1 <!doctype linuxdoc system> <!-- -*- text-mode -*- -->
4 <title>ld65 Users Guide
5 <author><url url="mailto:uz@cc65.org" name="Ullrich von Bassewitz">
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
70 -u sym Force an import of symbol `sym'
75 --cfg-path path Specify a config file search path
76 --config name Use linker config file
77 --dbgfile name Generate debug information
78 --define sym=val Define a symbol
79 --end-group End a library group
80 --force-import sym Force an import of symbol `sym'
81 --help Help (this text)
82 --lib file Link this library
83 --lib-path path Specify a library search path
84 --mapfile name Create a map file
85 --module-id id Specify a module id
86 --obj file Link this object file
87 --obj-path path Specify an object file search path
88 --start-addr addr Set the default start address
89 --start-group Start a library group
90 --target sys Set the target system
91 --version Print the linker version
92 ---------------------------------------------------------------------------
96 <sect1>Command line options in detail<p>
98 Here is a description of all the command line options:
102 <label id="option--start-group">
103 <tag><tt>-(, --start-group</tt></tag>
105 Start a library group. The libraries specified within a group are searched
106 multiple times to resolve crossreferences within the libraries. Normally,
107 crossreferences are only resolved within a library, that is the library is
108 searched multiple times. Libraries specified later on the command line
109 cannot reference otherwise unreferenced symbols in libraries specified
110 earlier, because the linker has already handled them. Library groups are
111 a solution for this problem, because the linker will search repeatedly
112 through all libraries specified in the group, until all possible open
113 symbol references have been satisfied.
116 <tag><tt>-), --end-group</tt></tag>
118 End a library group. See the explanation of the <tt><ref
119 id="option--start-group" name="--start-group"></tt> option.
122 <tag><tt>-h, --help</tt></tag>
124 Print the short option summary shown above.
127 <label id="option-m">
128 <tag><tt>-m name, --mapfile name</tt></tag>
130 This option (which needs an argument that will used as a filename for
131 the generated map file) will cause the linker to generate a map file.
132 The map file does contain a detailed overview over the modules used, the
133 sizes for the different segments, and a table containing exported
137 <label id="option-o">
138 <tag><tt>-o name</tt></tag>
140 The -o switch is used to give the name of the default output file.
141 Depending on your output configuration, this name may NOT be used as
142 name for the output file. However, for the default configurations, this
143 name is used for the output file name.
146 <label id="option-t">
147 <tag><tt>-t sys, --target sys</tt></tag>
149 The argument for the -t switch is the name of the target system. Since this
150 switch will activate a default configuration, it may not be used together
151 with the <tt><ref id="option-C" name="-C"></tt> option. The following target
152 systems are currently supported:
162 <item>c16 (works also for the c116 with memory up to 32K)
165 <item>cbm510 (CBM-II series with 40 column video)
166 <item>cbm610 (all CBM series-II computers with 80 column video)
172 <item>pet (all CBM PET systems except the 2001)
180 There are a few more targets defined but neither of them is actually
184 <tag><tt>-u sym[:addrsize], --force-import sym[:addrsize]</tt></tag>
186 Force an import of a symbol. While object files are always linked to the
187 output file, regardless if there are any references, object modules from
188 libraries get only linked in if an import can be satisfied by this module.
189 The <tt/--fore-import/ option may be used to add a reference to a symbol and
190 as a result force linkage of the module that exports the identifier.
192 The name of the symbol may optionally be followed by a colon and an address
193 size specifier. If no address size is specified, the default address size
194 for the target machine is used.
196 Please note that the symbol name needs to have the internal representation,
197 meaning you have to prepend an underline for C identifiers.
200 <label id="option-v">
201 <tag><tt>-v, --verbose</tt></tag>
203 Using the -v option, you may enable more output that may help you to
204 locate problems. If an undefined symbol is encountered, -v causes the
205 linker to print a detailed list of the references (that is, source file
206 and line) for this symbol.
209 <tag><tt>-vm</tt></tag>
211 Must be used in conjunction with <tt><ref id="option-m" name="-m"></tt>
212 (generate map file). Normally the map file will not include empty segments
213 and sections, or unreferenced symbols. Using this option, you can force the
214 linker to include all this information into the map file.
