1 <!doctype linuxdoc system> <!-- -*- text-mode -*- -->
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
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 --dump-config name Dump a builtin configuration
80 --end-group End a library group
81 --force-import sym Force an import of symbol `sym'
82 --help Help (this text)
83 --lib file Link this library
84 --lib-path path Specify a library search path
85 --mapfile name Create a map file
86 --module-id id Specify a module id
87 --obj file Link this object file
88 --obj-path path Specify an object file search path
89 --start-addr addr Set the default start address
90 --start-group Start a library group
91 --target sys Set the target system
92 --version Print the linker version
93 ---------------------------------------------------------------------------
97 <sect1>Command line options in detail<p>
99 Here is a description of all the command line options:
103 <label id="option--start-group">
104 <tag><tt>-(, --start-group</tt></tag>
106 Start a library group. The libraries specified within a group are searched
107 multiple times to resolve crossreferences within the libraries. Normally,
108 crossreferences are only resolved within a library, that is the library is
109 searched multiple times. Libraries specified later on the command line
110 cannot reference otherwise unreferenced symbols in libraries specified
111 earlier, because the linker has already handled them. Library groups are
112 a solution for this problem, because the linker will search repeatedly
113 through all libraries specified in the group, until all possible open
114 symbol references have been satisfied.
117 <tag><tt>-), --end-group</tt></tag>
119 End a library group. See the explanation of the <tt><ref
120 id="option--start-group" name="--start-group"></tt> option.
123 <tag><tt>-h, --help</tt></tag>
125 Print the short option summary shown above.
128 <label id="option-m">
129 <tag><tt>-m name, --mapfile name</tt></tag>
131 This option (which needs an argument that will used as a filename for
132 the generated map file) will cause the linker to generate a map file.
133 The map file does contain a detailed overview over the modules used, the
134 sizes for the different segments, and a table containing exported
138 <label id="option-o">
139 <tag><tt>-o name</tt></tag>
141 The -o switch is used to give the name of the default output file.
142 Depending on your output configuration, this name may NOT be used as
143 name for the output file. However, for the builtin configurations, this
144 name is used for the output file name.
147 <label id="option-t">
148 <tag><tt>-t sys, --target sys</tt></tag>
150 The argument for the -t switch is the name of the target system. Since this
151 switch will activate a builtin configuration, it may not be used together
152 with the <tt><ref id="option-C" name="-C"></tt> option. The following target
153 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)
178 There are a few more targets defined but neither of them is actually
182 <tag><tt>-u sym[:addrsize], --force-import sym[:addrsize]</tt></tag>
184 Force an import of a symbol. While object files are always linked to the
185 output file, regardless if there are any references, object modules from
186 libraries get only linked in if an import can be satisfied by this module.
187 The <tt/--fore-import/ option may be used to add a reference to a symbol and
188 as a result force linkage of the module that exports the identifier.
190 The name of the symbol may optionally be followed by a colon and an address
191 size specifier. If no address size is specified, the default address size
192 for the target machine is used.
194 Please note that the symbol name needs to have the internal representation,
195 meaning you have to prepend an underline for C identifiers.
198 <label id="option-v">
199 <tag><tt>-v, --verbose</tt></tag>
201 Using the -v option, you may enable more output that may help you to
202 locate problems. If an undefined symbol is encountered, -v causes the
203 linker to print a detailed list of the references (that is, source file
204 and line) for this symbol.
207 <tag><tt>-vm</tt></tag>
209 Must be used in conjunction with <tt><ref id="option-m" name="-m"></tt>
210 (generate map file). Normally the map file will not include empty segments
211 and sections, or unreferenced symbols. Using this option, you can force the
212 linker to include all this information into the map file.
215 <label id="option-C">
216 <tag><tt>-C</tt></tag>
218 This gives the name of an output config file to use. See section 4 for more
219 information about config files. -C may not be used together with <tt><ref
220 id="option-t" name="-t"></tt>.
