.
<item>A compiled in directory which is often <tt>/usr/lib/cc65/lib</tt> on
      Linux systems.
<item>The value of the environment variable <tt/LD65_OBJ/ if it is defined.
<item>The value of the environment variable <tt/CC65_LIB/ if it is defined.
      Please note that use of this environment variable is obsolete and may
      get removed in future versions.
<item>Any directory added with the <tt><ref id="option--obj-path"
      name="--obj-path"></tt> option on the command line.
</enum>


<sect1>Config file search path<p>

The config file search path contains in this order:

<enum>
<item>The current directory.
<item>A compiled in directory which is often <tt>/usr/lib/cc65/cfg</tt> on
      Linux systems.
<item>The value of the environment variable <tt/LD65_CFG/ if it is defined.
<item>The value of the environment variable <tt/CC65_LIB/ if it is defined.
      Please note that use of this environment variable is obsolete and may
      get removed in future versions.
<item>Any directory added with the <tt><ref id="option--cfg-path"
      name="--cfg-path"></tt> option on the command line.
</enum>



<sect>Detailed workings<p>

The linker does several things when combining object modules:

First, the command line is parsed from left to right. For each object file
encountered (object files are recognized by a magic word in the header, so
the linker does not care about the name), imported and exported
identifiers are read from the file and inserted in a table. If a library
name is given (libraries are also recognized by a magic word, there are no
special naming conventions), all modules in the library are checked if an
export from this module would satisfy an import from other modules. All
modules where this is the case are marked. If duplicate identifiers are
found, the linker issues a warning.

This procedure (parsing and reading from left to right) does mean, that a
library may only satisfy references for object modules (given directly or from
a library) named <em/before/ that library. With the command line

<tscreen><verb>
        ld65 crt0.o clib.lib test.o
</verb></tscreen>

the module test.o may not contain references to modules in the library
clib.lib. If this is the case, you have to change the order of the modules
on the command line:

<tscreen><verb>
        ld65 crt0.o test.o clib.lib
</verb></tscreen>

Step two is, to read the configuration file, and assign start addresses
for the segments and define any linker symbols (see <ref id="config-files"
name="Configuration files">).

After that, the linker is ready to produce an output file. Before doing that,
it checks it's data for consistency. That is, it checks for unresolved
externals (if the output format is not relocatable) and for symbol type
mismatches (for example a zero page symbol is imported by a module as absolute
symbol).

Step four is, to write the actual target files. In this step, the linker will
resolve any expressions contained in the segment data. Circular references are
also detected in this step (a symbol may have a circular reference that goes
unnoticed if the symbol is not used).

Step five is to output a map file with a detailed list of all modules,
segments and symbols encountered.

And, last step, if you give the <tt><ref id="option-v" name="-v"></tt> switch
twice, you get a dump of the segment data. However, this may be quite
unreadable if you're not a developer:-)



<sect>Configuration files<label id="config-files"><p>

Configuration files are used to describe the layout of the output file(s). Two
major topics are covered in a config file: The memory layout of the target
architecture, and the assignment of segments to memory areas. In addition,
several other attributes may be specified.

Case is ignored for keywords, that is, section or attribute names, but it is
<em/not/ ignored for names and strings.



<sect1>Memory areas<p>

Memory areas are specified in a <tt/MEMORY/ section. Lets have a look at an
example (this one describes the usable memory layout of the C64):

<tscreen><verb>
        MEMORY {
            RAM1:  start = $0800, size = $9800;
            ROM1:  start = $A000, size = $2000;
            RAM2:  start = $C000, size = $1000;
            ROM2:  start = $E000, size = $2000;
        }
</verb></tscreen>

As you can see, there are two ram areas and two rom areas. The names
(before the colon) are arbitrary names that must start with a letter, with
the remaining characters being letters or digits. The names of the memory
areas are used when assigning segments. As mentioned above, case is
significant for these names.