217 <label id="option-C">
218 <tag><tt>-C</tt></tag>
220 This gives the name of an output config file to use. See section 4 for more
221 information about config files. -C may not be used together with <tt><ref
222 id="option-t" name="-t"></tt>.
225 <label id="option-D">
226 <tag><tt>-D sym=value, --define sym=value</tt></tag>
228 This option allows to define an external symbol on the command line. Value
229 may start with a '$' sign or with <tt/0x/ for hexadecimal values,
230 otherwise a leading zero denotes octal values. See also the <ref
231 id="SYMBOLS" name="SYMBOLS section"> in the configuration file.
234 <label id="option--lib-path">
235 <tag><tt>-L path, --lib-path path</tt></tag>
237 Specify a library search path. This option may be used more than once. It
238 adds a directory to the search path for library files. Libraries specified
239 without a path are searched in current directory, in the directory given in
240 the <tt/LD65_LIB/ environment variable, and in the list of directories
241 specified using <tt/--lib-path/.
244 <tag><tt>-Ln</tt></tag>
246 This option allows you to create a file that contains all global labels and
247 may be loaded into VICE emulator using the <tt/ll/ (load label) command. You
248 may use this to debug your code with VICE. Note: Older versions had some
249 bugs in the label code. If you have problems, please get the latest VICE
253 <label id="option-S">
254 <tag><tt>-S addr, --start-addr addr</tt></tag>
256 Using -S you may define the default starting address. If and how this
257 address is used depends on the config file in use. For the default
258 configurations, only the "none", "apple2" and "apple2enh" systems honor an
259 explicit start address, all other default configs provide their own.
262 <tag><tt>-V, --version</tt></tag>
264 This option print the version number of the linker. If you send any
265 suggestions or bugfixes, please include this number.
268 <label id="option--cfg-path">
269 <tag><tt>--cfg-path path</tt></tag>
271 Specify a config file search path. This option may be used more than once.
272 It adds a directory to the search path for config files. A config file given
273 with the <tt><ref id="option-C" name="-C"></tt> option that has no path in
274 its name is searched in the current directory, in the directory given in the
275 <tt/LD65_CFG/ environment variable, and in the list of directories specified
276 using <tt/--cfg-path/.
279 <label id="option--dbgfile">
280 <tag><tt>--dbgfile name</tt></tag>
282 Specify an output file for debug information. Available information will be
283 written to this file. Using the <tt/-g/ option for the compiler and assembler
284 will increase the amount of information available. Please note that debug
285 information generation is currently being developed, so the format of the
286 file and its contents are subject to change without further notice.
289 <tag><tt>--lib file</tt></tag>
291 Links a library to the output. Use this command line option instead of just
292 naming the library file, if the linker is not able to determine the file
293 type because of an unusual extension.
296 <tag><tt>--obj file</tt></tag>
298 Links an object file to the output. Use this command line option instead
299 of just naming the object file, if the linker is not able to determine the
300 file type because of an unusual extension.
303 <label id="option--obj-path">
304 <tag><tt>--obj-path path</tt></tag>
306 Specify an object file search path. This option may be used more than once.
307 It adds a directory to the search path for object files. An object file
308 passed to the linker that has no path in its name is searched in current
309 directory, in the directory given in the <tt/LD65_OBJ/ environment variable,
310 and in the list of directories specified using <tt/--obj-path/.
316 <sect>Search paths<p>
318 Starting with version 2.10, there are now several search-path lists for files needed
319 by the linker: one for libraries, one for object files, and one for config
323 <sect1>Library search path<p>
325 The library search-path list contains in this order:
328 <item>The current directory.
329 <item>Any directory added with the <tt><ref id="option--lib-path"
330 name="--lib-path"></tt> option on the command line.
331 <item>The value of the environment variable <tt/LD65_LIB/ if it is defined.
332 <item>A subdirectory named <tt/lib/ of the directory defined in the environment
333 variable <tt/CC65_HOME/, if it is defined.
334 <item>An optionally compiled-in library path.
338 <sect1>Object file search path<p>
340 The object file search-path list contains in this order:
343 <item>The current directory.
344 <item>Any directory added with the <tt><ref id="option--obj-path"
345 name="--obj-path"></tt> option on the command line.
