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)
172 There are a few more targets defined but neither of them is actually
176 <label id="option-v">
177 <tag><tt>-v, --verbose</tt></tag>
179 Using the -v option, you may enable more output that may help you to
180 locate problems. If an undefined symbol is encountered, -v causes the
181 linker to print a detailed list of the references (that is, source file
182 and line) for this symbol.
185 <tag><tt>-vm</tt></tag>
187 Must be used in conjunction with <tt><ref id="option-m" name="-m"></tt>
188 (generate map file). Normally the map file will not include empty segments
189 and sections, or unreferenced symbols. Using this option, you can force the
190 linker to include all this information into the map file.
193 <label id="option-C">
194 <tag><tt>-C</tt></tag>
196 This gives the name of an output config file to use. See section 4 for more
197 information about config files. -C may not be used together with <tt><ref
198 id="option-t" name="-t"></tt>.
201 <label id="option-D">
202 <tag><tt>-D sym=value, --define sym=value</tt></tag>
204 This option allows to define an external symbol on the command line. Value
205 may start with a '$' sign or with <tt/0x/ for hexadecimal values,
206 otherwise a leading zero denotes octal values. See also the <ref
207 id="SYMBOLS" name="SYMBOLS section"> in the configuration file.
210 <label id="option--lib-path">
211 <tag><tt>-L path, --lib-path path</tt></tag>
213 Specify a library search path. This option may be used more than once. It
214 adds a directory to the search path for library files. Libraries specified
215 without a path are searched in current directory, in the directory given in
216 the <tt/LD65_LIB/ environment variable, and in the list of directories
217 specified using <tt/--lib-path/.
220 <tag><tt>-Ln</tt></tag>
222 This option allows you to create a file that contains all global labels and
223 may be loaded into VICE emulator using the <tt/ll/ (load label) command. You
224 may use this to debug your code with VICE. Note: Older versions had some
225 bugs in the label code. If you have problems, please get the latest VICE
229 <label id="option-S">
230 <tag><tt>-S addr, --start-addr addr</tt></tag>
232 Using -S you may define the default starting address. If and how this
233 address is used depends on the config file in use. For the builtin
234 configurations, only the "none", "apple2" and "apple2enh" systems honor an
235 explicit start address, all other builtin config provide their own.
238 <tag><tt>-V, --version</tt></tag>
240 This option print the version number of the linker. If you send any
241 suggestions or bugfixes, please include this number.
244 <label id="option--cfg-path">
245 <tag><tt>--cfg-path path</tt></tag>
247 Specify a config file search path. This option may be used more than once.
248 It adds a directory to the search path for config files. A config file given
249 with the <tt><ref id="option-C" name="-C"></tt> option that has no path in
250 its name is searched in the current directory, in the directory given in the
251 <tt/LD65_CFG/ environment variable, and in the list of directories specified
252 using <tt/--cfg-path/.
255 <label id="option--dbgfile">
256 <tag><tt>--dbgfile name</tt></tag>
258 Specify an output file for debug information. Available information will be
259 written to this file. Using the <tt/-g/ option for the compiler and assembler
260 will increase the amount of information available. Please note that debug
261 information generation is currently being developed, so the format of the
262 file and it's contents are subject to change without further notice.
265 <tag><tt>--lib file</tt></tag>
267 Links a library to the output. Use this command line option instead of just
268 naming the library file, if the linker is not able to determine the file
269 type because of an unusual extension.
272 <tag><tt>--obj file</tt></tag>
274 Links an object file to the output. Use this command line option instead
275 of just naming the object file, if the linker is not able to determine the
276 file type because of an unusual extension.
279 <label id="option--obj-path">
280 <tag><tt>--obj-path path</tt></tag>
282 Specify an object file search path. This option may be used more than once.
283 It adds a directory to the search path for object files. An object file
284 passed to the linker that has no path in its name is searched in current
285 directory, in the directory given in the <tt/LD65_OBJ/ environment variable,
286 and in the list of directories specified using <tt/--obj-path/.