The syntax above is used in all sections of the config file. The name
(<tt/ROM1/ etc.) is said to be an identifier, the remaining tokens up to the
semicolon specify attributes for this identifier. You may use the equal sign
to assign values to attributes, and you may use a comma to separate
attributes, you may also leave both out. But you <em/must/ use a semicolon to
mark the end of the attributes for one identifier. The section above may also
have looked like this:

<tscreen><verb>
        # Start of memory section
        MEMORY
        {
            RAM1:
                start $0800
                size $9800;
            ROM1:
                start $A000
                size $2000;
            RAM2:
                start $C000
                size $1000;
            ROM2:
                start $E000
                size $2000;
        }
</verb></tscreen>

There are of course more attributes for a memory section than just start and
size. Start and size are mandatory attributes, that means, each memory area
defined <em/must/ have these attributes given (the linker will check that). I
will cover other attributes later. As you may have noticed, I've used a
comment in the example above. Comments start with a hash mark (`#'), the
remainder of the line is ignored if this character is found.


<sect1>Segments<p>

Let's assume you have written a program for your trusty old C64, and you would
like to run it. For testing purposes, it should run in the <tt/RAM/ area. So
we will start to assign segments to memory sections in the <tt/SEGMENTS/
section:

<tscreen><verb>
        SEGMENTS {
            CODE:   load = RAM1, type = ro;
            RODATA: load = RAM1, type = ro;
            DATA:   load = RAM1, type = rw;
            BSS:    load = RAM1, type = bss, define = yes;
        }
</verb></tscreen>

What we are doing here is telling the linker, that all segments go into the
<tt/RAM1/ memory area in the order specified in the <tt/SEGMENTS/ section. So
the linker will first write the <tt/CODE/ segment, then the <tt/RODATA/
segment, then the <tt/DATA/ segment - but it will not write the <tt/BSS/
segment. Why? Enter the segment type: For each segment specified, you may also
specify a segment attribute. There are four possible segment attributes:

<tscreen><verb>
        ro      means readonly
        rw      means read/write
        bss     means that this is an uninitialized segment
        zp      a zeropage segment
</verb></tscreen>

So, because we specified that the segment with the name BSS is of type bss,
the linker knows that this is uninitialized data, and will not write it to an
output file. This is an important point: For the assembler, the <tt/BSS/
segment has no special meaning. You specify, which segments have the bss
attribute when linking. This approach is much more flexible than having one
fixed bss segment, and is a result of the design decision to supporting an
arbitrary segment count.

If you specify "<tt/type = bss/" for a segment, the linker will make sure that
this segment does only contain uninitialized data (that is, zeroes), and issue
a warning if this is not the case.

For a <tt/bss/ type segment to be useful, it must be cleared somehow by your
program (this happens usually in the startup code - for example the startup
code for cc65 generated programs takes care about that). But how does your
code know, where the segment starts, and how big it is? The linker is able to
give that information, but you must request it. This is, what we're doing with
the "<tt/define = yes/" attribute in the <tt/BSS/ definitions. For each
segment, where this attribute is true, the linker will export three symbols.

<tscreen><verb>
        __NAME_LOAD__   This is set to the address where the
                        segment is loaded.
        __NAME_RUN__    This is set to the run address of the
                        segment. We will cover run addresses
                        later.
        __NAME_SIZE__   This is set to the segment size.
</verb></tscreen>

Replace <tt/NAME/ by the name of the segment, in the example above, this would
be <tt/BSS/. These symbols may be accessed by your code.

Now, as we've configured the linker to write the first three segments and
create symbols for the last one, there's only one question left: Where does
the linker put the data? It would be very convenient to have the data in a
file, wouldn't it?

<sect1>Output files<p>

We don't have any files specified above, and indeed, this is not needed in a
simple configuration like the one above. There is an additional attribute
"file" that may be specified for a memory area, that gives a file name to
write the area data into. If there is no file name given, the linker will
assign the default file name. This is "a.out" or the one given with the
<tt><ref id="option-o" name="-o"></tt> option on the command line. Since the
default behaviour is ok for our purposes, I did not use the attribute in the
example above. Let's have a look at it now.

