1 <!doctype linuxdoc system>
4 <title>ld65 Users Guide
5 <author>Ullrich von Bassewitz, <htmlurl url="mailto:uz@cc65.org" name="uz@cc65.org">
6 <date>02.12.2000, 02.10.2001
9 The ld65 linker combines object files into an executable file. ld65 is highly
10 configurable and uses configuration files for high flexibility.
13 <!-- Table of contents -->
16 <!-- Begin the document -->
20 The ld65 linker combines several object modules created by the ca65
21 assembler, producing an executable file. The object modules may be read
22 from a library created by the ar65 archiver (this is somewhat faster and
23 more convenient). The linker was designed to be as flexible as possible.
24 It complements the features that are built into the ca65 macroassembler:
28 <item> Accept any number of segments to form an executable module.
30 <item> Resolve arbitrary expressions stored in the object files.
32 <item> In case of errors, use the meta information stored in the object files
33 to produce helpful error messages. In case of undefined symbols,
34 expression range errors, or symbol type mismatches, ld65 is able to
35 tell you the exact location in the original assembler source, where
36 the symbol was referenced.
38 <item> Flexible output. The output of ld65 is highly configurable by a config
39 file. More common platforms are supported by builtin configurations
40 that may be activated by naming the target system. The output
41 generation was designed with different output formats in mind, so
42 adding other formats shouldn't be a great problem.
50 <sect1>Command line option overview<p>
52 The linker is called as follows:
55 ---------------------------------------------------------------------------
56 Usage: ld65 [options] module ...
58 -( Start a library group
59 -) End a library group
60 -C name Use linker config file
61 -D sym=val Define a symbol
62 -L path Specify a library search path
63 -Ln name Create a VICE label file
64 -S addr Set the default start address
65 -V Print the linker version
67 -m name Create a map file
68 -o name Name the default output file
69 -t sys Set the target system
74 --cfg-path path Specify a config file search path
75 --config name Use linker config file
76 --dbgfile name Generate debug information
77 --define sym=val Define a symbol
78 --dump-config name Dump a builtin configuration
79 --end-group End a library group
80 --help Help (this text)
81 --lib file Link this library
82 --lib-path path Specify a library search path
83 --mapfile name Create a map file
84 --module-id id Specify a module id
85 --obj file Link this object file
86 --obj-path path Specify an object file search path
87 --start-addr addr Set the default start address
88 --start-group Start a library group
89 --target sys Set the target system
90 --version Print the linker version
91 ---------------------------------------------------------------------------
95 <sect1>Command line options in detail<p>
97 Here is a description of all the command line options:
101 <label id="option--start-group">
102 <tag><tt>-(, --start-group</tt></tag>
104 Start a library group. The libraries specified within a group are searched
105 multiple times to resolve crossreferences within the libraries. Normally,
106 crossreferences are only resolved within a library, that is the library is
107 searched multiple times. Libraries specified later on the command line
108 cannot reference otherwise unreferenced symbols in libraries specified
109 earlier, because the linker has already handled them. Library groups are
110 a solution for this problem, because the linker will search repeatedly
111 through all libraries specified in the group, until all possible open
112 symbol references have been satisfied.
115 <tag><tt>-), --end-group</tt></tag>
117 End a library group. See the explanation of the <tt><ref
118 id="option--start-group" name="--start-group"></tt> option.
121 <tag><tt>-h, --help</tt></tag>
123 Print the short option summary shown above.
126 <label id="option-m">
127 <tag><tt>-m name, --mapfile name</tt></tag>
129 This option (which needs an argument that will used as a filename for
130 the generated map file) will cause the linker to generate a map file.
131 The map file does contain a detailed overview over the modules used, the
132 sizes for the different segments, and a table containing exported
136 <label id="option-o">
137 <tag><tt>-o name</tt></tag>
139 The -o switch is used to give the name of the default output file.
140 Depending on your output configuration, this name may NOT be used as
141 name for the output file. However, for the builtin configurations, this
142 name is used for the output file name.