292 <sect>Search paths<p>
294 Starting with version 2.10 there are now several search paths for files needed
295 by the linker: One for libraries, one for object files and one for config
299 <sect1>Library search path<p>
301 The library search path contains in this order:
304 <item>The current directory.
305 <item>A compiled in library path which is often <tt>/usr/lib/cc65/lib</tt> on
307 <item>The value of the environment variable <tt/LD65_LIB/ if it is defined.
308 <item>The value of the environment variable <tt/CC65_LIB/ if it is defined.
309 Please note that use of this environment variable is obsolete and may
310 get removed in future versions.
311 <item>Any directory added with the <tt><ref id="option--lib-path"
312 name="--lib-path"></tt> option on the command line.
316 <sect1>Object file search path<p>
318 The object file search path contains in this order:
321 <item>The current directory.
322 <item>A compiled in directory which is often <tt>/usr/lib/cc65/lib</tt> on
324 <item>The value of the environment variable <tt/LD65_OBJ/ if it is defined.
325 <item>The value of the environment variable <tt/CC65_LIB/ if it is defined.
326 Please note that use of this environment variable is obsolete and may
327 get removed in future versions.
328 <item>Any directory added with the <tt><ref id="option--obj-path"
329 name="--obj-path"></tt> option on the command line.
333 <sect1>Config file search path<p>
335 The config file search path contains in this order:
338 <item>The current directory.
339 <item>A compiled in directory which is often <tt>/usr/lib/cc65/lib</tt> on
341 <item>The value of the environment variable <tt/LD65_CFG/ if it is defined.
342 <item>Any directory added with the <tt><ref id="option--cfg-path"
343 name="--cfg-path"></tt> option on the command line.
348 <sect>Detailed workings<p>
350 The linker does several things when combining object modules:
352 First, the command line is parsed from left to right. For each object file
353 encountered (object files are recognized by a magic word in the header, so
354 the linker does not care about the name), imported and exported
355 identifiers are read from the file and inserted in a table. If a library
356 name is given (libraries are also recognized by a magic word, there are no
357 special naming conventions), all modules in the library are checked if an
358 export from this module would satisfy an import from other modules. All
359 modules where this is the case are marked. If duplicate identifiers are
360 found, the linker issues a warning.
362 This procedure (parsing and reading from left to right) does mean, that a
363 library may only satisfy references for object modules (given directly or from
364 a library) named <em/before/ that library. With the command line
367 ld65 crt0.o clib.lib test.o
370 the module test.o may not contain references to modules in the library
371 clib.lib. If this is the case, you have to change the order of the modules
375 ld65 crt0.o test.o clib.lib
378 Step two is, to read the configuration file, and assign start addresses
379 for the segments and define any linker symbols (see <ref id="config-files"
380 name="Configuration files">).
382 After that, the linker is ready to produce an output file. Before doing that,
383 it checks it's data for consistency. That is, it checks for unresolved
384 externals (if the output format is not relocatable) and for symbol type
385 mismatches (for example a zero page symbol is imported by a module as absolute
388 Step four is, to write the actual target files. In this step, the linker will
389 resolve any expressions contained in the segment data. Circular references are
390 also detected in this step (a symbol may have a circular reference that goes
391 unnoticed if the symbol is not used).
393 Step five is to output a map file with a detailed list of all modules,
394 segments and symbols encountered.
396 And, last step, if you give the <tt><ref id="option-v" name="-v"></tt> switch
397 twice, you get a dump of the segment data. However, this may be quite
398 unreadable if you're not a developer:-)
402 <sect>Configuration files<label id="config-files"><p>
404 Configuration files are used to describe the layout of the output file(s). Two
405 major topics are covered in a config file: The memory layout of the target
406 architecture, and the assignment of segments to memory areas. In addition,
407 several other attributes may be specified.
409 Case is ignored for keywords, that is, section or attribute names, but it is
410 <em/not/ ignored for names and strings.