223 <label id="option-D">
224 <tag><tt>-D sym=value, --define sym=value</tt></tag>
226 This option allows to define an external symbol on the command line. Value
227 may start with a '$' sign or with <tt/0x/ for hexadecimal values,
228 otherwise a leading zero denotes octal values. See also the <ref
229 id="SYMBOLS" name="SYMBOLS section"> in the configuration file.
232 <label id="option--lib-path">
233 <tag><tt>-L path, --lib-path path</tt></tag>
235 Specify a library search path. This option may be used more than once. It
236 adds a directory to the search path for library files. Libraries specified
237 without a path are searched in current directory, in the directory given in
238 the <tt/LD65_LIB/ environment variable, and in the list of directories
239 specified using <tt/--lib-path/.
242 <tag><tt>-Ln</tt></tag>
244 This option allows you to create a file that contains all global labels and
245 may be loaded into VICE emulator using the <tt/ll/ (load label) command. You
246 may use this to debug your code with VICE. Note: Older versions had some
247 bugs in the label code. If you have problems, please get the latest VICE
251 <label id="option-S">
252 <tag><tt>-S addr, --start-addr addr</tt></tag>
254 Using -S you may define the default starting address. If and how this
255 address is used depends on the config file in use. For the builtin
256 configurations, only the "none", "apple2" and "apple2enh" systems honor an
257 explicit start address, all other builtin config provide their own.
260 <tag><tt>-V, --version</tt></tag>
262 This option print the version number of the linker. If you send any
263 suggestions or bugfixes, please include this number.
266 <label id="option--cfg-path">
267 <tag><tt>--cfg-path path</tt></tag>
269 Specify a config file search path. This option may be used more than once.
270 It adds a directory to the search path for config files. A config file given
271 with the <tt><ref id="option-C" name="-C"></tt> option that has no path in
272 its name is searched in the current directory, in the directory given in the
273 <tt/LD65_CFG/ environment variable, and in the list of directories specified
274 using <tt/--cfg-path/.
277 <label id="option--dbgfile">
278 <tag><tt>--dbgfile name</tt></tag>
280 Specify an output file for debug information. Available information will be
281 written to this file. Using the <tt/-g/ option for the compiler and assembler
282 will increase the amount of information available. Please note that debug
283 information generation is currently being developed, so the format of the
284 file and its contents are subject to change without further notice.
287 <tag><tt>--lib file</tt></tag>
289 Links a library to the output. Use this command line option instead of just
290 naming the library file, if the linker is not able to determine the file
291 type because of an unusual extension.
294 <tag><tt>--obj file</tt></tag>
296 Links an object file to the output. Use this command line option instead
297 of just naming the object file, if the linker is not able to determine the
298 file type because of an unusual extension.
301 <label id="option--obj-path">
302 <tag><tt>--obj-path path</tt></tag>
304 Specify an object file search path. This option may be used more than once.
305 It adds a directory to the search path for object files. An object file
306 passed to the linker that has no path in its name is searched in current
307 directory, in the directory given in the <tt/LD65_OBJ/ environment variable,
308 and in the list of directories specified using <tt/--obj-path/.
314 <sect>Search paths<p>
316 Starting with version 2.10 there are now several search paths for files needed
317 by the linker: One for libraries, one for object files and one for config
321 <sect1>Library search path<p>
323 The library search path contains in this order:
326 <item>The current directory.
327 <item>A compiled in library path which is often <tt>/usr/lib/cc65/lib</tt> on
329 <item>The value of the environment variable <tt/LD65_LIB/ if it is defined.
330 <item>A subdirectory named <tt/lib/ of the directory defined in the environment
331 variable <tt/CC65_HOME/, if it is defined.
332 <item>Any directory added with the <tt><ref id="option--lib-path"
333 name="--lib-path"></tt> option on the command line.
337 <sect1>Object file search path<p>
339 The object file search path contains in this order:
342 <item>The current directory.
343 <item>A compiled in directory which is often <tt>/usr/lib/cc65/obj</tt> on
345 <item>The value of the environment variable <tt/LD65_OBJ/ if it is defined.
346 <item>A subdirectory named <tt/obj/ of the directory defined in the environment
347 variable <tt/CC65_HOME/, if it is defined.