346 <item>The value of the environment variable <tt/LD65_OBJ/ if it is defined.
347 <item>A subdirectory named <tt/obj/ of the directory defined in the environment
348 variable <tt/CC65_HOME/, if it is defined.
349 <item>An optionally compiled-in directory.
353 <sect1>Config file search path<p>
355 The config file search-path list contains in this order:
358 <item>The current directory.
359 <item>Any directory added with the <tt><ref id="option--cfg-path"
360 name="--cfg-path"></tt> option on the command line.
361 <item>The value of the environment variable <tt/LD65_CFG/ if it is defined.
362 <item>A subdirectory named <tt/cfg/ of the directory defined in the environment
363 variable <tt/CC65_HOME/, if it is defined.
364 <item>An optionally compiled-in directory.
369 <sect>Detailed workings<p>
371 The linker does several things when combining object modules:
373 First, the command line is parsed from left to right. For each object file
374 encountered (object files are recognized by a magic word in the header, so
375 the linker does not care about the name), imported and exported
376 identifiers are read from the file and inserted in a table. If a library
377 name is given (libraries are also recognized by a magic word, there are no
378 special naming conventions), all modules in the library are checked if an
379 export from this module would satisfy an import from other modules. All
380 modules where this is the case are marked. If duplicate identifiers are
381 found, the linker issues a warning.
383 This procedure (parsing and reading from left to right) does mean, that a
384 library may only satisfy references for object modules (given directly or from
385 a library) named <em/before/ that library. With the command line
388 ld65 crt0.o clib.lib test.o
391 the module test.o may not contain references to modules in the library
392 clib.lib. If this is the case, you have to change the order of the modules
396 ld65 crt0.o test.o clib.lib
399 Step two is, to read the configuration file, and assign start addresses
400 for the segments and define any linker symbols (see <ref id="config-files"
401 name="Configuration files">).
403 After that, the linker is ready to produce an output file. Before doing that,
404 it checks its data for consistency. That is, it checks for unresolved
405 externals (if the output format is not relocatable) and for symbol type
406 mismatches (for example a zero page symbol is imported by a module as absolute
409 Step four is, to write the actual target files. In this step, the linker will
410 resolve any expressions contained in the segment data. Circular references are
411 also detected in this step (a symbol may have a circular reference that goes
412 unnoticed if the symbol is not used).
414 Step five is to output a map file with a detailed list of all modules,
415 segments and symbols encountered.
417 And, last step, if you give the <tt><ref id="option-v" name="-v"></tt> switch
418 twice, you get a dump of the segment data. However, this may be quite
419 unreadable if you're not a developer:-)
423 <sect>Configuration files<label id="config-files"><p>
425 Configuration files are used to describe the layout of the output file(s). Two
426 major topics are covered in a config file: The memory layout of the target
427 architecture, and the assignment of segments to memory areas. In addition,
428 several other attributes may be specified.
430 Case is ignored for keywords, that is, section or attribute names, but it is
431 <em/not/ ignored for names and strings.
435 <sect1>Memory areas<p>
437 Memory areas are specified in a <tt/MEMORY/ section. Lets have a look at an
438 example (this one describes the usable memory layout of the C64):
442 RAM1: start = $0800, size = $9800;
443 ROM1: start = $A000, size = $2000;
444 RAM2: start = $C000, size = $1000;
445 ROM2: start = $E000, size = $2000;
449 As you can see, there are two ram areas and two rom areas. The names
450 (before the colon) are arbitrary names that must start with a letter, with
451 the remaining characters being letters or digits. The names of the memory
452 areas are used when assigning segments. As mentioned above, case is
453 significant for these names.
455 The syntax above is used in all sections of the config file. The name
456 (<tt/ROM1/ etc.) is said to be an identifier, the remaining tokens up to the
457 semicolon specify attributes for this identifier. You may use the equal sign
458 to assign values to attributes, and you may use a comma to separate
459 attributes, you may also leave both out. But you <em/must/ use a semicolon to
460 mark the end of the attributes for one identifier. The section above may also
461 have looked like this:
464 # Start of memory section
482 There are of course more attributes for a memory section than just start and
483 size. Start and size are mandatory attributes, that means, each memory area
484 defined <em/must/ have these attributes given (the linker will check that). I
485 will cover other attributes later. As you may have noticed, I've used a
486 comment in the example above. Comments start with a hash mark (`#'), the
487 remainder of the line is ignored if this character is found.