414 <sect1>Memory areas<p>
416 Memory areas are specified in a <tt/MEMORY/ section. Lets have a look at an
417 example (this one describes the usable memory layout of the C64):
421 RAM1: start = $0800, size = $9800;
422 ROM1: start = $A000, size = $2000;
423 RAM2: start = $C000, size = $1000;
424 ROM2: start = $E000, size = $2000;
428 As you can see, there are two ram areas and two rom areas. The names
429 (before the colon) are arbitrary names that must start with a letter, with
430 the remaining characters being letters or digits. The names of the memory
431 areas are used when assigning segments. As mentioned above, case is
432 significant for these names.
434 The syntax above is used in all sections of the config file. The name
435 (<tt/ROM1/ etc.) is said to be an identifier, the remaining tokens up to the
436 semicolon specify attributes for this identifier. You may use the equal sign
437 to assign values to attributes, and you may use a comma to separate
438 attributes, you may also leave both out. But you <em/must/ use a semicolon to
439 mark the end of the attributes for one identifier. The section above may also
440 have looked like this:
443 # Start of memory section
461 There are of course more attributes for a memory section than just start and
462 size. Start and size are mandatory attributes, that means, each memory area
463 defined <em/must/ have these attributes given (the linker will check that). I
464 will cover other attributes later. As you may have noticed, I've used a
465 comment in the example above. Comments start with a hash mark (`#'), the
466 remainder of the line is ignored if this character is found.
471 Let's assume you have written a program for your trusty old C64, and you would
472 like to run it. For testing purposes, it should run in the <tt/RAM/ area. So
473 we will start to assign segments to memory sections in the <tt/SEGMENTS/
478 CODE: load = RAM1, type = ro;
479 RODATA: load = RAM1, type = ro;
480 DATA: load = RAM1, type = rw;
481 BSS: load = RAM1, type = bss, define = yes;
485 What we are doing here is telling the linker, that all segments go into the
486 <tt/RAM1/ memory area in the order specified in the <tt/SEGMENTS/ section. So
487 the linker will first write the <tt/CODE/ segment, then the <tt/RODATA/
488 segment, then the <tt/DATA/ segment - but it will not write the <tt/BSS/
489 segment. Why? Enter the segment type: For each segment specified, you may also
490 specify a segment attribute. There are four possible segment attributes:
495 bss means that this is an uninitialized segment
496 zp a zeropage segment
499 So, because we specified that the segment with the name BSS is of type bss,
500 the linker knows that this is uninitialized data, and will not write it to an
501 output file. This is an important point: For the assembler, the <tt/BSS/
502 segment has no special meaning. You specify, which segments have the bss
503 attribute when linking. This approach is much more flexible than having one
504 fixed bss segment, and is a result of the design decision to supporting an
505 arbitrary segment count.
507 If you specify "<tt/type = bss/" for a segment, the linker will make sure that
508 this segment does only contain uninitialized data (that is, zeroes), and issue
509 a warning if this is not the case.
511 For a <tt/bss/ type segment to be useful, it must be cleared somehow by your
512 program (this happens usually in the startup code - for example the startup
513 code for cc65 generated programs takes care about that). But how does your
514 code know, where the segment starts, and how big it is? The linker is able to
515 give that information, but you must request it. This is, what we're doing with
516 the "<tt/define = yes/" attribute in the <tt/BSS/ definitions. For each
517 segment, where this attribute is true, the linker will export three symbols.
520 __NAME_LOAD__ This is set to the address where the
522 __NAME_RUN__ This is set to the run address of the
523 segment. We will cover run addresses
525 __NAME_SIZE__ This is set to the segment size.
528 Replace <tt/NAME/ by the name of the segment, in the example above, this would
529 be <tt/BSS/. These symbols may be accessed by your code.