348 <item>Any directory added with the <tt><ref id="option--obj-path"
349 name="--obj-path"></tt> option on the command line.
353 <sect1>Config file search path<p>
355 The config file search path contains in this order:
358 <item>The current directory.
359 <item>A compiled in directory which is often <tt>/usr/lib/cc65/cfg</tt> on
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>Any directory added with the <tt><ref id="option--cfg-path"
365 name="--cfg-path"></tt> option on the command line.
370 <sect>Detailed workings<p>
372 The linker does several things when combining object modules:
374 First, the command line is parsed from left to right. For each object file
375 encountered (object files are recognized by a magic word in the header, so
376 the linker does not care about the name), imported and exported
377 identifiers are read from the file and inserted in a table. If a library
378 name is given (libraries are also recognized by a magic word, there are no
379 special naming conventions), all modules in the library are checked if an
380 export from this module would satisfy an import from other modules. All
381 modules where this is the case are marked. If duplicate identifiers are
382 found, the linker issues a warning.
384 This procedure (parsing and reading from left to right) does mean, that a
385 library may only satisfy references for object modules (given directly or from
386 a library) named <em/before/ that library. With the command line
389 ld65 crt0.o clib.lib test.o
392 the module test.o may not contain references to modules in the library
393 clib.lib. If this is the case, you have to change the order of the modules
397 ld65 crt0.o test.o clib.lib
400 Step two is, to read the configuration file, and assign start addresses
401 for the segments and define any linker symbols (see <ref id="config-files"
402 name="Configuration files">).
404 After that, the linker is ready to produce an output file. Before doing that,
405 it checks its data for consistency. That is, it checks for unresolved
406 externals (if the output format is not relocatable) and for symbol type
407 mismatches (for example a zero page symbol is imported by a module as absolute
410 Step four is, to write the actual target files. In this step, the linker will
411 resolve any expressions contained in the segment data. Circular references are
412 also detected in this step (a symbol may have a circular reference that goes
413 unnoticed if the symbol is not used).
415 Step five is to output a map file with a detailed list of all modules,
416 segments and symbols encountered.
418 And, last step, if you give the <tt><ref id="option-v" name="-v"></tt> switch
419 twice, you get a dump of the segment data. However, this may be quite
420 unreadable if you're not a developer:-)
424 <sect>Configuration files<label id="config-files"><p>
426 Configuration files are used to describe the layout of the output file(s). Two
427 major topics are covered in a config file: The memory layout of the target
428 architecture, and the assignment of segments to memory areas. In addition,
429 several other attributes may be specified.
431 Case is ignored for keywords, that is, section or attribute names, but it is
432 <em/not/ ignored for names and strings.
436 <sect1>Memory areas<p>
438 Memory areas are specified in a <tt/MEMORY/ section. Lets have a look at an
439 example (this one describes the usable memory layout of the C64):
443 RAM1: start = $0800, size = $9800;
444 ROM1: start = $A000, size = $2000;
445 RAM2: start = $C000, size = $1000;
446 ROM2: start = $E000, size = $2000;
450 As you can see, there are two ram areas and two rom areas. The names
451 (before the colon) are arbitrary names that must start with a letter, with
452 the remaining characters being letters or digits. The names of the memory
453 areas are used when assigning segments. As mentioned above, case is
454 significant for these names.
456 The syntax above is used in all sections of the config file. The name
457 (<tt/ROM1/ etc.) is said to be an identifier, the remaining tokens up to the
458 semicolon specify attributes for this identifier. You may use the equal sign
459 to assign values to attributes, and you may use a comma to separate
460 attributes, you may also leave both out. But you <em/must/ use a semicolon to
461 mark the end of the attributes for one identifier. The section above may also
462 have looked like this:
465 # Start of memory section
483 There are of course more attributes for a memory section than just start and
484 size. Start and size are mandatory attributes, that means, each memory area
485 defined <em/must/ have these attributes given (the linker will check that). I
486 will cover other attributes later. As you may have noticed, I've used a
487 comment in the example above. Comments start with a hash mark (`#'), the
488 remainder of the line is ignored if this character is found.