145 <label id="option-t">
146 <tag><tt>-t sys, --target sys</tt></tag>
148 The argument for the -t switch is the name of the target system. Since this
149 switch will activate a builtin configuration, it may not be used together
150 with the <tt><ref id="option-C" name="-C"></tt> option. The following target
151 systems are currently supported:
158 <item>c16 (works also for the c116 with memory up to 32K)
162 <item>cbm510 (CBM-II series with 40 column video)
163 <item>cbm610 (all CBM series-II computers with 80 column video)
164 <item>pet (all CBM PET systems except the 2001)
171 There are a few more targets defined but neither of them is actually
175 <label id="option-v">
176 <tag><tt>-v, --verbose</tt></tag>
178 Using the -v option, you may enable more output that may help you to
179 locate problems. If an undefined symbol is encountered, -v causes the
180 linker to print a detailed list of the references (that is, source file
181 and line) for this symbol.
184 <tag><tt>-vm</tt></tag>
186 Must be used in conjunction with <tt><ref id="option-m" name="-m"></tt>
187 (generate map file). Normally the map file will not include empty segments
188 and sections, or unreferenced symbols. Using this option, you can force the
189 linker to include all this information into the map file.
192 <label id="option-C">
193 <tag><tt>-C</tt></tag>
195 This gives the name of an output config file to use. See section 4 for more
196 information about config files. -C may not be used together with <tt><ref
197 id="option-t" name="-t"></tt>.
200 <label id="option-D">
201 <tag><tt>-D sym=value, --define sym=value</tt></tag>
203 This option allows to define an external symbol on the command line. Value
204 may start with a '$' sign or with <tt/0x/ for hexadecimal values,
205 otherwise a leading zero denotes octal values. See also the <ref
206 id="SYMBOLS" name="SYMBOLS section"> in the configuration file.
209 <label id="option--lib-path">
210 <tag><tt>-L path, --lib-path path</tt></tag>
212 Specify a library search path. This option may be used more than once. It
213 adds a directory to the search path for library files. Libraries specified
214 without a path are searched in current directory, in the directory given in
215 the <tt/LD65_LIB/ environment variable, and in the list of directories
216 specified using <tt/--lib-path/.
219 <tag><tt>-Ln</tt></tag>
221 This option allows you to create a file that contains all global labels and
222 may be loaded into VICE emulator using the <tt/ll/ (load label) command. You
223 may use this to debug your code with VICE. Note: Older versions had some
224 bugs in the label code. If you have problems, please get the latest VICE
228 <label id="option-S">
229 <tag><tt>-S addr, --start-addr addr</tt></tag>
231 Using -S you may define the default starting address. If and how this
232 address is used depends on the config file in use. For the builtin
233 configurations, only the "none" system honors an explicit start address,
234 all other builtin config provide their own.
237 <tag><tt>-V, --version</tt></tag>
239 This option print the version number of the linker. If you send any
240 suggestions or bugfixes, please include this number.
243 <label id="option--cfg-path">
244 <tag><tt>--cfg-path path</tt></tag>
246 Specify a config file search path. This option may be used more than once.
247 It adds a directory to the search path for config files. A config file given
248 with the <tt><ref id="option-C" name="-C"></tt> option that has no path in
249 its name is searched in the current directory, in the directory given in the
250 <tt/LD65_CFG/ environment variable, and in the list of directories specified
251 using <tt/--cfg-path/.
254 <label id="option--dbgfile">
255 <tag><tt>--dbgfile name</tt></tag>
257 Specify an output file for debug information. Available information will be
258 written to this file. Using the <tt/-g/ option for the compiler and assembler
259 will increase the amount of information available. Please note that debug
260 information generation is currently being developed, so the format of the
261 file and it's contents are subject to change without further notice.
264 <tag><tt>--lib file</tt></tag>
266 Links a library to the output. Use this command line option instead of just
267 naming the library file, if the linker is not able to determine the file
268 type because of an unusual extension.
271 <tag><tt>--obj file</tt></tag>
273 Links an object file to the output. Use this command line option instead
274 of just naming the object file, if the linker is not able to determine the
275 file type because of an unusual extension.