531 Now, as we've configured the linker to write the first three segments and
532 create symbols for the last one, there's only one question left: Where does
533 the linker put the data? It would be very convenient to have the data in a
536 <sect1>Output files<p>
538 We don't have any files specified above, and indeed, this is not needed in a
539 simple configuration like the one above. There is an additional attribute
540 "file" that may be specified for a memory area, that gives a file name to
541 write the area data into. If there is no file name given, the linker will
542 assign the default file name. This is "a.out" or the one given with the
543 <tt><ref id="option-o" name="-o"></tt> option on the command line. Since the
544 default behaviour is ok for our purposes, I did not use the attribute in the
545 example above. Let's have a look at it now.
547 The "file" attribute (the keyword may also be written as "FILE" if you like
548 that better) takes a string enclosed in double quotes (`"') that specifies the
549 file, where the data is written. You may specify the same file several times,
550 in that case the data for all memory areas having this file name is written
551 into this file, in the order of the memory areas defined in the <tt/MEMORY/
552 section. Let's specify some file names in the <tt/MEMORY/ section used above:
556 RAM1: start = $0800, size = $9800, file = %O;
557 ROM1: start = $A000, size = $2000, file = "rom1.bin";
558 RAM2: start = $C000, size = $1000, file = %O;
559 ROM2: start = $E000, size = $2000, file = "rom2.bin";
563 The <tt/%O/ used here is a way to specify the default behaviour explicitly:
564 <tt/%O/ is replaced by a string (including the quotes) that contains the
565 default output name, that is, "a.out" or the name specified with the <tt><ref
566 id="option-o" name="-o"></tt> option on the command line. Into this file, the
567 linker will first write any segments that go into <tt/RAM1/, and will append
568 then the segments for <tt/RAM2/, because the memory areas are given in this
569 order. So, for the RAM areas, nothing has really changed.
571 We've not used the ROM areas, but we will do that below, so we give the file
572 names here. Segments that go into <tt/ROM1/ will be written to a file named
573 "rom1.bin", and segments that go into <tt/ROM2/ will be written to a file
574 named "rom2.bin". The name given on the command line is ignored in both cases.
576 Assigning an empty file name for a memory area will discard the data written
577 to it. This is useful, if the a memory area has segments assigned that are
578 empty (for example because they are of type bss). In that case, the linker
579 will create an empty output file. This may be suppressed by assigning an empty
580 file name to that memory area.
583 <sect1>LOAD and RUN addresses (ROMable code)<p>
585 Let us look now at a more complex example. Say, you've successfully tested
586 your new "Super Operating System" (SOS for short) for the C64, and you
587 will now go and replace the ROMs by your own code. When doing that, you
588 face a new problem: If the code runs in RAM, we need not to care about
589 read/write data. But now, if the code is in ROM, we must care about it.
590 Remember the default segments (you may of course specify your own):
594 RODATA read only data
596 BSS uninitialized data, read/write
599 Since <tt/BSS/ is not initialized, we must not care about it now, but what
600 about <tt/DATA/? <tt/DATA/ contains initialized data, that is, data that was
601 explicitly assigned a value. And your program will rely on these values on
602 startup. Since there's no other way to remember the contents of the data
603 segment, than storing it into one of the ROMs, we have to put it there. But
604 unfortunately, ROM is not writable, so we have to copy it into RAM before
605 running the actual code.
607 The linker cannot help you copying the data from ROM into RAM (this must be
608 done by the startup code of your program), but it has some features that will
609 help you in this process.
611 First, you may not only specify a "<tt/load/" attribute for a segment, but
612 also a "<tt/run/" attribute. The "<tt/load/" attribute is mandatory, and, if
613 you don't specify a "<tt/run/" attribute, the linker assumes that load area
614 and run area are the same. We will use this feature for our data area:
618 CODE: load = ROM1, type = ro;
619 RODATA: load = ROM2, type = ro;
620 DATA: load = ROM2, run = RAM2, type = rw, define = yes;
621 BSS: load = RAM2, type = bss, define = yes;
625 Let's have a closer look at this <tt/SEGMENTS/ section. We specify that the
626 <tt/CODE/ segment goes into <tt/ROM1/ (the one at $A000). The readonly data
627 goes into <tt/ROM2/. Read/write data will be loaded into <tt/ROM2/ but is run
628 in <tt/RAM2/. That means that all references to labels in the <tt/DATA/
629 segment are relocated to be in <tt/RAM2/, but the segment is written to
630 <tt/ROM2/. All your startup code has to do is, to copy the data from it's
631 location in <tt/ROM2/ to the final location in <tt/RAM2/.