492 Let's assume you have written a program for your trusty old C64, and you would
493 like to run it. For testing purposes, it should run in the <tt/RAM/ area. So
494 we will start to assign segments to memory sections in the <tt/SEGMENTS/
499 CODE: load = RAM1, type = ro;
500 RODATA: load = RAM1, type = ro;
501 DATA: load = RAM1, type = rw;
502 BSS: load = RAM1, type = bss, define = yes;
506 What we are doing here is telling the linker, that all segments go into the
507 <tt/RAM1/ memory area in the order specified in the <tt/SEGMENTS/ section. So
508 the linker will first write the <tt/CODE/ segment, then the <tt/RODATA/
509 segment, then the <tt/DATA/ segment - but it will not write the <tt/BSS/
510 segment. Why? Enter the segment type: For each segment specified, you may also
511 specify a segment attribute. There are four possible segment attributes:
516 bss means that this is an uninitialized segment
517 zp a zeropage segment
520 So, because we specified that the segment with the name BSS is of type bss,
521 the linker knows that this is uninitialized data, and will not write it to an
522 output file. This is an important point: For the assembler, the <tt/BSS/
523 segment has no special meaning. You specify, which segments have the bss
524 attribute when linking. This approach is much more flexible than having one
525 fixed bss segment, and is a result of the design decision to supporting an
526 arbitrary segment count.
528 If you specify "<tt/type = bss/" for a segment, the linker will make sure that
529 this segment does only contain uninitialized data (that is, zeroes), and issue
530 a warning if this is not the case.
532 For a <tt/bss/ type segment to be useful, it must be cleared somehow by your
533 program (this happens usually in the startup code - for example the startup
534 code for cc65 generated programs takes care about that). But how does your
535 code know, where the segment starts, and how big it is? The linker is able to
536 give that information, but you must request it. This is, what we're doing with
537 the "<tt/define = yes/" attribute in the <tt/BSS/ definitions. For each
538 segment, where this attribute is true, the linker will export three symbols.
541 __NAME_LOAD__ This is set to the address where the
543 __NAME_RUN__ This is set to the run address of the
544 segment. We will cover run addresses
546 __NAME_SIZE__ This is set to the segment size.
549 Replace <tt/NAME/ by the name of the segment, in the example above, this would
550 be <tt/BSS/. These symbols may be accessed by your code.
552 Now, as we've configured the linker to write the first three segments and
553 create symbols for the last one, there's only one question left: Where does
554 the linker put the data? It would be very convenient to have the data in a
557 <sect1>Output files<p>
559 We don't have any files specified above, and indeed, this is not needed in a
560 simple configuration like the one above. There is an additional attribute
561 "file" that may be specified for a memory area, that gives a file name to
562 write the area data into. If there is no file name given, the linker will
563 assign the default file name. This is "a.out" or the one given with the
564 <tt><ref id="option-o" name="-o"></tt> option on the command line. Since the
565 default behaviour is ok for our purposes, I did not use the attribute in the
566 example above. Let's have a look at it now.
568 The "file" attribute (the keyword may also be written as "FILE" if you like
569 that better) takes a string enclosed in double quotes (`"') that specifies the
570 file, where the data is written. You may specify the same file several times,
571 in that case the data for all memory areas having this file name is written
572 into this file, in the order of the memory areas defined in the <tt/MEMORY/
573 section. Let's specify some file names in the <tt/MEMORY/ section used above:
577 RAM1: start = $0800, size = $9800, file = %O;
578 ROM1: start = $A000, size = $2000, file = "rom1.bin";
579 RAM2: start = $C000, size = $1000, file = %O;
580 ROM2: start = $E000, size = $2000, file = "rom2.bin";
584 The <tt/%O/ used here is a way to specify the default behaviour explicitly:
585 <tt/%O/ is replaced by a string (including the quotes) that contains the
586 default output name, that is, "a.out" or the name specified with the <tt><ref
587 id="option-o" name="-o"></tt> option on the command line. Into this file, the
588 linker will first write any segments that go into <tt/RAM1/, and will append
589 then the segments for <tt/RAM2/, because the memory areas are given in this
590 order. So, for the RAM areas, nothing has really changed.