278 <label id="option--obj-path">
279 <tag><tt>--obj-path path</tt></tag>
281 Specify an object file search path. This option may be used more than once.
282 It adds a directory to the search path for object files. An object file
283 passed to the linker that has no path in its name is searched in current
284 directory, in the directory given in the <tt/LD65_OBJ/ environment variable,
285 and in the list of directories specified using <tt/--obj-path/.
291 <sect>Search paths<p>
293 Starting with version 2.10 there are now several search paths for files needed
294 by the linker: One for libraries, one for object files and one for config
298 <sect1>Library search path<p>
300 The library search path contains in this order:
303 <item>The current directory.
304 <item>A compiled in library path which is often <tt>/usr/lib/cc65/lib</tt> on
306 <item>The value of the environment variable <tt/LD65_LIB/ if it is defined.
307 <item>The value of the environment variable <tt/CC65_LIB/ if it is defined.
308 Please note that use of this environment variable is obsolete and may
309 get removed in future versions.
310 <item>Any directory added with the <tt><ref id="option--lib-path"
311 name="--lib-path"></tt> option on the command line.
315 <sect1>Object file search path<p>
317 The object file search path contains in this order:
320 <item>The current directory.
321 <item>A compiled in directory which is often <tt>/usr/lib/cc65/lib</tt> on
323 <item>The value of the environment variable <tt/LD65_OBJ/ if it is defined.
324 <item>The value of the environment variable <tt/CC65_LIB/ if it is defined.
325 Please note that use of this environment variable is obsolete and may
326 get removed in future versions.
327 <item>Any directory added with the <tt><ref id="option--obj-path"
328 name="--obj-path"></tt> option on the command line.
332 <sect1>Config file search path<p>
334 The config file search path contains in this order:
337 <item>The current directory.
338 <item>A compiled in directory which is often <tt>/usr/lib/cc65/lib</tt> on
340 <item>The value of the environment variable <tt/LD65_CFG/ if it is defined.
341 <item>Any directory added with the <tt><ref id="option--cfg-path"
342 name="--cfg-path"></tt> option on the command line.
347 <sect>Detailed workings<p>
349 The linker does several things when combining object modules:
351 First, the command line is parsed from left to right. For each object file
352 encountered (object files are recognized by a magic word in the header, so
353 the linker does not care about the name), imported and exported
354 identifiers are read from the file and inserted in a table. If a library
355 name is given (libraries are also recognized by a magic word, there are no
356 special naming conventions), all modules in the library are checked if an
357 export from this module would satisfy an import from other modules. All
358 modules where this is the case are marked. If duplicate identifiers are
359 found, the linker issues a warning.
361 This procedure (parsing and reading from left to right) does mean, that a
362 library may only satisfy references for object modules (given directly or from
363 a library) named <em/before/ that library. With the command line
366 ld65 crt0.o clib.lib test.o
369 the module test.o may not contain references to modules in the library
370 clib.lib. If this is the case, you have to change the order of the modules
374 ld65 crt0.o test.o clib.lib
377 Step two is, to read the configuration file, and assign start addresses
378 for the segments and define any linker symbols (see <ref id="config-files"
379 name="Configuration files">).
381 After that, the linker is ready to produce an output file. Before doing that,
382 it checks it's data for consistency. That is, it checks for unresolved
383 externals (if the output format is not relocatable) and for symbol type
384 mismatches (for example a zero page symbol is imported by a module as absolute
387 Step four is, to write the actual target files. In this step, the linker will
388 resolve any expressions contained in the segment data. Circular references are
389 also detected in this step (a symbol may have a circular reference that goes
390 unnoticed if the symbol is not used).
392 Step five is to output a map file with a detailed list of all modules,
393 segments and symbols encountered.
395 And, last step, if you give the <tt><ref id="option-v" name="-v"></tt> switch
396 twice, you get a dump of the segment data. However, this may be quite
397 unreadable if you're not a developer:-)
401 <sect>Configuration files<label id="config-files"><p>
403 Configuration files are used to describe the layout of the output file(s). Two
404 major topics are covered in a config file: The memory layout of the target
405 architecture, and the assignment of segments to memory areas. In addition,
406 several other attributes may be specified.
