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 --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 builtin 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 builtin 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:
160 <item>c16 (works also for the c116 with memory up to 32K)
164 <item>cbm510 (CBM-II series with 40 column video)
165 <item>cbm610 (all CBM series-II computers with 80 column video)
166 <item>pet (all CBM PET systems except the 2001)
174 There are a few more targets defined but neither of them is actually
178 <label id="option-v">
179 <tag><tt>-v, --verbose</tt></tag>
181 Using the -v option, you may enable more output that may help you to
182 locate problems. If an undefined symbol is encountered, -v causes the
183 linker to print a detailed list of the references (that is, source file
184 and line) for this symbol.
187 <tag><tt>-vm</tt></tag>
189 Must be used in conjunction with <tt><ref id="option-m" name="-m"></tt>
190 (generate map file). Normally the map file will not include empty segments
191 and sections, or unreferenced symbols. Using this option, you can force the
192 linker to include all this information into the map file.
195 <label id="option-C">
196 <tag><tt>-C</tt></tag>
198 This gives the name of an output config file to use. See section 4 for more
199 information about config files. -C may not be used together with <tt><ref
200 id="option-t" name="-t"></tt>.
203 <label id="option-D">
204 <tag><tt>-D sym=value, --define sym=value</tt></tag>
206 This option allows to define an external symbol on the command line. Value
207 may start with a '$' sign or with <tt/0x/ for hexadecimal values,
208 otherwise a leading zero denotes octal values. See also the <ref
209 id="SYMBOLS" name="SYMBOLS section"> in the configuration file.
212 <label id="option--lib-path">
213 <tag><tt>-L path, --lib-path path</tt></tag>
215 Specify a library search path. This option may be used more than once. It
216 adds a directory to the search path for library files. Libraries specified
217 without a path are searched in current directory, in the directory given in
218 the <tt/LD65_LIB/ environment variable, and in the list of directories
219 specified using <tt/--lib-path/.
222 <tag><tt>-Ln</tt></tag>
224 This option allows you to create a file that contains all global labels and
225 may be loaded into VICE emulator using the <tt/ll/ (load label) command. You
226 may use this to debug your code with VICE. Note: Older versions had some
227 bugs in the label code. If you have problems, please get the latest VICE
231 <label id="option-S">
232 <tag><tt>-S addr, --start-addr addr</tt></tag>
234 Using -S you may define the default starting address. If and how this
235 address is used depends on the config file in use. For the builtin
236 configurations, only the "none", "apple2" and "apple2enh" systems honor an
237 explicit start address, all other builtin config provide their own.
240 <tag><tt>-V, --version</tt></tag>
242 This option print the version number of the linker. If you send any
243 suggestions or bugfixes, please include this number.
246 <label id="option--cfg-path">
247 <tag><tt>--cfg-path path</tt></tag>
249 Specify a config file search path. This option may be used more than once.
250 It adds a directory to the search path for config files. A config file given
251 with the <tt><ref id="option-C" name="-C"></tt> option that has no path in
252 its name is searched in the current directory, in the directory given in the
253 <tt/LD65_CFG/ environment variable, and in the list of directories specified
254 using <tt/--cfg-path/.
257 <label id="option--dbgfile">
258 <tag><tt>--dbgfile name</tt></tag>
260 Specify an output file for debug information. Available information will be
261 written to this file. Using the <tt/-g/ option for the compiler and assembler
262 will increase the amount of information available. Please note that debug
263 information generation is currently being developed, so the format of the
264 file and it's contents are subject to change without further notice.
267 <tag><tt>--force-import sym[:addrsize]</tt></tag>
269 Force an import of a symbol. While object files are always linked to the
270 output file, regardless if there are any references, object modules from
271 libraries get only linked in if an import can be satisfied by this module.
272 The <tt/--fore-import/ option may be used to add a reference to a symbol and
273 as a result force linkage of the module that exports the identifier.
275 The name of the symbol may be followed by a colon and an address size
276 specifier. If no address size is specified, the default address size
277 for the target machine is used.