The "file" attribute (the keyword may also be written as "FILE" if you like
that better) takes a string enclosed in double quotes (`"') that specifies the
file, where the data is written. You may specify the same file several times,
in that case the data for all memory areas having this file name is written
into this file, in the order of the memory areas defined in the <tt/MEMORY/
section. Let's specify some file names in the <tt/MEMORY/ section used above:

<tscreen><verb>
        MEMORY {
            RAM1:  start = $0800, size = $9800, file = %O;
            ROM1:  start = $A000, size = $2000, file = "rom1.bin";
            RAM2:  start = $C000, size = $1000, file = %O;
            ROM2:  start = $E000, size = $2000, file = "rom2.bin";
        }
</verb></tscreen>

The <tt/%O/ used here is a way to specify the default behaviour explicitly:
<tt/%O/ is replaced by a string (including the quotes) that contains the
default output name, that is, "a.out" or the name specified with the <tt><ref
id="option-o" name="-o"></tt> option on the command line. Into this file, the
linker will first write any segments that go into <tt/RAM1/, and will append
then the segments for <tt/RAM2/, because the memory areas are given in this
order. So, for the RAM areas, nothing has really changed.

We've not used the ROM areas, but we will do that below, so we give the file
names here. Segments that go into <tt/ROM1/ will be written to a file named
"rom1.bin", and segments that go into <tt/ROM2/ will be written to a file
named "rom2.bin". The name given on the command line is ignored in both cases.

Assigning an empty file name for a memory area will discard the data written
to it. This is useful, if the a memory area has segments assigned that are
empty (for example because they are of type bss). In that case, the linker
will create an empty output file. This may be suppressed by assigning an empty
file name to that memory area.


<sect1>LOAD and RUN addresses (ROMable code)<p>

Let us look now at a more complex example. Say, you've successfully tested
your new "Super Operating System" (SOS for short) for the C64, and you
will now go and replace the ROMs by your own code. When doing that, you
face a new problem: If the code runs in RAM, we need not to care about
read/write data. But now, if the code is in ROM, we must care about it.
Remember the default segments (you may of course specify your own):

<tscreen><verb>
        CODE            read only code
        RODATA          read only data
        DATA            read/write data
        BSS             uninitialized data, read/write
</verb></tscreen>

Since <tt/BSS/ is not initialized, we must not care about it now, but what
about <tt/DATA/? <tt/DATA/ contains initialized data, that is, data that was
explicitly assigned a value. And your program will rely on these values on
startup. Since there's no other way to remember the contents of the data
segment, than storing it into one of the ROMs, we have to put it there. But
unfortunately, ROM is not writable, so we have to copy it into RAM before
running the actual code.

The linker cannot help you copying the data from ROM into RAM (this must be
done by the startup code of your program), but it has some features that will
help you in this process.

First, you may not only specify a "<tt/load/" attribute for a segment, but
also a "<tt/run/" attribute. The "<tt/load/" attribute is mandatory, and, if
you don't specify a "<tt/run/" attribute, the linker assumes that load area
and run area are the same. We will use this feature for our data area:

<tscreen><verb>
        SEGMENTS {
            CODE:   load = ROM1, type = ro;
            RODATA: load = ROM2, type = ro;
            DATA:   load = ROM2, run = RAM2, type = rw, define = yes;
            BSS:    load = RAM2, type = bss, define = yes;
        }
</verb></tscreen>

Let's have a closer look at this <tt/SEGMENTS/ section. We specify that the
<tt/CODE/ segment goes into <tt/ROM1/ (the one at $A000). The readonly data
goes into <tt/ROM2/. Read/write data will be loaded into <tt/ROM2/ but is run
in <tt/RAM2/. That means that all references to labels in the <tt/DATA/
segment are relocated to be in <tt/RAM2/, but the segment is written to
<tt/ROM2/. All your startup code has to do is, to copy the data from it's
location in <tt/ROM2/ to the final location in <tt/RAM2/.