408 Case is ignored for keywords, that is, section or attribute names, but it is
409 <em/not/ ignored for names and strings.
413 <sect1>Memory areas<p>
415 Memory areas are specified in a <tt/MEMORY/ section. Lets have a look at an
416 example (this one describes the usable memory layout of the C64):
420 RAM1: start = $0800, size = $9800;
421 ROM1: start = $A000, size = $2000;
422 RAM2: start = $C000, size = $1000;
423 ROM2: start = $E000, size = $2000;
427 As you can see, there are two ram areas and two rom areas. The names
428 (before the colon) are arbitrary names that must start with a letter, with
429 the remaining characters being letters or digits. The names of the memory
430 areas are used when assigning segments. As mentioned above, case is
431 significant for these names.
433 The syntax above is used in all sections of the config file. The name
434 (<tt/ROM1/ etc.) is said to be an identifier, the remaining tokens up to the
435 semicolon specify attributes for this identifier. You may use the equal sign
436 to assign values to attributes, and you may use a comma to separate
437 attributes, you may also leave both out. But you <em/must/ use a semicolon to
438 mark the end of the attributes for one identifier. The section above may also
439 have looked like this:
442 # Start of memory section
460 There are of course more attributes for a memory section than just start and
461 size. Start and size are mandatory attributes, that means, each memory area
462 defined <em/must/ have these attributes given (the linker will check that). I
463 will cover other attributes later. As you may have noticed, I've used a
464 comment in the example above. Comments start with a hash mark (`#'), the
465 remainder of the line is ignored if this character is found.
470 Let's assume you have written a program for your trusty old C64, and you would
471 like to run it. For testing purposes, it should run in the <tt/RAM/ area. So
472 we will start to assign segments to memory sections in the <tt/SEGMENTS/
477 CODE: load = RAM1, type = ro;
478 RODATA: load = RAM1, type = ro;
479 DATA: load = RAM1, type = rw;
480 BSS: load = RAM1, type = bss, define = yes;
484 What we are doing here is telling the linker, that all segments go into the
485 <tt/RAM1/ memory area in the order specified in the <tt/SEGMENTS/ section. So
486 the linker will first write the <tt/CODE/ segment, then the <tt/RODATA/
487 segment, then the <tt/DATA/ segment - but it will not write the <tt/BSS/
488 segment. Why? Enter the segment type: For each segment specified, you may also
489 specify a segment attribute. There are five possible segment attributes:
494 bss means that this is an uninitialized segment
495 zp a zeropage segment
498 So, because we specified that the segment with the name BSS is of type bss,
499 the linker knows that this is uninitialized data, and will not write it to an
500 output file. This is an important point: For the assembler, the <tt/BSS/
501 segment has no special meaning. You specify, which segments have the bss
502 attribute when linking. This approach is much more flexible than having one
503 fixed bss segment, and is a result of the design decision to supporting an
504 arbitrary segment count.
506 If you specify "<tt/type = bss/" for a segment, the linker will make sure that
507 this segment does only contain uninitialized data (that is, zeroes), and issue
508 a warning if this is not the case.
510 For a <tt/bss/ type segment to be useful, it must be cleared somehow by your
511 program (this happens usually in the startup code - for example the startup
512 code for cc65 generated programs takes care about that). But how does your
513 code know, where the segment starts, and how big it is? The linker is able to
514 give that information, but you must request it. This is, what we're doing with
515 the "<tt/define = yes/" attribute in the <tt/BSS/ definitions. For each
516 segment, where this attribute is true, the linker will export three symbols.
519 __NAME_LOAD__ This is set to the address where the
521 __NAME_RUN__ This is set to the run address of the
522 segment. We will cover run addresses
524 __NAME_SIZE__ This is set to the segment size.
527 Replace <tt/NAME/ by the name of the segment, in the example above, this would
528 be <tt/BSS/. These symbols may be accessed by your code.