279 Please note that the symbol name needs to have the internal representation,
280 meaning you have to prepend an underline for C identifiers.
283 <tag><tt>--lib file</tt></tag>
285 Links a library to the output. Use this command line option instead of just
286 naming the library file, if the linker is not able to determine the file
287 type because of an unusual extension.
290 <tag><tt>--obj file</tt></tag>
292 Links an object file to the output. Use this command line option instead
293 of just naming the object file, if the linker is not able to determine the
294 file type because of an unusual extension.
297 <label id="option--obj-path">
298 <tag><tt>--obj-path path</tt></tag>
300 Specify an object file search path. This option may be used more than once.
301 It adds a directory to the search path for object files. An object file
302 passed to the linker that has no path in its name is searched in current
303 directory, in the directory given in the <tt/LD65_OBJ/ environment variable,
304 and in the list of directories specified using <tt/--obj-path/.
310 <sect>Search paths<p>
312 Starting with version 2.10 there are now several search paths for files needed
313 by the linker: One for libraries, one for object files and one for config
317 <sect1>Library search path<p>
319 The library search path contains in this order:
322 <item>The current directory.
323 <item>A compiled in library path which is often <tt>/usr/lib/cc65/lib</tt> on
325 <item>The value of the environment variable <tt/LD65_LIB/ 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--lib-path"
330 name="--lib-path"></tt> option on the command line.
334 <sect1>Object file search path<p>
336 The object 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_OBJ/ if it is defined.
343 <item>The value of the environment variable <tt/CC65_LIB/ if it is defined.
344 Please note that use of this environment variable is obsolete and may
345 get removed in future versions.
346 <item>Any directory added with the <tt><ref id="option--obj-path"
347 name="--obj-path"></tt> option on the command line.
351 <sect1>Config file search path<p>
353 The config file search path contains in this order:
356 <item>The current directory.
357 <item>A compiled in directory which is often <tt>/usr/lib/cc65/lib</tt> on
359 <item>The value of the environment variable <tt/LD65_CFG/ if it is defined.
360 <item>Any directory added with the <tt><ref id="option--cfg-path"
361 name="--cfg-path"></tt> option on the command line.
366 <sect>Detailed workings<p>
368 The linker does several things when combining object modules:
370 First, the command line is parsed from left to right. For each object file
371 encountered (object files are recognized by a magic word in the header, so
372 the linker does not care about the name), imported and exported
373 identifiers are read from the file and inserted in a table. If a library
374 name is given (libraries are also recognized by a magic word, there are no
375 special naming conventions), all modules in the library are checked if an
376 export from this module would satisfy an import from other modules. All
377 modules where this is the case are marked. If duplicate identifiers are
378 found, the linker issues a warning.
380 This procedure (parsing and reading from left to right) does mean, that a
381 library may only satisfy references for object modules (given directly or from
382 a library) named <em/before/ that library. With the command line
385 ld65 crt0.o clib.lib test.o
388 the module test.o may not contain references to modules in the library
389 clib.lib. If this is the case, you have to change the order of the modules
393 ld65 crt0.o test.o clib.lib
396 Step two is, to read the configuration file, and assign start addresses
397 for the segments and define any linker symbols (see <ref id="config-files"
398 name="Configuration files">).
400 After that, the linker is ready to produce an output file. Before doing that,
401 it checks it's data for consistency. That is, it checks for unresolved
402 externals (if the output format is not relocatable) and for symbol type
403 mismatches (for example a zero page symbol is imported by a module as absolute
406 Step four is, to write the actual target files. In this step, the linker will
407 resolve any expressions contained in the segment data. Circular references are
408 also detected in this step (a symbol may have a circular reference that goes
409 unnoticed if the symbol is not used).
411 Step five is to output a map file with a detailed list of all modules,
412 segments and symbols encountered.
414 And, last step, if you give the <tt><ref id="option-v" name="-v"></tt> switch
415 twice, you get a dump of the segment data. However, this may be quite
416 unreadable if you're not a developer:-)
420 <sect>Configuration files<label id="config-files"><p>
422 Configuration files are used to describe the layout of the output file(s). Two
423 major topics are covered in a config file: The memory layout of the target
424 architecture, and the assignment of segments to memory areas. In addition,
425 several other attributes may be specified.