633 So, how do you know, where the data is located? This is the second point,
634 where you get help from the linker. Remember the "<tt/define/" attribute?
635 Since we have set this attribute to true, the linker will define three
636 external symbols for the data segment that may be accessed from your code:
639 __DATA_LOAD__ This is set to the address where the segment
640 is loaded, in this case, it is an address in
642 __DATA_RUN__ This is set to the run address of the segment,
643 in this case, it is an address in RAM2.
644 __DATA_SIZE__ This is set to the segment size.
647 So, what your startup code must do, is to copy <tt/__DATA_SIZE__/ bytes from
648 <tt/__DATA_LOAD__/ to <tt/__DATA_RUN__/ before any other routines are called.
649 All references to labels in the <tt/DATA/ segment are relocated to <tt/RAM2/
650 by the linker, so things will work properly.
653 <sect1>Other MEMORY area attributes<p>
655 There are some other attributes not covered above. Before starting the
656 reference section, I will discuss the remaining things here.
658 You may request symbols definitions also for memory areas. This may be
659 useful for things like a software stack, or an i/o area.
663 STACK: start = $C000, size = $1000, define = yes;
667 This will define three external symbols that may be used in your code:
670 __STACK_START__ This is set to the start of the memory
671 area, $C000 in this example.
672 __STACK_SIZE__ The size of the area, here $1000.
673 __STACK_LAST__ This is NOT the same as START+SIZE.
674 Instead, it it defined as the first
675 address that is not used by data. If we
676 don't define any segments for this area,
677 the value will be the same as START.
680 A memory section may also have a type. Valid types are
683 ro for readonly memory
684 rw for read/write memory.
687 The linker will assure, that no segment marked as read/write or bss is put
688 into a memory area that is marked as readonly.
690 Unused memory in a memory area may be filled. Use the "<tt/fill = yes/"
691 attribute to request this. The default value to fill unused space is zero. If
692 you don't like this, you may specify a byte value that is used to fill these
693 areas with the "<tt/fillval/" attribute. This value is also used to fill unfilled
694 areas generated by the assemblers <tt/.ALIGN/ and <tt/.RES/ directives.
696 The symbol <tt/%S/ may be used to access the default start address (that is,
697 the one defined in the <ref id="FEATURES" name="FEATURES"> section, or the
698 value given on the command line with the <tt><ref id="option-S" name="-S"></tt>
702 <sect1>Other SEGMENT attributes<p>
704 Segments may be aligned to some memory boundary. Specify "<tt/align = num/" to
705 request this feature. Num must be a power of two. To align all segments on a
710 CODE: load = ROM1, type = ro, align = $100;
711 RODATA: load = ROM2, type = ro, align = $100;
712 DATA: load = ROM2, run = RAM2, type = rw, define = yes,
714 BSS: load = RAM2, type = bss, define = yes, align = $100;
718 If an alignment is requested, the linker will add enough space to the output
719 file, so that the new segment starts at an address that is divideable by the
720 given number without a remainder. All addresses are adjusted accordingly. To
721 fill the unused space, bytes of zero are used, or, if the memory area has a
722 "<tt/fillval/" attribute, that value. Alignment is always needed, if you have
723 used the <tt/.ALIGN/ command in the assembler. The alignment of a segment
724 must be equal or greater than the alignment used in the <tt/.ALIGN/ command.
725 The linker will check that, and issue a warning, if the alignment of a segment
726 is lower than the alignment requested in an <tt/.ALIGN/ command of one of the
727 modules making up this segment.