592 We've not used the ROM areas, but we will do that below, so we give the file
593 names here. Segments that go into <tt/ROM1/ will be written to a file named
594 "rom1.bin", and segments that go into <tt/ROM2/ will be written to a file
595 named "rom2.bin". The name given on the command line is ignored in both cases.
597 Assigning an empty file name for a memory area will discard the data written
598 to it. This is useful, if the memory area has segments assigned that are empty
599 (for example because they are of type bss). In that case, the linker will
600 create an empty output file. This may be suppressed by assigning an empty file
601 name to that memory area.
603 The <tt/%O/ sequence is also allowed inside a string. So using
607 ROM1: start = $A000, size = $2000, file = "%O-1.bin";
608 ROM2: start = $E000, size = $2000, file = "%O-2.bin";
612 would write two files that start with the name of the output file specified on
613 the command line, with "-1.bin" and "-2.bin" appended respectively. Because
614 '%' is used as an escape char, the sequence "%%" has to be used if a single
615 percent sign is required.
617 <sect1>LOAD and RUN addresses (ROMable code)<p>
619 Let us look now at a more complex example. Say, you've successfully tested
620 your new "Super Operating System" (SOS for short) for the C64, and you
621 will now go and replace the ROMs by your own code. When doing that, you
622 face a new problem: If the code runs in RAM, we need not to care about
623 read/write data. But now, if the code is in ROM, we must care about it.
624 Remember the default segments (you may of course specify your own):
628 RODATA read only data
630 BSS uninitialized data, read/write
633 Since <tt/BSS/ is not initialized, we must not care about it now, but what
634 about <tt/DATA/? <tt/DATA/ contains initialized data, that is, data that was
635 explicitly assigned a value. And your program will rely on these values on
636 startup. Since there's no other way to remember the contents of the data
637 segment, than storing it into one of the ROMs, we have to put it there. But
638 unfortunately, ROM is not writable, so we have to copy it into RAM before
639 running the actual code.
641 The linker won't copy the data from ROM into RAM for you (this must be done by
642 the startup code of your program), but it has some features that will help you
645 First, you may not only specify a "<tt/load/" attribute for a segment, but
646 also a "<tt/run/" attribute. The "<tt/load/" attribute is mandatory, and, if
647 you don't specify a "<tt/run/" attribute, the linker assumes that load area
648 and run area are the same. We will use this feature for our data area:
652 CODE: load = ROM1, type = ro;
653 RODATA: load = ROM2, type = ro;
654 DATA: load = ROM2, run = RAM2, type = rw, define = yes;
655 BSS: load = RAM2, type = bss, define = yes;
659 Let's have a closer look at this <tt/SEGMENTS/ section. We specify that the
660 <tt/CODE/ segment goes into <tt/ROM1/ (the one at $A000). The readonly data
661 goes into <tt/ROM2/. Read/write data will be loaded into <tt/ROM2/ but is run
662 in <tt/RAM2/. That means that all references to labels in the <tt/DATA/
663 segment are relocated to be in <tt/RAM2/, but the segment is written to
664 <tt/ROM2/. All your startup code has to do is, to copy the data from its
665 location in <tt/ROM2/ to the final location in <tt/RAM2/.
667 So, how do you know, where the data is located? This is the second point,
668 where you get help from the linker. Remember the "<tt/define/" attribute?
669 Since we have set this attribute to true, the linker will define three
670 external symbols for the data segment that may be accessed from your code:
673 __DATA_LOAD__ This is set to the address where the segment
674 is loaded, in this case, it is an address in
676 __DATA_RUN__ This is set to the run address of the segment,
677 in this case, it is an address in RAM2.
678 __DATA_SIZE__ This is set to the segment size.
681 So, what your startup code must do, is to copy <tt/__DATA_SIZE__/ bytes from
682 <tt/__DATA_LOAD__/ to <tt/__DATA_RUN__/ before any other routines are called.
683 All references to labels in the <tt/DATA/ segment are relocated to <tt/RAM2/
684 by the linker, so things will work properly.