427 Case is ignored for keywords, that is, section or attribute names, but it is
428 <em/not/ ignored for names and strings.
432 <sect1>Memory areas<p>
434 Memory areas are specified in a <tt/MEMORY/ section. Lets have a look at an
435 example (this one describes the usable memory layout of the C64):
439 RAM1: start = $0800, size = $9800;
440 ROM1: start = $A000, size = $2000;
441 RAM2: start = $C000, size = $1000;
442 ROM2: start = $E000, size = $2000;
446 As you can see, there are two ram areas and two rom areas. The names
447 (before the colon) are arbitrary names that must start with a letter, with
448 the remaining characters being letters or digits. The names of the memory
449 areas are used when assigning segments. As mentioned above, case is
450 significant for these names.
452 The syntax above is used in all sections of the config file. The name
453 (<tt/ROM1/ etc.) is said to be an identifier, the remaining tokens up to the
454 semicolon specify attributes for this identifier. You may use the equal sign
455 to assign values to attributes, and you may use a comma to separate
456 attributes, you may also leave both out. But you <em/must/ use a semicolon to
457 mark the end of the attributes for one identifier. The section above may also
458 have looked like this:
461 # Start of memory section
479 There are of course more attributes for a memory section than just start and
480 size. Start and size are mandatory attributes, that means, each memory area
481 defined <em/must/ have these attributes given (the linker will check that). I
482 will cover other attributes later. As you may have noticed, I've used a
483 comment in the example above. Comments start with a hash mark (`#'), the
484 remainder of the line is ignored if this character is found.
489 Let's assume you have written a program for your trusty old C64, and you would
490 like to run it. For testing purposes, it should run in the <tt/RAM/ area. So
491 we will start to assign segments to memory sections in the <tt/SEGMENTS/
496 CODE: load = RAM1, type = ro;
497 RODATA: load = RAM1, type = ro;
498 DATA: load = RAM1, type = rw;
499 BSS: load = RAM1, type = bss, define = yes;
503 What we are doing here is telling the linker, that all segments go into the
504 <tt/RAM1/ memory area in the order specified in the <tt/SEGMENTS/ section. So
505 the linker will first write the <tt/CODE/ segment, then the <tt/RODATA/
506 segment, then the <tt/DATA/ segment - but it will not write the <tt/BSS/
507 segment. Why? Enter the segment type: For each segment specified, you may also
508 specify a segment attribute. There are four possible segment attributes:
513 bss means that this is an uninitialized segment
514 zp a zeropage segment
517 So, because we specified that the segment with the name BSS is of type bss,
518 the linker knows that this is uninitialized data, and will not write it to an
519 output file. This is an important point: For the assembler, the <tt/BSS/
520 segment has no special meaning. You specify, which segments have the bss
521 attribute when linking. This approach is much more flexible than having one
522 fixed bss segment, and is a result of the design decision to supporting an
523 arbitrary segment count.
525 If you specify "<tt/type = bss/" for a segment, the linker will make sure that
526 this segment does only contain uninitialized data (that is, zeroes), and issue
527 a warning if this is not the case.
529 For a <tt/bss/ type segment to be useful, it must be cleared somehow by your
530 program (this happens usually in the startup code - for example the startup
531 code for cc65 generated programs takes care about that). But how does your
532 code know, where the segment starts, and how big it is? The linker is able to
533 give that information, but you must request it. This is, what we're doing with
534 the "<tt/define = yes/" attribute in the <tt/BSS/ definitions. For each
535 segment, where this attribute is true, the linker will export three symbols.
538 __NAME_LOAD__ This is set to the address where the
540 __NAME_RUN__ This is set to the run address of the
541 segment. We will cover run addresses
543 __NAME_SIZE__ This is set to the segment size.
546 Replace <tt/NAME/ by the name of the segment, in the example above, this would
547 be <tt/BSS/. These symbols may be accessed by your code.