530 Now, as we've configured the linker to write the first three segments and
531 create symbols for the last one, there's only one question left: Where does
532 the linker put the data? It would be very convenient to have the data in a
535 <sect1>Output files<p>
537 We don't have any files specified above, and indeed, this is not needed in a
538 simple configuration like the one above. There is an additional attribute
539 "file" that may be specified for a memory area, that gives a file name to
540 write the area data into. If there is no file name given, the linker will
541 assign the default file name. This is "a.out" or the one given with the
542 <tt><ref id="option-o" name="-o"></tt> option on the command line. Since the
543 default behaviour is ok for our purposes, I did not use the attribute in the
544 example above. Let's have a look at it now.
546 The "file" attribute (the keyword may also be written as "FILE" if you like
547 that better) takes a string enclosed in double quotes (`"') that specifies the
548 file, where the data is written. You may specify the same file several times,
549 in that case the data for all memory areas having this file name is written
550 into this file, in the order of the memory areas defined in the <tt/MEMORY/
551 section. Let's specify some file names in the <tt/MEMORY/ section used above:
555 RAM1: start = $0800, size = $9800, file = %O;
556 ROM1: start = $A000, size = $2000, file = "rom1.bin";
557 RAM2: start = $C000, size = $1000, file = %O;
558 ROM2: start = $E000, size = $2000, file = "rom2.bin";
562 The <tt/%O/ used here is a way to specify the default behaviour explicitly:
563 <tt/%O/ is replaced by a string (including the quotes) that contains the
564 default output name, that is, "a.out" or the name specified with the <tt><ref
565 id="option-o" name="-o"></tt> option on the command line. Into this file, the
566 linker will first write any segments that go into <tt/RAM1/, and will append
567 then the segments for <tt/RAM2/, because the memory areas are given in this
568 order. So, for the RAM areas, nothing has really changed.
570 We've not used the ROM areas, but we will do that below, so we give the file
571 names here. Segments that go into <tt/ROM1/ will be written to a file named
572 "rom1.bin", and segments that go into <tt/ROM2/ will be written to a file
573 named "rom2.bin". The name given on the command line is ignored in both cases.
575 Assigning an empty file name for a memory area will discard the data written
576 to it. This is useful, if the a memory area has segments assigned that are
577 empty (for example because they are of type bss). In that case, the linker
578 will create an empty output file. This may be suppressed by assigning an empty
579 file name to that memory area.
582 <sect1>LOAD and RUN addresses (ROMable code)<p>
584 Let us look now at a more complex example. Say, you've successfully tested
585 your new "Super Operating System" (SOS for short) for the C64, and you
586 will now go and replace the ROMs by your own code. When doing that, you
587 face a new problem: If the code runs in RAM, we need not to care about
588 read/write data. But now, if the code is in ROM, we must care about it.
589 Remember the default segments (you may of course specify your own):
593 RODATA read only data
595 BSS uninitialized data, read/write
598 Since <tt/BSS/ is not initialized, we must not care about it now, but what
599 about <tt/DATA/? <tt/DATA/ contains initialized data, that is, data that was
600 explicitly assigned a value. And your program will rely on these values on
601 startup. Since there's no other way to remember the contents of the data
602 segment, than storing it into one of the ROMs, we have to put it there. But
603 unfortunately, ROM is not writable, so we have to copy it into RAM before
604 running the actual code.
606 The linker cannot help you copying the data from ROM into RAM (this must be
607 done by the startup code of your program), but it has some features that will
608 help you in this process.