729 For a given segment you may also specify a fixed offset into a memory area or
730 a fixed start address. Use this if you want the code to run at a specific
731 address (a prominent case is the interrupt vector table which must go at
732 address $FFFA). Only one of <tt/ALIGN/ or <tt/OFFSET/ or <tt/START/ may be
733 specified. If the directive creates empty space, it will be filled with zero,
734 of with the value specified with the "<tt/fillval/" attribute if one is given.
735 The linker will warn you if it is not possible to put the code at the
736 specified offset (this may happen if other segments in this area are too
737 large). Here's an example:
741 VECTORS: load = ROM2, type = ro, start = $FFFA;
745 or (for the segment definitions from above)
749 VECTORS: load = ROM2, type = ro, offset = $1FFA;
753 The "<tt/align/", "<tt/start/" and "<tt/offset/" attributes change placement
754 of the segment in the run memory area, because this is what is usually
755 desired. If load and run memory areas are equal (which is the case if only the
756 load memory area has been specified), the attributes will also work. There is
757 also a "<tt/align_load/" attribute that may be used to align the start of the
758 segment in the load memory area, in case different load and run areas have
759 been specified. There are no special attributes to set start or offset for
760 just the load memory area.
762 To suppress the warning, the linker issues if it encounters a segment that is
763 not found in any of the input files, use "<tt/optional=yes/" as additional
764 segment attribute. Be careful when using this attribute, because a missing
765 segment may be a sign of a problem, and if you're suppressing the warning,
766 there is no one left to tell you about it.
768 <sect1>The FILES section<p>
770 The <tt/FILES/ section is used to support other formats than straight binary
771 (which is the default, so binary output files do not need an explicit entry
772 in the <tt/FILES/ section).
774 The <tt/FILES/ section lists output files and as only attribute the format of
775 each output file. Assigning binary format to the default output file would
784 The only other available output format is the o65 format specified by Andre
785 Fachat (see the <htmlurl url="http://www.6502.org/users/andre/o65/fileformat.html"
786 name="6502 binary relocation format specification">). It is defined like this:
794 The necessary o65 attributes are defined in a special section labeled
799 <sect1>The FORMAT section<p>
801 The <tt/FORMAT/ section is used to describe file formats. The default (binary)
802 format has currently no attributes, so, while it may be listed in this
803 section, the attribute list is empty. The second supported format, o65, has
804 several attributes that may be defined here.
808 o65: os = lunix, version = 0, type = small,
809 import = LUNIXKERNEL,
816 <sect1>The FEATURES section<label id="FEATURES"><p>
818 In addition to the <tt/MEMORY/ and <tt/SEGMENTS/ sections described above, the
819 linker has features that may be enabled by an additional section labeled
823 <sect2>The CONDES feature<p>
825 <tt/CONDES/ is used to tell the linker to emit module constructor/destructor
830 CONDES: segment = RODATA,
832 label = __CONSTRUCTOR_TABLE__,
833 count = __CONSTRUCTOR_COUNT__;
837 The <tt/CONDES/ feature has several attributes:
841 <tag><tt>segment</tt></tag>
843 This attribute tells the linker into which segment the table should be
844 placed. If the segment does not exist, it is created.
847 <tag><tt>type</tt></tag>
849 Describes the type of the routines to place in the table. Type may be one of
850 the predefined types <tt/constructor/, <tt/destructor/, <tt/interruptor/, or
851 a numeric value between 0 and 6.
854 <tag><tt>label</tt></tag>
856 This specifies the label to use for the table. The label points to the start
857 of the table in memory and may be used from within user written code.
860 <tag><tt>count</tt></tag>
862 This is an optional attribute. If specified, an additional symbol is defined
863 by the linker using the given name. The value of this symbol is the number
864 of entries (<em/not/ bytes) in the table. While this attribute is optional,
865 it is often useful to define it.
868 <tag><tt>order</tt></tag>
870 Optional attribute that takes one of the keywords <tt/increasing/ or
871 <tt/decreasing/ as an argument. Specifies the sorting order of the entries
872 within the table. The default is <tt/increasing/, which means that the
873 entries are sorted with increasing priority (the first entry has the lowest
874 priority). "Priority" is the priority specified when declaring a symbol as
875 <tt/.CONDES/ with the assembler, higher values mean higher priority. You may
876 change this behaviour by specifying <tt/decreasing/ as the argument, the
877 order of entries is reversed in this case.
