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:
159 <item>c16 (works also for the c116 with memory up to 32K)
163 <item>cbm510 (CBM-II series with 40 column video)
164 <item>cbm610 (all CBM series-II computers with 80 column video)
165 <item>pet (all CBM PET systems except the 2001)
173 There are a few more targets defined but neither of them is actually
177 <label id="option-v">
178 <tag><tt>-v, --verbose</tt></tag>
180 Using the -v option, you may enable more output that may help you to
181 locate problems. If an undefined symbol is encountered, -v causes the
182 linker to print a detailed list of the references (that is, source file
183 and line) for this symbol.
186 <tag><tt>-vm</tt></tag>
188 Must be used in conjunction with <tt><ref id="option-m" name="-m"></tt>
189 (generate map file). Normally the map file will not include empty segments
190 and sections, or unreferenced symbols. Using this option, you can force the
191 linker to include all this information into the map file.
194 <label id="option-C">
195 <tag><tt>-C</tt></tag>
197 This gives the name of an output config file to use. See section 4 for more
198 information about config files. -C may not be used together with <tt><ref
199 id="option-t" name="-t"></tt>.
202 <label id="option-D">
203 <tag><tt>-D sym=value, --define sym=value</tt></tag>
205 This option allows to define an external symbol on the command line. Value
206 may start with a '$' sign or with <tt/0x/ for hexadecimal values,
207 otherwise a leading zero denotes octal values. See also the <ref
208 id="SYMBOLS" name="SYMBOLS section"> in the configuration file.
211 <label id="option--lib-path">
212 <tag><tt>-L path, --lib-path path</tt></tag>
214 Specify a library search path. This option may be used more than once. It
215 adds a directory to the search path for library files. Libraries specified
216 without a path are searched in current directory, in the directory given in
217 the <tt/LD65_LIB/ environment variable, and in the list of directories
218 specified using <tt/--lib-path/.
221 <tag><tt>-Ln</tt></tag>
223 This option allows you to create a file that contains all global labels and
224 may be loaded into VICE emulator using the <tt/ll/ (load label) command. You
225 may use this to debug your code with VICE. Note: Older versions had some
226 bugs in the label code. If you have problems, please get the latest VICE
230 <label id="option-S">
231 <tag><tt>-S addr, --start-addr addr</tt></tag>
233 Using -S you may define the default starting address. If and how this
234 address is used depends on the config file in use. For the builtin
235 configurations, only the "none", "apple2" and "apple2enh" systems honor an
236 explicit start address, all other builtin config provide their own.
239 <tag><tt>-V, --version</tt></tag>
241 This option print the version number of the linker. If you send any
242 suggestions or bugfixes, please include this number.
245 <label id="option--cfg-path">
246 <tag><tt>--cfg-path path</tt></tag>
248 Specify a config file search path. This option may be used more than once.
249 It adds a directory to the search path for config files. A config file given
250 with the <tt><ref id="option-C" name="-C"></tt> option that has no path in
251 its name is searched in the current directory, in the directory given in the
252 <tt/LD65_CFG/ environment variable, and in the list of directories specified
253 using <tt/--cfg-path/.
256 <label id="option--dbgfile">
257 <tag><tt>--dbgfile name</tt></tag>
259 Specify an output file for debug information. Available information will be
260 written to this file. Using the <tt/-g/ option for the compiler and assembler
261 will increase the amount of information available. Please note that debug
262 information generation is currently being developed, so the format of the
263 file and it's contents are subject to change without further notice.
266 <tag><tt>--lib file</tt></tag>
268 Links a library to the output. Use this command line option instead of just
269 naming the library file, if the linker is not able to determine the file
270 type because of an unusual extension.
273 <tag><tt>--obj file</tt></tag>
275 Links an object file to the output. Use this command line option instead
276 of just naming the object file, if the linker is not able to determine the
277 file type because of an unusual extension.