610 First, you may not only specify a "<tt/load/" attribute for a segment, but
611 also a "<tt/run/" attribute. The "<tt/load/" attribute is mandatory, and, if
612 you don't specify a "<tt/run/" attribute, the linker assumes that load area
613 and run area are the same. We will use this feature for our data area:
617 CODE: load = ROM1, type = ro;
618 RODATA: load = ROM2, type = ro;
619 DATA: load = ROM2, run = RAM2, type = rw, define = yes;
620 BSS: load = RAM2, type = bss, define = yes;
624 Let's have a closer look at this <tt/SEGMENTS/ section. We specify that the
625 <tt/CODE/ segment goes into <tt/ROM1/ (the one at $A000). The readonly data
626 goes into <tt/ROM2/. Read/write data will be loaded into <tt/ROM2/ but is run
627 in <tt/RAM2/. That means that all references to labels in the <tt/DATA/
628 segment are relocated to be in <tt/RAM2/, but the segment is written to
629 <tt/ROM2/. All your startup code has to do is, to copy the data from it's
630 location in <tt/ROM2/ to the final location in <tt/RAM2/.
632 So, how do you know, where the data is located? This is the second point,
633 where you get help from the linker. Remember the "<tt/define/" attribute?
634 Since we have set this attribute to true, the linker will define three
635 external symbols for the data segment that may be accessed from your code:
638 __DATA_LOAD__ This is set to the address where the segment
639 is loaded, in this case, it is an address in
641 __DATA_RUN__ This is set to the run address of the segment,
642 in this case, it is an address in RAM2.
643 __DATA_SIZE__ This is set to the segment size.
646 So, what your startup code must do, is to copy <tt/__DATA_SIZE__/ bytes from
647 <tt/__DATA_LOAD__/ to <tt/__DATA_RUN__/ before any other routines are called.
648 All references to labels in the <tt/DATA/ segment are relocated to <tt/RAM2/
649 by the linker, so things will work properly.
652 <sect1>Other MEMORY area attributes<p>
654 There are some other attributes not covered above. Before starting the
655 reference section, I will discuss the remaining things here.
657 You may request symbols definitions also for memory areas. This may be
658 useful for things like a software stack, or an i/o area.
662 STACK: start = $C000, size = $1000, define = yes;
666 This will define three external symbols that may be used in your code:
669 __STACK_START__ This is set to the start of the memory
670 area, $C000 in this example.
671 __STACK_SIZE__ The size of the area, here $1000.
672 __STACK_LAST__ This is NOT the same as START+SIZE.
673 Instead, it it defined as the first
674 address that is not used by data. If we
675 don't define any segments for this area,
676 the value will be the same as START.
679 A memory section may also have a type. Valid types are
682 ro for readonly memory
683 rw for read/write memory.
686 The linker will assure, that no segment marked as read/write or bss is put
687 into a memory area that is marked as readonly.
689 Unused memory in a memory area may be filled. Use the "<tt/fill = yes/"
690 attribute to request this. The default value to fill unused space is zero. If
691 you don't like this, you may specify a byte value that is used to fill these
692 areas with the "<tt/fillval/" attribute. This value is also used to fill unfilled
693 areas generated by the assemblers <tt/.ALIGN/ and <tt/.RES/ directives.
695 The symbol <tt/%S/ may be used to access the default start address (that is,
696 the one defined in the <ref id="FEATURES" name="FEATURES"> section, or the
697 value given on the command line with the <tt><ref id="option-S" name="-S"></tt>
701 <sect1>Other SEGMENT attributes<p>
703 Segments may be aligned to some memory boundary. Specify "<tt/align = num/" to
704 request this feature. Num must be a power of two. To align all segments on a
709 CODE: load = ROM1, type = ro, align = $100;
710 RODATA: load = ROM2, type = ro, align = $100;
711 DATA: load = ROM2, run = RAM2, type = rw, define = yes,
713 BSS: load = RAM2, type = bss, define = yes, align = $100;
717 If an alignment is requested, the linker will add enough space to the output
718 file, so that the new segment starts at an address that is divideable by the
719 given number without a remainder. All addresses are adjusted accordingly. To
720 fill the unused space, bytes of zero are used, or, if the memory area has a
721 "<tt/fillval/" attribute, that value. Alignment is always needed, if you have
722 the used the <tt/.ALIGN/ command in the assembler. The alignment of a segment
723 must be equal or greater than the alignment used in the <tt/.ALIGN/ command.
724 The linker will check that, and issue a warning, if the alignment of a segment
725 is lower than the alignment requested in a <tt/.ALIGN/ command of one of the
726 modules making up this segment.