549 Now, as we've configured the linker to write the first three segments and
550 create symbols for the last one, there's only one question left: Where does
551 the linker put the data? It would be very convenient to have the data in a
554 <sect1>Output files<p>
556 We don't have any files specified above, and indeed, this is not needed in a
557 simple configuration like the one above. There is an additional attribute
558 "file" that may be specified for a memory area, that gives a file name to
559 write the area data into. If there is no file name given, the linker will
560 assign the default file name. This is "a.out" or the one given with the
561 <tt><ref id="option-o" name="-o"></tt> option on the command line. Since the
562 default behaviour is ok for our purposes, I did not use the attribute in the
563 example above. Let's have a look at it now.
565 The "file" attribute (the keyword may also be written as "FILE" if you like
566 that better) takes a string enclosed in double quotes (`"') that specifies the
567 file, where the data is written. You may specify the same file several times,
568 in that case the data for all memory areas having this file name is written
569 into this file, in the order of the memory areas defined in the <tt/MEMORY/
570 section. Let's specify some file names in the <tt/MEMORY/ section used above:
574 RAM1: start = $0800, size = $9800, file = %O;
575 ROM1: start = $A000, size = $2000, file = "rom1.bin";
576 RAM2: start = $C000, size = $1000, file = %O;
577 ROM2: start = $E000, size = $2000, file = "rom2.bin";
581 The <tt/%O/ used here is a way to specify the default behaviour explicitly:
582 <tt/%O/ is replaced by a string (including the quotes) that contains the
583 default output name, that is, "a.out" or the name specified with the <tt><ref
584 id="option-o" name="-o"></tt> option on the command line. Into this file, the
585 linker will first write any segments that go into <tt/RAM1/, and will append
586 then the segments for <tt/RAM2/, because the memory areas are given in this
587 order. So, for the RAM areas, nothing has really changed.
589 We've not used the ROM areas, but we will do that below, so we give the file
590 names here. Segments that go into <tt/ROM1/ will be written to a file named
591 "rom1.bin", and segments that go into <tt/ROM2/ will be written to a file
592 named "rom2.bin". The name given on the command line is ignored in both cases.
594 Assigning an empty file name for a memory area will discard the data written
595 to it. This is useful, if the a memory area has segments assigned that are
596 empty (for example because they are of type bss). In that case, the linker
597 will create an empty output file. This may be suppressed by assigning an empty
598 file name to that memory area.
601 <sect1>LOAD and RUN addresses (ROMable code)<p>
603 Let us look now at a more complex example. Say, you've successfully tested
604 your new "Super Operating System" (SOS for short) for the C64, and you
605 will now go and replace the ROMs by your own code. When doing that, you
606 face a new problem: If the code runs in RAM, we need not to care about
607 read/write data. But now, if the code is in ROM, we must care about it.
608 Remember the default segments (you may of course specify your own):
612 RODATA read only data
614 BSS uninitialized data, read/write
617 Since <tt/BSS/ is not initialized, we must not care about it now, but what
618 about <tt/DATA/? <tt/DATA/ contains initialized data, that is, data that was
619 explicitly assigned a value. And your program will rely on these values on
620 startup. Since there's no other way to remember the contents of the data
621 segment, than storing it into one of the ROMs, we have to put it there. But
622 unfortunately, ROM is not writable, so we have to copy it into RAM before
623 running the actual code.
625 The linker cannot help you copying the data from ROM into RAM (this must be
626 done by the startup code of your program), but it has some features that will
627 help you in this process.
629 First, you may not only specify a "<tt/load/" attribute for a segment, but
630 also a "<tt/run/" attribute. The "<tt/load/" attribute is mandatory, and, if
631 you don't specify a "<tt/run/" attribute, the linker assumes that load area
632 and run area are the same. We will use this feature for our data area:
636 CODE: load = ROM1, type = ro;
637 RODATA: load = ROM2, type = ro;
638 DATA: load = ROM2, run = RAM2, type = rw, define = yes;
639 BSS: load = RAM2, type = bss, define = yes;
643 Let's have a closer look at this <tt/SEGMENTS/ section. We specify that the
644 <tt/CODE/ segment goes into <tt/ROM1/ (the one at $A000). The readonly data
645 goes into <tt/ROM2/. Read/write data will be loaded into <tt/ROM2/ but is run
646 in <tt/RAM2/. That means that all references to labels in the <tt/DATA/
647 segment are relocated to be in <tt/RAM2/, but the segment is written to
648 <tt/ROM2/. All your startup code has to do is, to copy the data from it's
649 location in <tt/ROM2/ to the final location in <tt/RAM2/.