493 Let's assume you have written a program for your trusty old C64, and you would
494 like to run it. For testing purposes, it should run in the <tt/RAM/ area. So
495 we will start to assign segments to memory sections in the <tt/SEGMENTS/
500 CODE: load = RAM1, type = ro;
501 RODATA: load = RAM1, type = ro;
502 DATA: load = RAM1, type = rw;
503 BSS: load = RAM1, type = bss, define = yes;
507 What we are doing here is telling the linker, that all segments go into the
508 <tt/RAM1/ memory area in the order specified in the <tt/SEGMENTS/ section. So
509 the linker will first write the <tt/CODE/ segment, then the <tt/RODATA/
510 segment, then the <tt/DATA/ segment - but it will not write the <tt/BSS/
511 segment. Why? Enter the segment type: For each segment specified, you may also
512 specify a segment attribute. There are four possible segment attributes:
517 bss means that this is an uninitialized segment
518 zp a zeropage segment
521 So, because we specified that the segment with the name BSS is of type bss,
522 the linker knows that this is uninitialized data, and will not write it to an
523 output file. This is an important point: For the assembler, the <tt/BSS/
524 segment has no special meaning. You specify, which segments have the bss
525 attribute when linking. This approach is much more flexible than having one
526 fixed bss segment, and is a result of the design decision to supporting an
527 arbitrary segment count.
529 If you specify "<tt/type = bss/" for a segment, the linker will make sure that
530 this segment does only contain uninitialized data (that is, zeroes), and issue
531 a warning if this is not the case.
533 For a <tt/bss/ type segment to be useful, it must be cleared somehow by your
534 program (this happens usually in the startup code - for example the startup
535 code for cc65 generated programs takes care about that). But how does your
536 code know, where the segment starts, and how big it is? The linker is able to
537 give that information, but you must request it. This is, what we're doing with
538 the "<tt/define = yes/" attribute in the <tt/BSS/ definitions. For each
539 segment, where this attribute is true, the linker will export three symbols.
542 __NAME_LOAD__ This is set to the address where the
544 __NAME_RUN__ This is set to the run address of the
545 segment. We will cover run addresses
547 __NAME_SIZE__ This is set to the segment size.
550 Replace <tt/NAME/ by the name of the segment, in the example above, this would
551 be <tt/BSS/. These symbols may be accessed by your code.
553 Now, as we've configured the linker to write the first three segments and
554 create symbols for the last one, there's only one question left: Where does
555 the linker put the data? It would be very convenient to have the data in a
558 <sect1>Output files<p>
560 We don't have any files specified above, and indeed, this is not needed in a
561 simple configuration like the one above. There is an additional attribute
562 "file" that may be specified for a memory area, that gives a file name to
563 write the area data into. If there is no file name given, the linker will
564 assign the default file name. This is "a.out" or the one given with the
565 <tt><ref id="option-o" name="-o"></tt> option on the command line. Since the
566 default behaviour is ok for our purposes, I did not use the attribute in the
567 example above. Let's have a look at it now.
569 The "file" attribute (the keyword may also be written as "FILE" if you like
570 that better) takes a string enclosed in double quotes (`"') that specifies the
571 file, where the data is written. You may specify the same file several times,
572 in that case the data for all memory areas having this file name is written
573 into this file, in the order of the memory areas defined in the <tt/MEMORY/
574 section. Let's specify some file names in the <tt/MEMORY/ section used above:
578 RAM1: start = $0800, size = $9800, file = %O;
579 ROM1: start = $A000, size = $2000, file = "rom1.bin";
580 RAM2: start = $C000, size = $1000, file = %O;
581 ROM2: start = $E000, size = $2000, file = "rom2.bin";
585 The <tt/%O/ used here is a way to specify the default behaviour explicitly:
586 <tt/%O/ is replaced by a string (including the quotes) that contains the
587 default output name, that is, "a.out" or the name specified with the <tt><ref
588 id="option-o" name="-o"></tt> option on the command line. Into this file, the
589 linker will first write any segments that go into <tt/RAM1/, and will append
590 then the segments for <tt/RAM2/, because the memory areas are given in this
591 order. So, for the RAM areas, nothing has really changed.