280 <label id="option--obj-path">
281 <tag><tt>--obj-path path</tt></tag>
283 Specify an object file search path. This option may be used more than once.
284 It adds a directory to the search path for object files. An object file
285 passed to the linker that has no path in its name is searched in current
286 directory, in the directory given in the <tt/LD65_OBJ/ environment variable,
287 and in the list of directories specified using <tt/--obj-path/.
293 <sect>Search paths<p>
295 Starting with version 2.10 there are now several search paths for files needed
296 by the linker: One for libraries, one for object files and one for config
300 <sect1>Library search path<p>
302 The library search path contains in this order:
305 <item>The current directory.
306 <item>A compiled in library path which is often <tt>/usr/lib/cc65/lib</tt> on
308 <item>The value of the environment variable <tt/LD65_LIB/ if it is defined.
309 <item>The value of the environment variable <tt/CC65_LIB/ if it is defined.
310 Please note that use of this environment variable is obsolete and may
311 get removed in future versions.
312 <item>Any directory added with the <tt><ref id="option--lib-path"
313 name="--lib-path"></tt> option on the command line.
317 <sect1>Object file search path<p>
319 The object file search path contains in this order:
322 <item>The current directory.
323 <item>A compiled in directory which is often <tt>/usr/lib/cc65/lib</tt> on
325 <item>The value of the environment variable <tt/LD65_OBJ/ if it is defined.
326 <item>The value of the environment variable <tt/CC65_LIB/ if it is defined.
327 Please note that use of this environment variable is obsolete and may
328 get removed in future versions.
329 <item>Any directory added with the <tt><ref id="option--obj-path"
330 name="--obj-path"></tt> option on the command line.
334 <sect1>Config file search path<p>
336 The config file search path contains in this order:
339 <item>The current directory.
340 <item>A compiled in directory which is often <tt>/usr/lib/cc65/lib</tt> on
342 <item>The value of the environment variable <tt/LD65_CFG/ if it is defined.
343 <item>Any directory added with the <tt><ref id="option--cfg-path"
344 name="--cfg-path"></tt> option on the command line.
349 <sect>Detailed workings<p>
351 The linker does several things when combining object modules:
353 First, the command line is parsed from left to right. For each object file
354 encountered (object files are recognized by a magic word in the header, so
355 the linker does not care about the name), imported and exported
356 identifiers are read from the file and inserted in a table. If a library
357 name is given (libraries are also recognized by a magic word, there are no
358 special naming conventions), all modules in the library are checked if an
359 export from this module would satisfy an import from other modules. All
360 modules where this is the case are marked. If duplicate identifiers are
361 found, the linker issues a warning.
363 This procedure (parsing and reading from left to right) does mean, that a
364 library may only satisfy references for object modules (given directly or from
365 a library) named <em/before/ that library. With the command line
368 ld65 crt0.o clib.lib test.o
371 the module test.o may not contain references to modules in the library
372 clib.lib. If this is the case, you have to change the order of the modules
376 ld65 crt0.o test.o clib.lib
379 Step two is, to read the configuration file, and assign start addresses
380 for the segments and define any linker symbols (see <ref id="config-files"
381 name="Configuration files">).
383 After that, the linker is ready to produce an output file. Before doing that,
384 it checks it's data for consistency. That is, it checks for unresolved
385 externals (if the output format is not relocatable) and for symbol type
386 mismatches (for example a zero page symbol is imported by a module as absolute
389 Step four is, to write the actual target files. In this step, the linker will
390 resolve any expressions contained in the segment data. Circular references are
391 also detected in this step (a symbol may have a circular reference that goes
392 unnoticed if the symbol is not used).
394 Step five is to output a map file with a detailed list of all modules,
395 segments and symbols encountered.
397 And, last step, if you give the <tt><ref id="option-v" name="-v"></tt> switch
398 twice, you get a dump of the segment data. However, this may be quite
399 unreadable if you're not a developer:-)
403 <sect>Configuration files<label id="config-files"><p>
405 Configuration files are used to describe the layout of the output file(s). Two
406 major topics are covered in a config file: The memory layout of the target
407 architecture, and the assignment of segments to memory areas. In addition,
408 several other attributes may be specified.