686 There's a library subroutine called <tt/copydata/ (in a module named
687 <tt/copydata.s/) that might be used to do actual copying. Be sure to have a
688 look at it's inner workings before using it!
691 <sect1>Other MEMORY area attributes<p>
693 There are some other attributes not covered above. Before starting the
694 reference section, I will discuss the remaining things here.
696 You may request symbols definitions also for memory areas. This may be
697 useful for things like a software stack, or an i/o area.
701 STACK: start = $C000, size = $1000, define = yes;
705 This will define some external symbols that may be used in your code:
708 __STACK_START__ This is set to the start of the memory
709 area, $C000 in this example.
710 __STACK_SIZE__ The size of the area, here $1000.
711 __STACK_LAST__ This is NOT the same as START+SIZE.
712 Instead, it it defined as the first
713 address that is not used by data. If we
714 don't define any segments for this area,
715 the value will be the same as START.
716 __STACK_FILEOFFS__ The binary offset in the output file. This
717 is not defined for relocatable output file
721 A memory section may also have a type. Valid types are
724 ro for readonly memory
725 rw for read/write memory.
728 The linker will assure, that no segment marked as read/write or bss is put
729 into a memory area that is marked as readonly.
731 Unused memory in a memory area may be filled. Use the "<tt/fill = yes/"
732 attribute to request this. The default value to fill unused space is zero. If
733 you don't like this, you may specify a byte value that is used to fill these
734 areas with the "<tt/fillval/" attribute. If there is no "<tt/fillval/"
735 attribute for the segment, the "<tt/fillval/" attribute of the memory area (or
736 its default) is used instead. This means that the value may also be used to
737 fill unfilled areas generated by the assemblers <tt/.ALIGN/ and <tt/.RES/
740 The symbol <tt/%S/ may be used to access the default start address (that is,
741 the one defined in the <ref id="FEATURES" name="FEATURES"> section, or the
742 value given on the command line with the <tt><ref id="option-S" name="-S"></tt>
745 To support systems with banked memory, a special attribute named <tt/bank/ is
746 available. The attribute value is an arbitrary 32 bit integer. The assembler
747 has a builtin function named <tt/.BANK/ which may be used with an argument
748 that has a segment reference (for example a symbol). The result of this
749 function is the value of the bank attribute for the run memory area of the
753 <sect1>Other SEGMENT attributes<p>
755 Segments may be aligned to some memory boundary. Specify "<tt/align = num/" to
756 request this feature. Num must be a power of two. To align all segments on a
761 CODE: load = ROM1, type = ro, align = $100;
762 RODATA: load = ROM2, type = ro, align = $100;
763 DATA: load = ROM2, run = RAM2, type = rw, define = yes,
765 BSS: load = RAM2, type = bss, define = yes, align = $100;
769 If an alignment is requested, the linker will add enough space to the output
770 file, so that the new segment starts at an address that is dividable by the
771 given number without a remainder. All addresses are adjusted accordingly. To
772 fill the unused space, bytes of zero are used, or, if the memory area has a
773 "<tt/fillval/" attribute, that value. Alignment is always needed, if you have
774 used the <tt/.ALIGN/ command in the assembler. The alignment of a segment
775 must be equal or greater than the alignment used in the <tt/.ALIGN/ command.
776 The linker will check that, and issue a warning, if the alignment of a segment
777 is lower than the alignment requested in an <tt/.ALIGN/ command of one of the
778 modules making up this segment.
780 For a given segment you may also specify a fixed offset into a memory area or
781 a fixed start address. Use this if you want the code to run at a specific
782 address (a prominent case is the interrupt vector table which must go at
783 address $FFFA). Only one of <tt/ALIGN/ or <tt/OFFSET/ or <tt/START/ may be
784 specified. If the directive creates empty space, it will be filled with zero,
785 of with the value specified with the "<tt/fillval/" attribute if one is given.