593 We've not used the ROM areas, but we will do that below, so we give the file
594 names here. Segments that go into <tt/ROM1/ will be written to a file named
595 "rom1.bin", and segments that go into <tt/ROM2/ will be written to a file
596 named "rom2.bin". The name given on the command line is ignored in both cases.
598 Assigning an empty file name for a memory area will discard the data written
599 to it. This is useful, if the memory area has segments assigned that are empty
600 (for example because they are of type bss). In that case, the linker will
601 create an empty output file. This may be suppressed by assigning an empty file
602 name to that memory area.
604 The <tt/%O/ sequence is also allowed inside a string. So using
608 ROM1: start = $A000, size = $2000, file = "%O-1.bin";
609 ROM2: start = $E000, size = $2000, file = "%O-2.bin";
613 would write two files that start with the name of the output file specified on
614 the command line, with "-1.bin" and "-2.bin" appended respectively. Because
615 '%' is used as an escape char, the sequence "%%" has to be used if a single
616 percent sign is required.
618 <sect1>LOAD and RUN addresses (ROMable code)<p>
620 Let us look now at a more complex example. Say, you've successfully tested
621 your new "Super Operating System" (SOS for short) for the C64, and you
622 will now go and replace the ROMs by your own code. When doing that, you
623 face a new problem: If the code runs in RAM, we need not to care about
624 read/write data. But now, if the code is in ROM, we must care about it.
625 Remember the default segments (you may of course specify your own):
629 RODATA read only data
631 BSS uninitialized data, read/write
634 Since <tt/BSS/ is not initialized, we must not care about it now, but what
635 about <tt/DATA/? <tt/DATA/ contains initialized data, that is, data that was
636 explicitly assigned a value. And your program will rely on these values on
637 startup. Since there's no other way to remember the contents of the data
638 segment, than storing it into one of the ROMs, we have to put it there. But
639 unfortunately, ROM is not writable, so we have to copy it into RAM before
640 running the actual code.
642 The linker won't copy the data from ROM into RAM for you (this must be done by
643 the startup code of your program), but it has some features that will help you
646 First, you may not only specify a "<tt/load/" attribute for a segment, but
647 also a "<tt/run/" attribute. The "<tt/load/" attribute is mandatory, and, if
648 you don't specify a "<tt/run/" attribute, the linker assumes that load area
649 and run area are the same. We will use this feature for our data area:
653 CODE: load = ROM1, type = ro;
654 RODATA: load = ROM2, type = ro;
655 DATA: load = ROM2, run = RAM2, type = rw, define = yes;
656 BSS: load = RAM2, type = bss, define = yes;
660 Let's have a closer look at this <tt/SEGMENTS/ section. We specify that the
661 <tt/CODE/ segment goes into <tt/ROM1/ (the one at $A000). The readonly data
662 goes into <tt/ROM2/. Read/write data will be loaded into <tt/ROM2/ but is run
663 in <tt/RAM2/. That means that all references to labels in the <tt/DATA/
664 segment are relocated to be in <tt/RAM2/, but the segment is written to
665 <tt/ROM2/. All your startup code has to do is, to copy the data from its
666 location in <tt/ROM2/ to the final location in <tt/RAM2/.
668 So, how do you know, where the data is located? This is the second point,
669 where you get help from the linker. Remember the "<tt/define/" attribute?
670 Since we have set this attribute to true, the linker will define three
671 external symbols for the data segment that may be accessed from your code:
674 __DATA_LOAD__ This is set to the address where the segment
675 is loaded, in this case, it is an address in
677 __DATA_RUN__ This is set to the run address of the segment,
678 in this case, it is an address in RAM2.
679 __DATA_SIZE__ This is set to the segment size.
682 So, what your startup code must do, is to copy <tt/__DATA_SIZE__/ bytes from
683 <tt/__DATA_LOAD__/ to <tt/__DATA_RUN__/ before any other routines are called.
684 All references to labels in the <tt/DATA/ segment are relocated to <tt/RAM2/
685 by the linker, so things will work properly.