So, how do you know, where the data is located? This is the second point,
where you get help from the linker. Remember the "<tt/define/" attribute?
Since we have set this attribute to true, the linker will define three
external symbols for the data segment that may be accessed from your code:

<tscreen><verb>
        __DATA_LOAD__   This is set to the address where the segment
                        is loaded, in this case, it is an address in
                        ROM2.
        __DATA_RUN__    This is set to the run address of the segment,
                        in this case, it is an address in RAM2.
        __DATA_SIZE__   This is set to the segment size.
</verb></tscreen>

So, what your startup code must do, is to copy <tt/__DATA_SIZE__/ bytes from
<tt/__DATA_LOAD__/ to <tt/__DATA_RUN__/ before any other routines are called.
All references to labels in the <tt/DATA/ segment are relocated to <tt/RAM2/
by the linker, so things will work properly.


<sect1>Other MEMORY area attributes<p>

There are some other attributes not covered above. Before starting the
reference section, I will discuss the remaining things here.

You may request symbols definitions also for memory areas. This may be
useful for things like a software stack, or an i/o area.

<tscreen><verb>
        MEMORY {
            STACK:  start = $C000, size = $1000, define = yes;
        }
</verb></tscreen>

This will define three external symbols that may be used in your code:

<tscreen><verb>
        __STACK_START__         This is set to the start of the memory
                                area, $C000 in this example.
        __STACK_SIZE__          The size of the area, here $1000.
        __STACK_LAST__          This is NOT the same as START+SIZE.
                                Instead, it it defined as the first
                                address that is not used by data. If we
                                don't define any segments for this area,
                                the value will be the same as START.
</verb></tscreen>

A memory section may also have a type. Valid types are

<tscreen><verb>
        ro      for readonly memory
        rw      for read/write memory.
</verb></tscreen>

The linker will assure, that no segment marked as read/write or bss is put
into a memory area that is marked as readonly.

Unused memory in a memory area may be filled. Use the "<tt/fill = yes/"
attribute to request this. The default value to fill unused space is zero. If
you don't like this, you may specify a byte value that is used to fill these
areas with the "<tt/fillval/" attribute. This value is also used to fill unfilled
areas generated by the assemblers <tt/.ALIGN/ and <tt/.RES/ directives.

The symbol <tt/%S/ may be used to access the default start address (that is,
the one defined in the <ref id="FEATURES" name="FEATURES"> section, or the
value given on the command line with the <tt><ref id="option-S" name="-S"></tt>
option).


<sect1>Other SEGMENT attributes<p>

Segments may be aligned to some memory boundary. Specify "<tt/align = num/" to
request this feature. Num must be a power of two. To align all segments on a
page boundary, use

<tscreen><verb>
        SEGMENTS {
            CODE:   load = ROM1, type = ro, align = $100;
            RODATA: load = ROM2, type = ro, align = $100;
            DATA:   load = ROM2, run = RAM2, type = rw, define = yes,
                    align = $100;
            BSS:    load = RAM2, type = bss, define = yes, align = $100;
        }
</verb></tscreen>

If an alignment is requested, the linker will add enough space to the output
file, so that the new segment starts at an address that is dividable by the
given number without a remainder. All addresses are adjusted accordingly. To
fill the unused space, bytes of zero are used, or, if the memory area has a
"<tt/fillval/" attribute, that value. Alignment is always needed, if you have
used the <tt/.ALIGN/ command in the assembler. The alignment of a segment
must be equal or greater than the alignment used in the <tt/.ALIGN/ command.
The linker will check that, and issue a warning, if the alignment of a segment
is lower than the alignment requested in an <tt/.ALIGN/ command of one of the
modules making up this segment.