786 The linker will warn you if it is not possible to put the code at the
787 specified offset (this may happen if other segments in this area are too
788 large). Here's an example:
792 VECTORS: load = ROM2, type = ro, start = $FFFA;
796 or (for the segment definitions from above)
800 VECTORS: load = ROM2, type = ro, offset = $1FFA;
804 The "<tt/align/", "<tt/start/" and "<tt/offset/" attributes change placement
805 of the segment in the run memory area, because this is what is usually
806 desired. If load and run memory areas are equal (which is the case if only the
807 load memory area has been specified), the attributes will also work. There is
808 also an "<tt/align_load/" attribute that may be used to align the start of the
809 segment in the load memory area, in case different load and run areas have
810 been specified. There are no special attributes to set start or offset for
811 just the load memory area.
813 A "<tt/fillval/" attribute may not only be specified for a memory area, but
814 also for a segment. The value must be an integer between 0 and 255. It is used
815 as fill value for space reserved by the assemblers <tt/.ALIGN/ and <tt/.RES/
816 commands. It is also used as fill value for space between sections (part of a
817 segment that comes from one object file) caused by alignment, but not for
818 space that preceeds the first section.
820 To suppress the warning, the linker issues if it encounters a segment that is
821 not found in any of the input files, use "<tt/optional=yes/" as additional
822 segment attribute. Be careful when using this attribute, because a missing
823 segment may be a sign of a problem, and if you're suppressing the warning,
824 there is no one left to tell you about it.
826 <sect1>The FILES section<p>
828 The <tt/FILES/ section is used to support other formats than straight binary
829 (which is the default, so binary output files do not need an explicit entry
830 in the <tt/FILES/ section).
832 The <tt/FILES/ section lists output files and as only attribute the format of
833 each output file. Assigning binary format to the default output file would
842 The only other available output format is the o65 format specified by Andre
843 Fachat (see the <url url="http://www.6502.org/users/andre/o65/fileformat.html"
844 name="6502 binary relocation format specification">). It is defined like this:
852 The necessary o65 attributes are defined in a special section labeled
857 <sect1>The FORMAT section<p>
859 The <tt/FORMAT/ section is used to describe file formats. The default (binary)
860 format has currently no attributes, so, while it may be listed in this
861 section, the attribute list is empty. The second supported format,
862 <url url="http://www.6502.org/users/andre/o65/fileformat.html" name="o65">,
863 has several attributes that may be defined here.
867 o65: os = lunix, version = 0, type = small,
868 import = LUNIXKERNEL,
875 <sect1>The FEATURES section<label id="FEATURES"><p>
877 In addition to the <tt/MEMORY/ and <tt/SEGMENTS/ sections described above, the
878 linker has features that may be enabled by an additional section labeled
882 <sect2>The CONDES feature<p>
884 <tt/CONDES/ is used to tell the linker to emit module constructor/destructor
889 CONDES: segment = RODATA,
891 label = __CONSTRUCTOR_TABLE__,
892 count = __CONSTRUCTOR_COUNT__;
896 The <tt/CONDES/ feature has several attributes:
900 <tag><tt>segment</tt></tag>
902 This attribute tells the linker into which segment the table should be
903 placed. If the segment does not exist, it is created.
906 <tag><tt>type</tt></tag>
908 Describes the type of the routines to place in the table. Type may be one of
909 the predefined types <tt/constructor/, <tt/destructor/, <tt/interruptor/, or
910 a numeric value between 0 and 6.
913 <tag><tt>label</tt></tag>
915 This specifies the label to use for the table. The label points to the start
916 of the table in memory and may be used from within user written code.
919 <tag><tt>count</tt></tag>
921 This is an optional attribute. If specified, an additional symbol is defined
922 by the linker using the given name. The value of this symbol is the number
923 of entries (<em/not/ bytes) in the table. While this attribute is optional,
924 it is often useful to define it.
927 <tag><tt>order</tt></tag>
929 Optional attribute that takes one of the keywords <tt/increasing/ or
930 <tt/decreasing/ as an argument. Specifies the sorting order of the entries
931 within the table. The default is <tt/increasing/, which means that the
932 entries are sorted with increasing priority (the first entry has the lowest
933 priority). "Priority" is the priority specified when declaring a symbol as
934 <tt/.CONDES/ with the assembler, higher values mean higher priority. You may
935 change this behaviour by specifying <tt/decreasing/ as the argument, the
936 order of entries is reversed in this case.
938 Please note that the order of entries with equal priority is undefined.
940 <tag><tt>import</tt></tag>
942 This attribute defines a valid symbol name, that is added as an import
943 to the modules defining a constructor/desctructor of the given type.