687 There's a library subroutine called <tt/copydata/ (in a module named
688 <tt/copydata.s/) that might be used to do actual copying. Be sure to have a
689 look at it's inner workings before using it!
692 <sect1>Other MEMORY area attributes<p>
694 There are some other attributes not covered above. Before starting the
695 reference section, I will discuss the remaining things here.
697 You may request symbols definitions also for memory areas. This may be
698 useful for things like a software stack, or an i/o area.
702 STACK: start = $C000, size = $1000, define = yes;
706 This will define some external symbols that may be used in your code:
709 __STACK_START__ This is set to the start of the memory
710 area, $C000 in this example.
711 __STACK_SIZE__ The size of the area, here $1000.
712 __STACK_LAST__ This is NOT the same as START+SIZE.
713 Instead, it it defined as the first
714 address that is not used by data. If we
715 don't define any segments for this area,
716 the value will be the same as START.
717 __STACK_FILEOFFS__ The binary offset in the output file. This
718 is not defined for relocatable output file
722 A memory section may also have a type. Valid types are
725 ro for readonly memory
726 rw for read/write memory.
729 The linker will assure, that no segment marked as read/write or bss is put
730 into a memory area that is marked as readonly.
732 Unused memory in a memory area may be filled. Use the "<tt/fill = yes/"
733 attribute to request this. The default value to fill unused space is zero. If
734 you don't like this, you may specify a byte value that is used to fill these
735 areas with the "<tt/fillval/" attribute. If there is no "<tt/fillval/"
736 attribute for the segment, the "<tt/fillval/" attribute of the memory area (or
737 its default) is used instead. This means that the value may also be used to
738 fill unfilled areas generated by the assemblers <tt/.ALIGN/ and <tt/.RES/
741 The symbol <tt/%S/ may be used to access the default start address (that is,
742 the one defined in the <ref id="FEATURES" name="FEATURES"> section, or the
743 value given on the command line with the <tt><ref id="option-S" name="-S"></tt>
746 To support systems with banked memory, a special attribute named <tt/bank/ is
747 available. The attribute value is an arbitrary 32 bit integer. The assembler
748 has a builtin function named <tt/.BANK/ which may be used with an argument
749 that has a segment reference (for example a symbol). The result of this
750 function is the value of the bank attribute for the run memory area of the
754 <sect1>Other SEGMENT attributes<p>
756 Segments may be aligned to some memory boundary. Specify "<tt/align = num/" to
757 request this feature. Num must be a power of two. To align all segments on a
762 CODE: load = ROM1, type = ro, align = $100;
763 RODATA: load = ROM2, type = ro, align = $100;
764 DATA: load = ROM2, run = RAM2, type = rw, define = yes,
766 BSS: load = RAM2, type = bss, define = yes, align = $100;
770 If an alignment is requested, the linker will add enough space to the output
771 file, so that the new segment starts at an address that is dividable by the
772 given number without a remainder. All addresses are adjusted accordingly. To
773 fill the unused space, bytes of zero are used, or, if the memory area has a
774 "<tt/fillval/" attribute, that value. Alignment is always needed, if you have
775 used the <tt/.ALIGN/ command in the assembler. The alignment of a segment
776 must be equal or greater than the alignment used in the <tt/.ALIGN/ command.
777 The linker will check that, and issue a warning, if the alignment of a segment
778 is lower than the alignment requested in an <tt/.ALIGN/ command of one of the
779 modules making up this segment.
781 For a given segment you may also specify a fixed offset into a memory area or
782 a fixed start address. Use this if you want the code to run at a specific
783 address (a prominent case is the interrupt vector table which must go at
784 address $FFFA). Only one of <tt/ALIGN/ or <tt/OFFSET/ or <tt/START/ may be
785 specified. If the directive creates empty space, it will be filled with zero,
786 of with the value specified with the "<tt/fillval/" attribute if one is given.