651 So, how do you know, where the data is located? This is the second point,
652 where you get help from the linker. Remember the "<tt/define/" attribute?
653 Since we have set this attribute to true, the linker will define three
654 external symbols for the data segment that may be accessed from your code:
657 __DATA_LOAD__ This is set to the address where the segment
658 is loaded, in this case, it is an address in
660 __DATA_RUN__ This is set to the run address of the segment,
661 in this case, it is an address in RAM2.
662 __DATA_SIZE__ This is set to the segment size.
665 So, what your startup code must do, is to copy <tt/__DATA_SIZE__/ bytes from
666 <tt/__DATA_LOAD__/ to <tt/__DATA_RUN__/ before any other routines are called.
667 All references to labels in the <tt/DATA/ segment are relocated to <tt/RAM2/
668 by the linker, so things will work properly.
671 <sect1>Other MEMORY area attributes<p>
673 There are some other attributes not covered above. Before starting the
674 reference section, I will discuss the remaining things here.
676 You may request symbols definitions also for memory areas. This may be
677 useful for things like a software stack, or an i/o area.
681 STACK: start = $C000, size = $1000, define = yes;
685 This will define three external symbols that may be used in your code:
688 __STACK_START__ This is set to the start of the memory
689 area, $C000 in this example.
690 __STACK_SIZE__ The size of the area, here $1000.
691 __STACK_LAST__ This is NOT the same as START+SIZE.
692 Instead, it it defined as the first
693 address that is not used by data. If we
694 don't define any segments for this area,
695 the value will be the same as START.
698 A memory section may also have a type. Valid types are
701 ro for readonly memory
702 rw for read/write memory.
705 The linker will assure, that no segment marked as read/write or bss is put
706 into a memory area that is marked as readonly.
708 Unused memory in a memory area may be filled. Use the "<tt/fill = yes/"
709 attribute to request this. The default value to fill unused space is zero. If
710 you don't like this, you may specify a byte value that is used to fill these
711 areas with the "<tt/fillval/" attribute. This value is also used to fill unfilled
712 areas generated by the assemblers <tt/.ALIGN/ and <tt/.RES/ directives.
714 The symbol <tt/%S/ may be used to access the default start address (that is,
715 the one defined in the <ref id="FEATURES" name="FEATURES"> section, or the
716 value given on the command line with the <tt><ref id="option-S" name="-S"></tt>
720 <sect1>Other SEGMENT attributes<p>
722 Segments may be aligned to some memory boundary. Specify "<tt/align = num/" to
723 request this feature. Num must be a power of two. To align all segments on a
728 CODE: load = ROM1, type = ro, align = $100;
729 RODATA: load = ROM2, type = ro, align = $100;
730 DATA: load = ROM2, run = RAM2, type = rw, define = yes,
732 BSS: load = RAM2, type = bss, define = yes, align = $100;
736 If an alignment is requested, the linker will add enough space to the output
737 file, so that the new segment starts at an address that is dividable by the
738 given number without a remainder. All addresses are adjusted accordingly. To
739 fill the unused space, bytes of zero are used, or, if the memory area has a
740 "<tt/fillval/" attribute, that value. Alignment is always needed, if you have
741 used the <tt/.ALIGN/ command in the assembler. The alignment of a segment
742 must be equal or greater than the alignment used in the <tt/.ALIGN/ command.
743 The linker will check that, and issue a warning, if the alignment of a segment
744 is lower than the alignment requested in an <tt/.ALIGN/ command of one of the
745 modules making up this segment.