944 This can be used to force linkage of a module if this module exports the
949 Without specifying the <tt/CONDES/ feature, the linker will not create any
950 tables, even if there are <tt/condes/ entries in the object files.
952 For more information see the <tt/.CONDES/ command in the <url
953 url="ca65.html" name="ca65 manual">.
956 <sect2>The STARTADDRESS feature<p>
958 <tt/STARTADDRESS/ is used to set the default value for the start address,
959 which can be referenced by the <tt/%S/ symbol. The builtin default for the
960 linker is $200.
964 # Default start address is $1000
965 STARTADDRESS: default = $1000;
969 Please note that order is important: The default start address must be defined
970 <em/before/ the <tt/%S/ symbol is used in the config file. This does usually
971 mean, that the <tt/FEATURES/ section has to go to the top of the config file.
975 <sect1>The SYMBOLS section<label id="SYMBOLS"><p>
977 The configuration file may also be used to define symbols used in the link
978 stage or to force symbols imports. This is done in the SYMBOLS section. The
979 symbol name is followed by a colon and symbol attributes.
981 The following symbol attributes are supported:
985 <tag><tt>addrsize</tt></tag>
987 The <tt/addrsize/ attribute specifies the address size of the symbol and
990 <item><tt/zp/, <tt/zeropage/ or <tt/direct/
991 <item><tt/abs/, <tt/absolute/ or <tt/near/
993 <item><tt/long/ or <tt/dword/.
996 Without this attribute, the default address size is <tt/abs/.
998 <tag><tt>type</tt></tag>
1000 This attribute is mandatory. Its value is one of <tt/export/, <tt/import/ or
1001 <tt/weak/. <tt/export/ means that the symbol is defined and exported from
1002 the linker config. <tt/import/ means that an import is generated for this
1003 symbol, eventually forcing a module that exports this symbol to be included
1004 in the output. <tt/weak/ is similar as <tt/export/. However, the symbol is
1005 only defined if it is not defined elsewhere.
1007 <tag><tt>value</tt></tag>
1009 This must only be given for symbols of type <tt/export/ or <tt/weak/. It
1010 defines the value of the symbol and may be an expression.
1014 The following example defines the stack size for an application, but allows
1015 the programmer to override the value by specifying <tt/--define
1016 __STACKSIZE__=xxx/ on the command line.
1020 # Define the stack size for the application
1021 __STACKSIZE__: type = weak, value = $800;
1027 <sect>Special segments<p>
1029 The builtin config files do contain segments that have a special meaning for
1030 the compiler and the libraries that come with it. If you replace the builtin
1031 config files, you will need the following information.
1035 The INIT segment is used for initialization code that may be reused once
1036 execution reaches main() - provided that the program runs in RAM. You
1037 may for example add the INIT segment to the heap in really memory
1038 constrained systems.
1042 For the LOWCODE segment, it is guaranteed that it won't be banked out, so it
1043 is reachable at any time by interrupt handlers or similar.
1047 This segment contains the startup code which initializes the C software stack
1048 and the libraries. It is placed in its own segment because it needs to be
1049 loaded at the lowest possible program address on several platforms.
1053 The ZPSAVE segment contains the original values of the zeropage locations used
1054 by the ZEROPAGE segment. It is placed in its own segment because it must not be
1061 ld65 (and all cc65 binutils) are (C) Copyright 1998-2005 Ullrich von
1062 Bassewitz. For usage of the binaries and/or sources the following
1063 conditions do apply:
1065 This software is provided 'as-is', without any expressed or implied
1066 warranty. In no event will the authors be held liable for any damages
1067 arising from the use of this software.
1069 Permission is granted to anyone to use this software for any purpose,
1070 including commercial applications, and to alter it and redistribute it
1071 freely, subject to the following restrictions:
1074 <item> The origin of this software must not be misrepresented; you must not
1075 claim that you wrote the original software. If you use this software
1076 in a product, an acknowledgment in the product documentation would be
1077 appreciated but is not required.
1078 <item> Altered source versions must be plainly marked as such, and must not
1079 be misrepresented as being the original software.
1080 <item> This notice may not be removed or altered from any source