787 The linker will warn you if it is not possible to put the code at the
788 specified offset (this may happen if other segments in this area are too
789 large). Here's an example:
793 VECTORS: load = ROM2, type = ro, start = $FFFA;
797 or (for the segment definitions from above)
801 VECTORS: load = ROM2, type = ro, offset = $1FFA;
805 The "<tt/align/", "<tt/start/" and "<tt/offset/" attributes change placement
806 of the segment in the run memory area, because this is what is usually
807 desired. If load and run memory areas are equal (which is the case if only the
808 load memory area has been specified), the attributes will also work. There is
809 also an "<tt/align_load/" attribute that may be used to align the start of the
810 segment in the load memory area, in case different load and run areas have
811 been specified. There are no special attributes to set start or offset for
812 just the load memory area.
814 A "<tt/fillval/" attribute may not only be specified for a memory area, but
815 also for a segment. The value must be an integer between 0 and 255. It is used
816 as fill value for space reserved by the assemblers <tt/.ALIGN/ and <tt/.RES/
817 commands. It is also used as fill value for space between sections (part of a
818 segment that comes from one object file) caused by alignment, but not for
819 space that preceeds the first section.
821 To suppress the warning, the linker issues if it encounters a segment that is
822 not found in any of the input files, use "<tt/optional=yes/" as additional
823 segment attribute. Be careful when using this attribute, because a missing
824 segment may be a sign of a problem, and if you're suppressing the warning,
825 there is no one left to tell you about it.
827 <sect1>The FILES section<p>
829 The <tt/FILES/ section is used to support other formats than straight binary
830 (which is the default, so binary output files do not need an explicit entry
831 in the <tt/FILES/ section).
833 The <tt/FILES/ section lists output files and as only attribute the format of
834 each output file. Assigning binary format to the default output file would
843 The only other available output format is the o65 format specified by Andre
844 Fachat (see the <htmlurl url="http://www.6502.org/users/andre/o65/fileformat.html"
845 name="6502 binary relocation format specification">). It is defined like this:
853 The necessary o65 attributes are defined in a special section labeled
858 <sect1>The FORMAT section<p>
860 The <tt/FORMAT/ section is used to describe file formats. The default (binary)
861 format has currently no attributes, so, while it may be listed in this
862 section, the attribute list is empty. The second supported format,
863 <htmlurl url="http://www.6502.org/users/andre/o65/fileformat.html" name="o65">,
864 has several attributes that may be defined here.
868 o65: os = lunix, version = 0, type = small,
869 import = LUNIXKERNEL,
876 <sect1>The FEATURES section<label id="FEATURES"><p>
878 In addition to the <tt/MEMORY/ and <tt/SEGMENTS/ sections described above, the
879 linker has features that may be enabled by an additional section labeled
883 <sect2>The CONDES feature<p>
885 <tt/CONDES/ is used to tell the linker to emit module constructor/destructor
890 CONDES: segment = RODATA,
892 label = __CONSTRUCTOR_TABLE__,
893 count = __CONSTRUCTOR_COUNT__;
897 The <tt/CONDES/ feature has several attributes:
901 <tag><tt>segment</tt></tag>
903 This attribute tells the linker into which segment the table should be
904 placed. If the segment does not exist, it is created.
907 <tag><tt>type</tt></tag>
909 Describes the type of the routines to place in the table. Type may be one of
910 the predefined types <tt/constructor/, <tt/destructor/, <tt/interruptor/, or
911 a numeric value between 0 and 6.
914 <tag><tt>label</tt></tag>
916 This specifies the label to use for the table. The label points to the start
917 of the table in memory and may be used from within user written code.
920 <tag><tt>count</tt></tag>
922 This is an optional attribute. If specified, an additional symbol is defined
923 by the linker using the given name. The value of this symbol is the number
924 of entries (<em/not/ bytes) in the table. While this attribute is optional,
925 it is often useful to define it.
928 <tag><tt>order</tt></tag>
930 Optional attribute that takes one of the keywords <tt/increasing/ or
931 <tt/decreasing/ as an argument. Specifies the sorting order of the entries
932 within the table. The default is <tt/increasing/, which means that the
933 entries are sorted with increasing priority (the first entry has the lowest
934 priority). "Priority" is the priority specified when declaring a symbol as
935 <tt/.CONDES/ with the assembler, higher values mean higher priority. You may
936 change this behaviour by specifying <tt/decreasing/ as the argument, the
937 order of entries is reversed in this case.