410 Case is ignored for keywords, that is, section or attribute names, but it is
411 <em/not/ ignored for names and strings.
415 <sect1>Memory areas<p>
417 Memory areas are specified in a <tt/MEMORY/ section. Lets have a look at an
418 example (this one describes the usable memory layout of the C64):
422 RAM1: start = $0800, size = $9800;
423 ROM1: start = $A000, size = $2000;
424 RAM2: start = $C000, size = $1000;
425 ROM2: start = $E000, size = $2000;
429 As you can see, there are two ram areas and two rom areas. The names
430 (before the colon) are arbitrary names that must start with a letter, with
431 the remaining characters being letters or digits. The names of the memory
432 areas are used when assigning segments. As mentioned above, case is
433 significant for these names.
435 The syntax above is used in all sections of the config file. The name
436 (<tt/ROM1/ etc.) is said to be an identifier, the remaining tokens up to the
437 semicolon specify attributes for this identifier. You may use the equal sign
438 to assign values to attributes, and you may use a comma to separate
439 attributes, you may also leave both out. But you <em/must/ use a semicolon to
440 mark the end of the attributes for one identifier. The section above may also
441 have looked like this:
444 # Start of memory section
462 There are of course more attributes for a memory section than just start and
463 size. Start and size are mandatory attributes, that means, each memory area
464 defined <em/must/ have these attributes given (the linker will check that). I
465 will cover other attributes later. As you may have noticed, I've used a
466 comment in the example above. Comments start with a hash mark (`#'), the
467 remainder of the line is ignored if this character is found.
472 Let's assume you have written a program for your trusty old C64, and you would
473 like to run it. For testing purposes, it should run in the <tt/RAM/ area. So
474 we will start to assign segments to memory sections in the <tt/SEGMENTS/
479 CODE: load = RAM1, type = ro;
480 RODATA: load = RAM1, type = ro;
481 DATA: load = RAM1, type = rw;
482 BSS: load = RAM1, type = bss, define = yes;
486 What we are doing here is telling the linker, that all segments go into the
487 <tt/RAM1/ memory area in the order specified in the <tt/SEGMENTS/ section. So
488 the linker will first write the <tt/CODE/ segment, then the <tt/RODATA/
489 segment, then the <tt/DATA/ segment - but it will not write the <tt/BSS/
490 segment. Why? Enter the segment type: For each segment specified, you may also
491 specify a segment attribute. There are four possible segment attributes:
496 bss means that this is an uninitialized segment
497 zp a zeropage segment
500 So, because we specified that the segment with the name BSS is of type bss,
501 the linker knows that this is uninitialized data, and will not write it to an
502 output file. This is an important point: For the assembler, the <tt/BSS/
503 segment has no special meaning. You specify, which segments have the bss
504 attribute when linking. This approach is much more flexible than having one
505 fixed bss segment, and is a result of the design decision to supporting an
506 arbitrary segment count.
508 If you specify "<tt/type = bss/" for a segment, the linker will make sure that
509 this segment does only contain uninitialized data (that is, zeroes), and issue
510 a warning if this is not the case.
512 For a <tt/bss/ type segment to be useful, it must be cleared somehow by your
513 program (this happens usually in the startup code - for example the startup
514 code for cc65 generated programs takes care about that). But how does your
515 code know, where the segment starts, and how big it is? The linker is able to
516 give that information, but you must request it. This is, what we're doing with
517 the "<tt/define = yes/" attribute in the <tt/BSS/ definitions. For each
518 segment, where this attribute is true, the linker will export three symbols.
521 __NAME_LOAD__ This is set to the address where the
523 __NAME_RUN__ This is set to the run address of the
524 segment. We will cover run addresses
526 __NAME_SIZE__ This is set to the segment size.
529 Replace <tt/NAME/ by the name of the segment, in the example above, this would
530 be <tt/BSS/. These symbols may be accessed by your code.