879 Please note that the order of entries with equal priority is undefined.
883 Without specifying the <tt/CONDES/ feature, the linker will not create any
884 tables, even if there are <tt/condes/ entries in the object files.
886 For more information see the <tt/.CONDES/ command in the <htmlurl
887 url="ca65.html" name="ca65 manual">.
890 <sect2>The STARTADDRESS feature<p>
892 <tt/STARTADDRESS/ is used to set the default value for the start address,
893 which can be referenced by the <tt/%S/ symbol. The builtin default for the
894 linker is $200.
898 # Default start address is $1000
899 STARTADDRESS: default = $1000;
903 Please note that order is important: The default start address must be defined
904 <em/before/ the <tt/%S/ symbol is used in the config file. This does usually
905 mean, that the <tt/FEATURES/ section has to go to the top of the config file.
909 <sect1>The SYMBOLS section<label id="SYMBOLS"><p>
911 The configuration file may also be used to define symbols used in the link
912 stage. The mandatory attribute for a symbol is its value. A second, boolean
913 attribute named <tt/weak/ is available. If a symbol is marked as weak, it may
914 be overridden by defining a symbol of the same name from the command line. The
915 default for symbols is that they're strong, which means that an attempt to
916 define a symbol with the same name from the command line will lead to an
919 The following example defines the stack size for an application, but allows
920 the programmer to override the value by specifying <tt/--define
921 __STACKSIZE__=xxx/ on the command line.
925 # Define the stack size for the application
926 __STACKSIZE__: value = $800, weak = yes;
932 <sect1>Builtin configurations<p>
934 The builtin configurations are part of the linker source. They are also
935 distributed together with the machine specific binary packages (usually in the
936 doc directory) and don't have a special format. So if you need a special
937 configuration, it's a good idea to start with the builtin configuration for
938 your system. In a first step, just replace <tt/-t target/ by <tt/-C
939 configfile/. Then go on and modify the config file to suit your needs.
943 <sect>Special segments<p>
945 The builtin config files do contain segments that have a special meaning for
946 the compiler and the libraries that come with it. If you replace the builtin
947 config files, you will need the following information.
951 The INIT segment is used for initialization code that may be reused once
952 execution reaches main() - provided that the program runs in RAM. You
953 may for example add the INIT segment to the heap in really memory
958 For the LOWCODE segment, it is guaranteed that it won't be banked out, so it
959 is reachable at any time by interrupt handlers or similar.
963 This segment contains the startup code which initializes the C software stack
964 and the libraries. It is placed in its own segment because it needs to be
965 loaded at the lowest possible program address on several platforms.
969 This segment defines the location of the memory heap used by the malloc
974 <sect>Bugs/Feedback<p>
976 If you have problems using the linker, if you find any bugs, or if you're
977 doing something interesting with it, I would be glad to hear from you. Feel
978 free to contact me by email (<htmlurl url="mailto:uz@cc65.org"
979 name="uz@cc65.org">).
985 ld65 (and all cc65 binutils) are (C) Copyright 1998-2005 Ullrich von
986 Bassewitz. For usage of the binaries and/or sources the following
989 This software is provided 'as-is', without any expressed or implied
990 warranty. In no event will the authors be held liable for any damages
991 arising from the use of this software.
993 Permission is granted to anyone to use this software for any purpose,
994 including commercial applications, and to alter it and redistribute it
995 freely, subject to the following restrictions:
998 <item> The origin of this software must not be misrepresented; you must not
999 claim that you wrote the original software. If you use this software
1000 in a product, an acknowledgment in the product documentation would be
1001 appreciated but is not required.
1002 <item> Altered source versions must be plainly marked as such, and must not
1003 be misrepresented as being the original software.
1004 <item> This notice may not be removed or altered from any source