For a given segment you may also specify a fixed offset into a memory area or
a fixed start address. Use this if you want the code to run at a specific
address (a prominent case is the interrupt vector table which must go at
address $FFFA). Only one of <tt/ALIGN/ or <tt/OFFSET/ or <tt/START/ may be
specified. If the directive creates empty space, it will be filled with zero,
of with the value specified with the "<tt/fillval/" attribute if one is given.
The linker will warn you if it is not possible to put the code at the
specified offset (this may happen if other segments in this area are too
large). Here's an example:

<tscreen><verb>
        SEGMENTS {
            VECTORS: load = ROM2, type = ro, start = $FFFA;
        }
</verb></tscreen>

or (for the segment definitions from above)

<tscreen><verb>
        SEGMENTS {
            VECTORS: load = ROM2, type = ro, offset = $1FFA;
        }
</verb></tscreen>

The "<tt/align/", "<tt/start/" and "<tt/offset/" attributes change placement
of the segment in the run memory area, because this is what is usually
desired. If load and run memory areas are equal (which is the case if only the
load memory area has been specified), the attributes will also work. There is
also an "<tt/align_load/" attribute that may be used to align the start of the
segment in the load memory area, in case different load and run areas have
been specified. There are no special attributes to set start or offset for
just the load memory area.

To suppress the warning, the linker issues if it encounters a segment that is
not found in any of the input files, use "<tt/optional=yes/" as additional
segment attribute. Be careful when using this attribute, because a missing
segment may be a sign of a problem, and if you're suppressing the warning,
there is no one left to tell you about it.

<sect1>The FILES section<p>

The <tt/FILES/ section is used to support other formats than straight binary
(which is the default, so binary output files do not need an explicit entry
in the <tt/FILES/ section).

The <tt/FILES/ section lists output files and as only attribute the format of
each output file. Assigning binary format to the default output file would
look like this:

<tscreen><verb>
        FILES {
            %O: format = bin;
        }
</verb></tscreen>

The only other available output format is the o65 format specified by Andre
Fachat (see the <htmlurl url="http://www.6502.org/users/andre/o65/fileformat.html"
name="6502 binary relocation format specification">). It is defined like this:

<tscreen><verb>
        FILES {
            %O: format = o65;
        }
</verb></tscreen>

The necessary o65 attributes are defined in a special section labeled
<tt/FORMAT/.



<sect1>The FORMAT section<p>

The <tt/FORMAT/ section is used to describe file formats. The default (binary)
format has currently no attributes, so, while it may be listed in this
section, the attribute list is empty. The second supported format, o65, has
several attributes that may be defined here.

<tscreen><verb>
    FORMATS {
        o65: os = lunix, version = 0, type = small,
             import = LUNIXKERNEL,
             export = _main;
    }
</verb></tscreen>



<sect1>The FEATURES section<label id="FEATURES"><p>

In addition to the <tt/MEMORY/ and <tt/SEGMENTS/ sections described above, the
linker has features that may be enabled by an additional section labeled
<tt/FEATURES/.


<sect2>The CONDES feature<p>

<tt/CONDES/ is used to tell the linker to emit module constructor/destructor
tables.

<tscreen><verb>
        FEATURES {
            CONDES: segment = RODATA,
                    type = constructor,
                    label = __CONSTRUCTOR_TABLE__,
                    count = __CONSTRUCTOR_COUNT__;
        }
</verb></tscreen>

The <tt/CONDES/ feature has several attributes:

<descrip>

  <tag><tt>segment</tt></tag>

  This attribute tells the linker into which segment the table should be
  placed. If the segment does not exist, it is created.


  <tag><tt>type</tt></tag>

  Describes the type of the routines to place in the table. Type may be one of
  the predefined types <tt/constructor/, <tt/destructor/, <tt/interruptor/, or
  a numeric value between 0 and 6.


  <tag><tt>label</tt></tag>

  This specifies the label to use for the table. The label points to the start
  of the table in memory and may be used from within user written code.


  <tag><tt>count</tt></tag>

  This is an optional attribute. If specified, an additional symbol is defined
  by the linker using the given name. The value of this symbol is the number
  of entries (<em/not/ bytes) in the table. While this attribute is optional,
  it is often useful to define it.


  <tag><tt>order</tt></tag>

  Optional attribute that takes one of the keywords <tt/increasing/ or
  <tt/decreasing/ as an argument. Specifies the sorting order of the entries
  within the table. The default is <tt/increasing/, which means that the
  entries are sorted with increasing priority (the first entry has the lowest
  priority). "Priority" is the priority specified when declaring a symbol as
  <tt/.CONDES/ with the assembler, higher values mean higher priority. You may
  change this behaviour by specifying <tt/decreasing/ as the argument, the
  order of entries is reversed in this case.