532 Now, as we've configured the linker to write the first three segments and
533 create symbols for the last one, there's only one question left: Where does
534 the linker put the data? It would be very convenient to have the data in a
537 <sect1>Output files<p>
539 We don't have any files specified above, and indeed, this is not needed in a
540 simple configuration like the one above. There is an additional attribute
541 "file" that may be specified for a memory area, that gives a file name to
542 write the area data into. If there is no file name given, the linker will
543 assign the default file name. This is "a.out" or the one given with the
544 <tt><ref id="option-o" name="-o"></tt> option on the command line. Since the
545 default behaviour is ok for our purposes, I did not use the attribute in the
546 example above. Let's have a look at it now.
548 The "file" attribute (the keyword may also be written as "FILE" if you like
549 that better) takes a string enclosed in double quotes (`"') that specifies the
550 file, where the data is written. You may specify the same file several times,
551 in that case the data for all memory areas having this file name is written
552 into this file, in the order of the memory areas defined in the <tt/MEMORY/
553 section. Let's specify some file names in the <tt/MEMORY/ section used above:
557 RAM1: start = $0800, size = $9800, file = %O;
558 ROM1: start = $A000, size = $2000, file = "rom1.bin";
559 RAM2: start = $C000, size = $1000, file = %O;
560 ROM2: start = $E000, size = $2000, file = "rom2.bin";
564 The <tt/%O/ used here is a way to specify the default behaviour explicitly:
565 <tt/%O/ is replaced by a string (including the quotes) that contains the
566 default output name, that is, "a.out" or the name specified with the <tt><ref
567 id="option-o" name="-o"></tt> option on the command line. Into this file, the
568 linker will first write any segments that go into <tt/RAM1/, and will append
569 then the segments for <tt/RAM2/, because the memory areas are given in this
570 order. So, for the RAM areas, nothing has really changed.
572 We've not used the ROM areas, but we will do that below, so we give the file
573 names here. Segments that go into <tt/ROM1/ will be written to a file named
574 "rom1.bin", and segments that go into <tt/ROM2/ will be written to a file
575 named "rom2.bin". The name given on the command line is ignored in both cases.
577 Assigning an empty file name for a memory area will discard the data written
578 to it. This is useful, if the a memory area has segments assigned that are
579 empty (for example because they are of type bss). In that case, the linker
580 will create an empty output file. This may be suppressed by assigning an empty
581 file name to that memory area.
584 <sect1>LOAD and RUN addresses (ROMable code)<p>
586 Let us look now at a more complex example. Say, you've successfully tested
587 your new "Super Operating System" (SOS for short) for the C64, and you
588 will now go and replace the ROMs by your own code. When doing that, you
589 face a new problem: If the code runs in RAM, we need not to care about
590 read/write data. But now, if the code is in ROM, we must care about it.
591 Remember the default segments (you may of course specify your own):
595 RODATA read only data
597 BSS uninitialized data, read/write
600 Since <tt/BSS/ is not initialized, we must not care about it now, but what
601 about <tt/DATA/? <tt/DATA/ contains initialized data, that is, data that was
602 explicitly assigned a value. And your program will rely on these values on
603 startup. Since there's no other way to remember the contents of the data
604 segment, than storing it into one of the ROMs, we have to put it there. But
605 unfortunately, ROM is not writable, so we have to copy it into RAM before
606 running the actual code.
608 The linker cannot help you copying the data from ROM into RAM (this must be
609 done by the startup code of your program), but it has some features that will
610 help you in this process.
612 First, you may not only specify a "<tt/load/" attribute for a segment, but
613 also a "<tt/run/" attribute. The "<tt/load/" attribute is mandatory, and, if
614 you don't specify a "<tt/run/" attribute, the linker assumes that load area
615 and run area are the same. We will use this feature for our data area:
619 CODE: load = ROM1, type = ro;
620 RODATA: load = ROM2, type = ro;
621 DATA: load = ROM2, run = RAM2, type = rw, define = yes;
622 BSS: load = RAM2, type = bss, define = yes;
626 Let's have a closer look at this <tt/SEGMENTS/ section. We specify that the
627 <tt/CODE/ segment goes into <tt/ROM1/ (the one at $A000). The readonly data
628 goes into <tt/ROM2/. Read/write data will be loaded into <tt/ROM2/ but is run
629 in <tt/RAM2/. That means that all references to labels in the <tt/DATA/
630 segment are relocated to be in <tt/RAM2/, but the segment is written to
631 <tt/ROM2/. All your startup code has to do is, to copy the data from it's
632 location in <tt/ROM2/ to the final location in <tt/RAM2/.