747 For a given segment you may also specify a fixed offset into a memory area or
748 a fixed start address. Use this if you want the code to run at a specific
749 address (a prominent case is the interrupt vector table which must go at
750 address $FFFA). Only one of <tt/ALIGN/ or <tt/OFFSET/ or <tt/START/ may be
751 specified. If the directive creates empty space, it will be filled with zero,
752 of with the value specified with the "<tt/fillval/" attribute if one is given.
753 The linker will warn you if it is not possible to put the code at the
754 specified offset (this may happen if other segments in this area are too
755 large). Here's an example:
759 VECTORS: load = ROM2, type = ro, start = $FFFA;
763 or (for the segment definitions from above)
767 VECTORS: load = ROM2, type = ro, offset = $1FFA;
771 The "<tt/align/", "<tt/start/" and "<tt/offset/" attributes change placement
772 of the segment in the run memory area, because this is what is usually
773 desired. If load and run memory areas are equal (which is the case if only the
774 load memory area has been specified), the attributes will also work. There is
775 also an "<tt/align_load/" attribute that may be used to align the start of the
776 segment in the load memory area, in case different load and run areas have
777 been specified. There are no special attributes to set start or offset for
778 just the load memory area.
780 To suppress the warning, the linker issues if it encounters a segment that is
781 not found in any of the input files, use "<tt/optional=yes/" as additional
782 segment attribute. Be careful when using this attribute, because a missing
783 segment may be a sign of a problem, and if you're suppressing the warning,
784 there is no one left to tell you about it.
786 <sect1>The FILES section<p>
788 The <tt/FILES/ section is used to support other formats than straight binary
789 (which is the default, so binary output files do not need an explicit entry
790 in the <tt/FILES/ section).
792 The <tt/FILES/ section lists output files and as only attribute the format of
793 each output file. Assigning binary format to the default output file would
802 The only other available output format is the o65 format specified by Andre
803 Fachat (see the <htmlurl url="http://www.6502.org/users/andre/o65/fileformat.html"
804 name="6502 binary relocation format specification">). It is defined like this:
812 The necessary o65 attributes are defined in a special section labeled
817 <sect1>The FORMAT section<p>
819 The <tt/FORMAT/ section is used to describe file formats. The default (binary)
820 format has currently no attributes, so, while it may be listed in this
821 section, the attribute list is empty. The second supported format, o65, has
822 several attributes that may be defined here.
826 o65: os = lunix, version = 0, type = small,
827 import = LUNIXKERNEL,
834 <sect1>The FEATURES section<label id="FEATURES"><p>
836 In addition to the <tt/MEMORY/ and <tt/SEGMENTS/ sections described above, the
837 linker has features that may be enabled by an additional section labeled
841 <sect2>The CONDES feature<p>
843 <tt/CONDES/ is used to tell the linker to emit module constructor/destructor
848 CONDES: segment = RODATA,
850 label = __CONSTRUCTOR_TABLE__,
851 count = __CONSTRUCTOR_COUNT__;
855 The <tt/CONDES/ feature has several attributes:
859 <tag><tt>segment</tt></tag>
861 This attribute tells the linker into which segment the table should be
862 placed. If the segment does not exist, it is created.
865 <tag><tt>type</tt></tag>
867 Describes the type of the routines to place in the table. Type may be one of
868 the predefined types <tt/constructor/, <tt/destructor/, <tt/interruptor/, or
869 a numeric value between 0 and 6.
872 <tag><tt>label</tt></tag>
874 This specifies the label to use for the table. The label points to the start
875 of the table in memory and may be used from within user written code.
878 <tag><tt>count</tt></tag>
880 This is an optional attribute. If specified, an additional symbol is defined
881 by the linker using the given name. The value of this symbol is the number
882 of entries (<em/not/ bytes) in the table. While this attribute is optional,
883 it is often useful to define it.
886 <tag><tt>order</tt></tag>
888 Optional attribute that takes one of the keywords <tt/increasing/ or
889 <tt/decreasing/ as an argument. Specifies the sorting order of the entries
890 within the table. The default is <tt/increasing/, which means that the
891 entries are sorted with increasing priority (the first entry has the lowest
892 priority). "Priority" is the priority specified when declaring a symbol as
893 <tt/.CONDES/ with the assembler, higher values mean higher priority. You may
894 change this behaviour by specifying <tt/decreasing/ as the argument, the
895 order of entries is reversed in this case.