728 For a given segment you may also specify a fixed offset into a memory area or
729 a fixed start address. Use this if you want the code to run at a specific
730 address (a prominent case is the interrupt vector table which must go at
731 address $FFFA). Only one of <tt/ALIGN/ or <tt/OFFSET/ or <tt/START/ may be
732 specified. If the directive creates empty space, it will be filled with zero,
733 of with the value specified with the "<tt/fillval/" attribute if one is given.
734 The linker will warn you if it is not possible to put the code at the
735 specified offset (this may happen if other segments in this area are too
736 large). Here's an example:
740 VECTORS: load = ROM2, type = ro, start = $FFFA;
744 or (for the segment definitions from above)
748 VECTORS: load = ROM2, type = ro, offset = $1FFA;
752 The "<tt/align/", "<tt/start/" and "<tt/offset/" attributes change placement
753 of the segment in the run memory area, because this is what is usually
754 desired. If load and run memory areas are equal (which is the case if only the
755 load memory area has been specified), the attributes will also work. There is
756 also a "<tt/align_load/" attribute that may be used to align the start of the
757 segment in the load memory area, in case different load and run areas have
758 been specified. There are no special attributes to set start or offset for
759 just the load memory area.
761 To suppress the warning, the linker issues if it encounters a segment that is
762 not found in any of the input files, use "<tt/optional=yes/" as additional
763 segment attribute. Be careful when using this attribute, because a missing
764 segment may be a sign of a problem, and if you're suppressing the warning,
765 there is no one left to tell you about it.
767 <sect1>The FILES section<p>
769 The <tt/FILES/ section is used to support other formats than straight binary
770 (which is the default, so binary output files do not need an explicit entry
771 in the <tt/FILES/ section).
773 The <tt/FILES/ section lists output files and as only attribute the format of
774 each output file. Assigning binary format to the default output file would
783 The only other available output format is the o65 format specified by Andre
784 Fachat. It is defined like this:
792 The necessary o65 attributes are defined in a special section labeled
797 <sect1>The FORMAT section<p>
799 The <tt/FORMAT/ section is used to describe file formats. The default (binary)
800 format has currently no attributes, so, while it may be listed in this
801 section, the attribute list is empty. The second supported format, o65, has
802 several attributes that may be defined here.
806 o65: os = lunix, version = 0, type = small,
807 import = LUNIXKERNEL,
814 <sect1>The FEATURES section<label id="FEATURES"><p>
816 In addition to the <tt/MEMORY/ and <tt/SEGMENTS/ sections described above, the
817 linker has features that may be enabled by an additional section labeled
821 <sect2>The CONDES feature<p>
823 <tt/CONDES/ is used to tell the linker to emit module constructor/destructor
828 CONDES: segment = RODATA,
830 label = __CONSTRUCTOR_TABLE__,
831 count = __CONSTRUCTOR_COUNT__;
835 The <tt/CONDES/ feature has several attributes:
839 <tag><tt>segment</tt></tag>
841 This attribute tells the linker into which segment the table should be
842 placed. If the segment does not exist, it is created.
845 <tag><tt>type</tt></tag>
847 Describes the type of the routines to place in the table. Type may be one of
848 the predefined types <tt/constructor/, <tt/destructor/, <tt/interruptor/, or
849 a numeric value between 0 and 6.
852 <tag><tt>label</tt></tag>
854 This specifies the label to use for the table. The label points to the start
855 of the table in memory and may be used from within user written code.
858 <tag><tt>count</tt></tag>
860 This is an optional attribute. If specified, an additional symbol is defined
861 by the linker using the given name. The value of this symbol is the number
862 of entries (<em/not/ bytes) in the table. While this attribute is optional,
863 it is often useful to define it.