634 So, how do you know, where the data is located? This is the second point,
635 where you get help from the linker. Remember the "<tt/define/" attribute?
636 Since we have set this attribute to true, the linker will define three
637 external symbols for the data segment that may be accessed from your code:
640 __DATA_LOAD__ This is set to the address where the segment
641 is loaded, in this case, it is an address in
643 __DATA_RUN__ This is set to the run address of the segment,
644 in this case, it is an address in RAM2.
645 __DATA_SIZE__ This is set to the segment size.
648 So, what your startup code must do, is to copy <tt/__DATA_SIZE__/ bytes from
649 <tt/__DATA_LOAD__/ to <tt/__DATA_RUN__/ before any other routines are called.
650 All references to labels in the <tt/DATA/ segment are relocated to <tt/RAM2/
651 by the linker, so things will work properly.
654 <sect1>Other MEMORY area attributes<p>
656 There are some other attributes not covered above. Before starting the
657 reference section, I will discuss the remaining things here.
659 You may request symbols definitions also for memory areas. This may be
660 useful for things like a software stack, or an i/o area.
664 STACK: start = $C000, size = $1000, define = yes;
668 This will define three external symbols that may be used in your code:
671 __STACK_START__ This is set to the start of the memory
672 area, $C000 in this example.
673 __STACK_SIZE__ The size of the area, here $1000.
674 __STACK_LAST__ This is NOT the same as START+SIZE.
675 Instead, it it defined as the first
676 address that is not used by data. If we
677 don't define any segments for this area,
678 the value will be the same as START.
681 A memory section may also have a type. Valid types are
684 ro for readonly memory
685 rw for read/write memory.
688 The linker will assure, that no segment marked as read/write or bss is put
689 into a memory area that is marked as readonly.
691 Unused memory in a memory area may be filled. Use the "<tt/fill = yes/"
692 attribute to request this. The default value to fill unused space is zero. If
693 you don't like this, you may specify a byte value that is used to fill these
694 areas with the "<tt/fillval/" attribute. This value is also used to fill unfilled
695 areas generated by the assemblers <tt/.ALIGN/ and <tt/.RES/ directives.
697 The symbol <tt/%S/ may be used to access the default start address (that is,
698 the one defined in the <ref id="FEATURES" name="FEATURES"> section, or the
699 value given on the command line with the <tt><ref id="option-S" name="-S"></tt>
703 <sect1>Other SEGMENT attributes<p>
705 Segments may be aligned to some memory boundary. Specify "<tt/align = num/" to
706 request this feature. Num must be a power of two. To align all segments on a
711 CODE: load = ROM1, type = ro, align = $100;
712 RODATA: load = ROM2, type = ro, align = $100;
713 DATA: load = ROM2, run = RAM2, type = rw, define = yes,
715 BSS: load = RAM2, type = bss, define = yes, align = $100;
719 If an alignment is requested, the linker will add enough space to the output
720 file, so that the new segment starts at an address that is dividable by the
721 given number without a remainder. All addresses are adjusted accordingly. To
722 fill the unused space, bytes of zero are used, or, if the memory area has a
723 "<tt/fillval/" attribute, that value. Alignment is always needed, if you have
724 used the <tt/.ALIGN/ command in the assembler. The alignment of a segment
725 must be equal or greater than the alignment used in the <tt/.ALIGN/ command.
726 The linker will check that, and issue a warning, if the alignment of a segment
727 is lower than the alignment requested in an <tt/.ALIGN/ command of one of the
728 modules making up this segment.