897 Please note that the order of entries with equal priority is undefined.
901 Without specifying the <tt/CONDES/ feature, the linker will not create any
902 tables, even if there are <tt/condes/ entries in the object files.
904 For more information see the <tt/.CONDES/ command in the <htmlurl
905 url="ca65.html" name="ca65 manual">.
908 <sect2>The STARTADDRESS feature<p>
910 <tt/STARTADDRESS/ is used to set the default value for the start address,
911 which can be referenced by the <tt/%S/ symbol. The builtin default for the
912 linker is $200.
916 # Default start address is $1000
917 STARTADDRESS: default = $1000;
921 Please note that order is important: The default start address must be defined
922 <em/before/ the <tt/%S/ symbol is used in the config file. This does usually
923 mean, that the <tt/FEATURES/ section has to go to the top of the config file.
927 <sect1>The SYMBOLS section<label id="SYMBOLS"><p>
929 The configuration file may also be used to define symbols used in the link
930 stage. The mandatory attribute for a symbol is its value. A second, boolean
931 attribute named <tt/weak/ is available. If a symbol is marked as weak, it may
932 be overridden by defining a symbol of the same name from the command line. The
933 default for symbols is that they're strong, which means that an attempt to
934 define a symbol with the same name from the command line will lead to an
937 The following example defines the stack size for an application, but allows
938 the programmer to override the value by specifying <tt/--define
939 __STACKSIZE__=xxx/ on the command line.
943 # Define the stack size for the application
944 __STACKSIZE__: value = $800, weak = yes;
950 <sect1>Builtin configurations<p>
952 The builtin configurations are part of the linker source. They are also
953 distributed together with the machine specific binary packages (usually in the
954 doc directory) and don't have a special format. So if you need a special
955 configuration, it's a good idea to start with the builtin configuration for
956 your system. In a first step, just replace <tt/-t target/ by <tt/-C
957 configfile/. Then go on and modify the config file to suit your needs.
961 <sect>Special segments<p>
963 The builtin config files do contain segments that have a special meaning for
964 the compiler and the libraries that come with it. If you replace the builtin
965 config files, you will need the following information.
969 The INIT segment is used for initialization code that may be reused once
970 execution reaches main() - provided that the program runs in RAM. You
971 may for example add the INIT segment to the heap in really memory
976 For the LOWCODE segment, it is guaranteed that it won't be banked out, so it
977 is reachable at any time by interrupt handlers or similar.
981 This segment contains the startup code which initializes the C software stack
982 and the libraries. It is placed in its own segment because it needs to be
983 loaded at the lowest possible program address on several platforms.
987 This segment defines the location of the memory heap used by the malloc
992 <sect>Bugs/Feedback<p>
994 If you have problems using the linker, if you find any bugs, or if you're
995 doing something interesting with it, I would be glad to hear from you. Feel
996 free to contact me by email (<htmlurl url="mailto:uz@cc65.org"
997 name="uz@cc65.org">).
1003 ld65 (and all cc65 binutils) are (C) Copyright 1998-2005 Ullrich von
1004 Bassewitz. For usage of the binaries and/or sources the following
1005 conditions do apply:
1007 This software is provided 'as-is', without any expressed or implied
1008 warranty. In no event will the authors be held liable for any damages
1009 arising from the use of this software.
1011 Permission is granted to anyone to use this software for any purpose,
1012 including commercial applications, and to alter it and redistribute it
1013 freely, subject to the following restrictions:
1016 <item> The origin of this software must not be misrepresented; you must not
1017 claim that you wrote the original software. If you use this software
1018 in a product, an acknowledgment in the product documentation would be
1019 appreciated but is not required.
1020 <item> Altered source versions must be plainly marked as such, and must not
1021 be misrepresented as being the original software.
1022 <item> This notice may not be removed or altered from any source