  Please note that the order of entries with equal priority is undefined.

</descrip>

Without specifying the <tt/CONDES/ feature, the linker will not create any
tables, even if there are <tt/condes/ entries in the object files.

For more information see the <tt/.CONDES/ command in the <htmlurl
url="ca65.html" name="ca65 manual">.


<sect2>The STARTADDRESS feature<p>

<tt/STARTADDRESS/ is used to set the default value for the start address,
which can be referenced by the <tt/%S/ symbol. The builtin default for the
linker is &dollar;200.

<tscreen><verb>
        FEATURES {
            # Default start address is $1000
            STARTADDRESS:       default = $1000;
        }
</verb></tscreen>

Please note that order is important: The default start address must be defined
<em/before/ the <tt/%S/ symbol is used in the config file. This does usually
mean, that the <tt/FEATURES/ section has to go to the top of the config file.



<sect1>The SYMBOLS section<label id="SYMBOLS"><p>

The configuration file may also be used to define symbols used in the link
stage. The mandatory attribute for a symbol is its value. A second, boolean
attribute named <tt/weak/ is available. If a symbol is marked as weak, it may
be overridden by defining a symbol of the same name from the command line. The
default for symbols is that they're strong, which means that an attempt to
define a symbol with the same name from the command line will lead to an
error.

The following example defines the stack size for an application, but allows
the programmer to override the value by specifying <tt/--define
__STACKSIZE__=xxx/ on the command line.

<tscreen><verb>
        SYMBOLS {
            # Define the stack size for the application
            __STACKSIZE__:      value = $800, weak = yes;
        }
</verb></tscreen>



<sect1>Builtin configurations<p>

The builtin configurations are part of the linker source. They are also
distributed together with the machine specific binary packages (usually in the
doc directory) and don't have a special format. So if you need a special
configuration, it's a good idea to start with the builtin configuration for
your system. In a first step, just replace <tt/-t target/ by <tt/-C
configfile/. Then go on and modify the config file to suit your needs.



<sect>Special segments<p>

The builtin config files do contain segments that have a special meaning for
the compiler and the libraries that come with it. If you replace the builtin
config files, you will need the following information.

<sect1>INIT<p>

The INIT segment is used for initialization code that may be reused once
execution reaches main() - provided that the program runs in RAM. You
may for example add the INIT segment to the heap in really memory
constrained systems.

<sect1>LOWCODE<p>

For the LOWCODE segment, it is guaranteed that it won't be banked out, so it
is reachable at any time by interrupt handlers or similar.

<sect1>STARTUP<p>

This segment contains the startup code which initializes the C software stack
and the libraries. It is placed in its own segment because it needs to be
loaded at the lowest possible program address on several platforms.

<sect1>HEAP<p>

This segment defines the location of the memory heap used by the malloc
routine.



<sect>Bugs/Feedback<p>

If you have problems using the linker, if you find any bugs, or if you're
doing something interesting with it, I would be glad to hear from you. Feel
free to contact me by email (<htmlurl url="mailto:uz@cc65.org"
name="uz@cc65.org">).



<sect>Copyright<p>

ld65 (and all cc65 binutils) are (C) Copyright 1998-2005 Ullrich von
Bassewitz. For usage of the binaries and/or sources the following
conditions do apply:

This software is provided 'as-is', without any expressed or implied
warranty.  In no event will the authors be held liable for any damages
arising from the use of this software.

Permission is granted to anyone to use this software for any purpose,
including commercial applications, and to alter it and redistribute it
freely, subject to the following restrictions:

<enum>
<item>  The origin of this software must not be misrepresented; you must not
        claim that you wrote the original software. If you use this software
        in a product, an acknowledgment in the product documentation would be
        appreciated but is not required.
<item>  Altered source versions must be plainly marked as such, and must not
        be misrepresented as being the original software.
<item>  This notice may not be removed or altered from any source
        distribution.
</enum>



</article>