730 For a given segment you may also specify a fixed offset into a memory area or
731 a fixed start address. Use this if you want the code to run at a specific
732 address (a prominent case is the interrupt vector table which must go at
733 address $FFFA). Only one of <tt/ALIGN/ or <tt/OFFSET/ or <tt/START/ may be
734 specified. If the directive creates empty space, it will be filled with zero,
735 of with the value specified with the "<tt/fillval/" attribute if one is given.
736 The linker will warn you if it is not possible to put the code at the
737 specified offset (this may happen if other segments in this area are too
738 large). Here's an example:
742 VECTORS: load = ROM2, type = ro, start = $FFFA;
746 or (for the segment definitions from above)
750 VECTORS: load = ROM2, type = ro, offset = $1FFA;
754 The "<tt/align/", "<tt/start/" and "<tt/offset/" attributes change placement
755 of the segment in the run memory area, because this is what is usually
756 desired. If load and run memory areas are equal (which is the case if only the
757 load memory area has been specified), the attributes will also work. There is
758 also an "<tt/align_load/" attribute that may be used to align the start of the
759 segment in the load memory area, in case different load and run areas have
760 been specified. There are no special attributes to set start or offset for
761 just the load memory area.
763 To suppress the warning, the linker issues if it encounters a segment that is
764 not found in any of the input files, use "<tt/optional=yes/" as additional
765 segment attribute. Be careful when using this attribute, because a missing
766 segment may be a sign of a problem, and if you're suppressing the warning,
767 there is no one left to tell you about it.
769 <sect1>The FILES section<p>
771 The <tt/FILES/ section is used to support other formats than straight binary
772 (which is the default, so binary output files do not need an explicit entry
773 in the <tt/FILES/ section).
775 The <tt/FILES/ section lists output files and as only attribute the format of
776 each output file. Assigning binary format to the default output file would
785 The only other available output format is the o65 format specified by Andre
786 Fachat (see the <htmlurl url="http://www.6502.org/users/andre/o65/fileformat.html"
787 name="6502 binary relocation format specification">). It is defined like this:
795 The necessary o65 attributes are defined in a special section labeled
800 <sect1>The FORMAT section<p>
802 The <tt/FORMAT/ section is used to describe file formats. The default (binary)
803 format has currently no attributes, so, while it may be listed in this
804 section, the attribute list is empty. The second supported format, o65, has
805 several attributes that may be defined here.
809 o65: os = lunix, version = 0, type = small,
810 import = LUNIXKERNEL,
817 <sect1>The FEATURES section<label id="FEATURES"><p>
819 In addition to the <tt/MEMORY/ and <tt/SEGMENTS/ sections described above, the
820 linker has features that may be enabled by an additional section labeled
824 <sect2>The CONDES feature<p>
826 <tt/CONDES/ is used to tell the linker to emit module constructor/destructor
831 CONDES: segment = RODATA,
833 label = __CONSTRUCTOR_TABLE__,
834 count = __CONSTRUCTOR_COUNT__;
838 The <tt/CONDES/ feature has several attributes:
842 <tag><tt>segment</tt></tag>
844 This attribute tells the linker into which segment the table should be
845 placed. If the segment does not exist, it is created.
848 <tag><tt>type</tt></tag>
850 Describes the type of the routines to place in the table. Type may be one of
851 the predefined types <tt/constructor/, <tt/destructor/, <tt/interruptor/, or
852 a numeric value between 0 and 6.
855 <tag><tt>label</tt></tag>
857 This specifies the label to use for the table. The label points to the start
858 of the table in memory and may be used from within user written code.
861 <tag><tt>count</tt></tag>
863 This is an optional attribute. If specified, an additional symbol is defined
864 by the linker using the given name. The value of this symbol is the number
865 of entries (<em/not/ bytes) in the table. While this attribute is optional,
866 it is often useful to define it.
869 <tag><tt>order</tt></tag>
871 Optional attribute that takes one of the keywords <tt/increasing/ or
872 <tt/decreasing/ as an argument. Specifies the sorting order of the entries
873 within the table. The default is <tt/increasing/, which means that the
874 entries are sorted with increasing priority (the first entry has the lowest
875 priority). "Priority" is the priority specified when declaring a symbol as
876 <tt/.CONDES/ with the assembler, higher values mean higher priority. You may
877 change this behaviour by specifying <tt/decreasing/ as the argument, the
878 order of entries is reversed in this case.
