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5. Configuration files

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 not ignored for names and strings.

5.1 Memory areas

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

        MEMORY {
            RAM1:  start = $0800, size = $9800;
            ROM1:  start = $A000, size = $2000;
            RAM2:  start = $C000, size = $1000;
            ROM2:  start = $E000, size = $2000;

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 (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 must use a semicolon to mark the end of the attributes for one identifier. The section above may also have looked like this:

        # Start of memory section
                start $0800
                size $9800;
                start $A000
                size $2000;
                start $C000
                size $1000;
                start $E000
                size $2000;

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 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.

5.2 Segments

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 RAM area. So we will start to assign segments to memory sections in the SEGMENTS section:

        SEGMENTS {
            CODE:   load = RAM1, type = ro;
            RODATA: load = RAM1, type = ro;
            DATA:   load = RAM1, type = rw;
            BSS:    load = RAM1, type = bss, define = yes;

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

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

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 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 "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 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 "define = yes" attribute in the BSS definitions. For each segment, where this attribute is true, the linker will export three symbols.

        __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
        __NAME_SIZE__   This is set to the segment size.

Replace NAME by the name of the segment, in the example above, this would be 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?

5.3 Output files

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 -o 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 MEMORY section. Let's specify some file names in the MEMORY section used above:

        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";

The %O used here is a way to specify the default behaviour explicitly: %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 -o option on the command line. Into this file, the linker will first write any segments that go into RAM1, and will append then the segments for 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 ROM1 will be written to a file named "rom1.bin", and segments that go into 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 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.

The %O sequence is also allowed inside a string. So using

        MEMORY {
            ROM1:  start = $A000, size = $2000, file = "%O-1.bin";
            ROM2:  start = $E000, size = $2000, file = "%O-2.bin";

would write two files that start with the name of the output file specified on the command line, with "-1.bin" and "-2.bin" appended respectively. Because '%' is used as an escape char, the sequence "%%" has to be used if a single percent sign is required.

5.4 LOAD and RUN addresses (ROMable code)

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):

        CODE            read only code
        RODATA          read only data
        DATA            read/write data
        BSS             uninitialized data, read/write

Since BSS is not initialized, we must not care about it now, but what about DATA? 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 won't copy the data from ROM into RAM for you (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 "load" attribute for a segment, but also a "run" attribute. The "load" attribute is mandatory, and, if you don't specify a "run" attribute, the linker assumes that load area and run area are the same. We will use this feature for our data area:

        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;

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

        __DATA_LOAD__   This is set to the address where the segment
                        is loaded, in this case, it is an address in
        __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.

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

There's a library subroutine called copydata (in a module named copydata.s) that might be used to do actual copying. Be sure to have a look at it's inner workings before using it!

5.5 Other MEMORY area attributes

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.

        MEMORY {
            STACK:  start = $C000, size = $1000, define = yes;

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

        __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.
        __STACK_FILEOFFS__      The binary offset in the output file. This
                                is not defined for relocatable output file
                                formats (o65).

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

        ro      for readonly memory
        rw      for read/write memory.

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 "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 "fillval" attribute. If there is no "fillval" attribute for the segment, the "fillval" attribute of the memory area (or its default) is used instead. This means that the value may also be used to fill unfilled areas generated by the assemblers .ALIGN and .RES directives.

The symbol %S may be used to access the default start address (that is, the one defined in the FEATURES section, or the value given on the command line with the -S option).

To support systems with banked memory, a special attribute named bank is available. The attribute value is an arbitrary 32 bit integer. The assembler has a builtin function named .BANK which may be used with an argument that has a segment reference (for example a symbol). The result of this function is the value of the bank attribute for the run memory area of the segment.

5.6 Other SEGMENT attributes

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

        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;

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 "fillval" attribute, that value. Alignment is always needed, if you have used the .ALIGN command in the assembler. The alignment of a segment must be equal or greater than the alignment used in the .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 .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 ALIGN or OFFSET or START may be specified. If the directive creates empty space, it will be filled with zero, of with the value specified with the "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:

        SEGMENTS {
            VECTORS: load = ROM2, type = ro, start = $FFFA;

or (for the segment definitions from above)

        SEGMENTS {
            VECTORS: load = ROM2, type = ro, offset = $1FFA;

The "align", "start" and "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 "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.

A "fillval" attribute may not only be specified for a memory area, but also for a segment. The value must be an integer between 0 and 255. It is used as fill value for space reserved by the assemblers .ALIGN and .RES commands. It is also used as fill value for space between sections (part of a segment that comes from one object file) caused by alignment, but not for space that preceeds the first section.

To suppress the warning, the linker issues if it encounters a segment that is not found in any of the input files, use "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.

5.7 The FILES section

The 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 FILES section).

The 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:

        FILES {
            %O: format = bin;

The only other available output format is the o65 format specified by Andre Fachat (see the 6502 binary relocation format specification). It is defined like this:

        FILES {
            %O: format = o65;

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

5.8 The FORMAT section

The 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.

        o65: os = lunix, version = 0, type = small,
             import = LUNIXKERNEL,
             export = _main;

5.9 The FEATURES section

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

The CONDES feature

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

        FEATURES {
            CONDES: segment = RODATA,
                    type = constructor,
                    label = __CONSTRUCTOR_TABLE__,
                    count = __CONSTRUCTOR_COUNT__;

The CONDES feature has several attributes:


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


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


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.


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 (not bytes) in the table. While this attribute is optional, it is often useful to define it.


Optional attribute that takes one of the keywords increasing or decreasing as an argument. Specifies the sorting order of the entries within the table. The default is 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 .CONDES with the assembler, higher values mean higher priority. You may change this behaviour by specifying 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.


This attribute defines a valid symbol name, that is added as an import to the modules defining a constructor/desctructor of the given type. This can be used to force linkage of a module if this module exports the requested symbol.

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

For more information see the .CONDES command in the ca65 manual.


STARTADDRESS is used to set the default value for the start address, which can be referenced by the %S symbol. The builtin default for the linker is $200.

        FEATURES {
            # Default start address is $1000
            STARTADDRESS:       default = $1000;

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

5.10 The SYMBOLS section

The configuration file may also be used to define symbols used in the link stage or to force symbols imports. This is done in the SYMBOLS section. The symbol name is followed by a colon and symbol attributes.

The following symbol attributes are supported:


The addrsize attribute specifies the address size of the symbol and may be one of

Without this attribute, the default address size is abs.


This attribute is mandatory. Its value is one of export, import or weak. export means that the symbol is defined and exported from the linker config. import means that an import is generated for this symbol, eventually forcing a module that exports this symbol to be included in the output. weak is similar as export. However, the symbol is only defined if it is not defined elsewhere.


This must only be given for symbols of type export or weak. It defines the value of the symbol and may be an expression.

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

        SYMBOLS {
            # Define the stack size for the application
            __STACKSIZE__:  type = weak, value = $800;

5.11 Builtin configurations

The builtin configurations are part of the linker source. They can be retrieved with --dump-config 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 -t target by -C configfile. Then go on and modify the config file to suit your needs.

5.12 Secondary configurations

Several machine specific binary packages are distributed together with secondary configurations (in the cfg directory). These configurations can be used with -C configfile too.

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