866 <tag><tt>order</tt></tag>
868 Optional attribute that takes one of the keywords <tt/increasing/ or
869 <tt/decreasing/ as an argument. Specifies the sorting order of the entries
870 within the table. The default is <tt/increasing/, which means that the
871 entries are sorted with increasing priority (the first entry has the lowest
872 priority). "Priority" is the priority specified when declaring a symbol as
873 <tt/.CONDES/ with the assembler, higher values mean higher priority. You may
874 change this behaviour by specifying <tt/decreasing/ as the argument, the
875 order of entries is reversed in this case.
877 Please note that the order of entries with equal priority is undefined.
881 Without specifying the <tt/CONDES/ feature, the linker will not create any
882 tables, even if there are <tt/condes/ entries in the object files.
884 For more information see the <tt/.CONDES/ command in the <htmlurl
885 url="ca65.html" name="ca65 manual">.
888 <sect2>The STARTADDRESS feature<p>
890 <tt/STARTADDRESS/ is used to set the default value for the start address,
891 which can be referenced by the <tt/%S/ symbol. The builtin default for the
892 linker is $200.
896 # Default start address is $1000
897 STARTADDRESS: default = $1000;
901 Please note that order is important: The default start address must be defined
902 <em/before/ the <tt/%S/ symbol is used in the config file. This does usually
903 mean, that the <tt/FEATURES/ section has to go to the top of the config file.
907 <sect1>The SYMBOLS section<label id="SYMBOLS"><p>
909 The configuration file may also be used to define symbols used in the link
910 stage. The mandatory attribute for a symbol is its value. A second, boolean
911 attribute named <tt/weak/ is available. If a symbol is marked as weak, it may
912 be overridden by defining a symbol of the same name from the command line. The
913 default for symbols is that they're strong, which means that an attempt to
914 define a symbol with the same name from the command line will lead to an
917 The following example defines the stack size for an application, but allows
918 the programmer to override the value by specifying <tt/--define
919 __STACKSIZE__=xxx/ on the command line.
923 # Define the stack size for the application
924 __STACKSIZE__: value = $800, weak = yes;
930 <sect1>Builtin configurations<p>
932 The builtin configurations are part of the linker source. They are also
933 distributed together with the machine specific binary packages (usually in the
934 doc directory) and don't have a special format. So if you need a special
935 configuration, it's a good idea to start with the builtin configuration for
936 your system. In a first step, just replace <tt/-t target/ by <tt/-C
937 configfile/. The go on and modify the config file to suit your needs.
941 <sect>Special segments<p>
943 The builtin config files do contain segments that have a special meaning for
944 the compiler and the libraries that come with it. If you replace the builtin
945 config files, you will need the following information.
949 The INIT segment is used for initialization code that may be reused once
950 execution reaches main() - provided that the program runs in RAM. You
951 may for example add the INIT segment to the heap in really memory
956 For the LOWCODE segment, it is guaranteed that it won't be banked out, so it
957 is reachable at any time by interrupt handlers or similar.
961 This segment contains the startup code which initializes the C software stack
962 and the libraries. It is placed in its own segment because it needs to be
963 loaded at the lowest possible program address on several platforms.
967 This segment defines the location of the memory heap used by the malloc
972 <sect>Bugs/Feedback<p>
974 If you have problems using the linker, if you find any bugs, or if you're
975 doing something interesting with it, I would be glad to hear from you. Feel
976 free to contact me by email (<htmlurl url="mailto:uz@cc65.org"
977 name="uz@cc65.org">).
983 ld65 (and all cc65 binutils) are (C) Copyright 1998-2005 Ullrich von
984 Bassewitz. For usage of the binaries and/or sources the following
987 This software is provided 'as-is', without any expressed or implied
988 warranty. In no event will the authors be held liable for any damages
989 arising from the use of this software.
991 Permission is granted to anyone to use this software for any purpose,
992 including commercial applications, and to alter it and redistribute it
993 freely, subject to the following restrictions:
996 <item> The origin of this software must not be misrepresented; you must not
997 claim that you wrote the original software. If you use this software
998 in a product, an acknowledgment in the product documentation would be
999 appreciated but is not required.
1000 <item> Altered source versions must be plainly marked as such, and must not
1001 be misrepresented as being the original software.
1002 <item> This notice may not be removed or altered from any source