880 Please note that the order of entries with equal priority is undefined.
884 Without specifying the <tt/CONDES/ feature, the linker will not create any
885 tables, even if there are <tt/condes/ entries in the object files.
887 For more information see the <tt/.CONDES/ command in the <htmlurl
888 url="ca65.html" name="ca65 manual">.
891 <sect2>The STARTADDRESS feature<p>
893 <tt/STARTADDRESS/ is used to set the default value for the start address,
894 which can be referenced by the <tt/%S/ symbol. The builtin default for the
895 linker is $200.
899 # Default start address is $1000
900 STARTADDRESS: default = $1000;
904 Please note that order is important: The default start address must be defined
905 <em/before/ the <tt/%S/ symbol is used in the config file. This does usually
906 mean, that the <tt/FEATURES/ section has to go to the top of the config file.
910 <sect1>The SYMBOLS section<label id="SYMBOLS"><p>
912 The configuration file may also be used to define symbols used in the link
913 stage. The mandatory attribute for a symbol is its value. A second, boolean
914 attribute named <tt/weak/ is available. If a symbol is marked as weak, it may
915 be overridden by defining a symbol of the same name from the command line. The
916 default for symbols is that they're strong, which means that an attempt to
917 define a symbol with the same name from the command line will lead to an
920 The following example defines the stack size for an application, but allows
921 the programmer to override the value by specifying <tt/--define
922 __STACKSIZE__=xxx/ on the command line.
926 # Define the stack size for the application
927 __STACKSIZE__: value = $800, weak = yes;
933 <sect1>Builtin configurations<p>
935 The builtin configurations are part of the linker source. They are also
936 distributed together with the machine specific binary packages (usually in the
937 doc directory) and don't have a special format. So if you need a special
938 configuration, it's a good idea to start with the builtin configuration for
939 your system. In a first step, just replace <tt/-t target/ by <tt/-C
940 configfile/. Then go on and modify the config file to suit your needs.
944 <sect>Special segments<p>
946 The builtin config files do contain segments that have a special meaning for
947 the compiler and the libraries that come with it. If you replace the builtin
948 config files, you will need the following information.
952 The INIT segment is used for initialization code that may be reused once
953 execution reaches main() - provided that the program runs in RAM. You
954 may for example add the INIT segment to the heap in really memory
959 For the LOWCODE segment, it is guaranteed that it won't be banked out, so it
960 is reachable at any time by interrupt handlers or similar.
964 This segment contains the startup code which initializes the C software stack
965 and the libraries. It is placed in its own segment because it needs to be
966 loaded at the lowest possible program address on several platforms.
970 This segment defines the location of the memory heap used by the malloc
975 <sect>Bugs/Feedback<p>
977 If you have problems using the linker, if you find any bugs, or if you're
978 doing something interesting with it, I would be glad to hear from you. Feel
979 free to contact me by email (<htmlurl url="mailto:uz@cc65.org"
980 name="uz@cc65.org">).
986 ld65 (and all cc65 binutils) are (C) Copyright 1998-2005 Ullrich von
987 Bassewitz. For usage of the binaries and/or sources the following
990 This software is provided 'as-is', without any expressed or implied
991 warranty. In no event will the authors be held liable for any damages
992 arising from the use of this software.
994 Permission is granted to anyone to use this software for any purpose,
995 including commercial applications, and to alter it and redistribute it
996 freely, subject to the following restrictions:
999 <item> The origin of this software must not be misrepresented; you must not
1000 claim that you wrote the original software. If you use this software
1001 in a product, an acknowledgment in the product documentation would be
1002 appreciated but is not required.
1003 <item> Altered source versions must be plainly marked as such, and must not
1004 be misrepresented as being the original software.
1005 <item> This notice may not be removed or altered from any source