939 Please note that the order of entries with equal priority is undefined.
941 <tag><tt>import</tt></tag>
943 This attribute defines a valid symbol name, that is added as an import
944 to the modules defining a constructor/desctructor of the given type.
945 This can be used to force linkage of a module if this module exports the
950 Without specifying the <tt/CONDES/ feature, the linker will not create any
951 tables, even if there are <tt/condes/ entries in the object files.
953 For more information see the <tt/.CONDES/ command in the <htmlurl
954 url="ca65.html" name="ca65 manual">.
957 <sect2>The STARTADDRESS feature<p>
959 <tt/STARTADDRESS/ is used to set the default value for the start address,
960 which can be referenced by the <tt/%S/ symbol. The builtin default for the
961 linker is $200.
965 # Default start address is $1000
966 STARTADDRESS: default = $1000;
970 Please note that order is important: The default start address must be defined
971 <em/before/ the <tt/%S/ symbol is used in the config file. This does usually
972 mean, that the <tt/FEATURES/ section has to go to the top of the config file.
976 <sect1>The SYMBOLS section<label id="SYMBOLS"><p>
978 The configuration file may also be used to define symbols used in the link
979 stage or to force symbols imports. This is done in the SYMBOLS section. The
980 symbol name is followed by a colon and symbol attributes.
982 The following symbol attributes are supported:
986 <tag><tt>addrsize</tt></tag>
988 The <tt/addrsize/ attribute specifies the address size of the symbol and
991 <item><tt/zp/, <tt/zeropage/ or <tt/direct/
992 <item><tt/abs/, <tt/absolute/ or <tt/near/
994 <item><tt/long/ or <tt/dword/.
997 Without this attribute, the default address size is <tt/abs/.
999 <tag><tt>type</tt></tag>
1001 This attribute is mandatory. Its value is one of <tt/export/, <tt/import/ or
1002 <tt/weak/. <tt/export/ means that the symbol is defined and exported from
1003 the linker config. <tt/import/ means that an import is generated for this
1004 symbol, eventually forcing a module that exports this symbol to be included
1005 in the output. <tt/weak/ is similar as <tt/export/. However, the symbol is
1006 only defined if it is not defined elsewhere.
1008 <tag><tt>value</tt></tag>
1010 This must only be given for symbols of type <tt/export/ or <tt/weak/. It
1011 defines the value of the symbol and may be an expression.
1015 The following example defines the stack size for an application, but allows
1016 the programmer to override the value by specifying <tt/--define
1017 __STACKSIZE__=xxx/ on the command line.
1021 # Define the stack size for the application
1022 __STACKSIZE__: type = weak, value = $800;
1028 <sect1>Builtin configurations<p>
1030 The builtin configurations are part of the linker source. They can be retrieved
1031 with <tt/--dump-config/ and don't have a special format. So if you need a
1032 special configuration, it's a good idea to start with the builtin configuration
1033 for your system. In a first step, just replace <tt/-t target/ by <tt/-C
1034 configfile/. Then go on and modify the config file to suit your needs.
1038 <sect1>Secondary configurations<p>
1040 Several machine specific binary packages are distributed together with secondary
1041 configurations (in the cfg directory). These configurations can be used with
1042 <tt/-C configfile/ too.
1046 <sect>Special segments<p>
1048 The builtin config files do contain segments that have a special meaning for
1049 the compiler and the libraries that come with it. If you replace the builtin
1050 config files, you will need the following information.
1054 The INIT segment is used for initialization code that may be reused once
1055 execution reaches main() - provided that the program runs in RAM. You
1056 may for example add the INIT segment to the heap in really memory
1057 constrained systems.
1061 For the LOWCODE segment, it is guaranteed that it won't be banked out, so it
1062 is reachable at any time by interrupt handlers or similar.
1066 This segment contains the startup code which initializes the C software stack
1067 and the libraries. It is placed in its own segment because it needs to be
1068 loaded at the lowest possible program address on several platforms.
1072 The ZPSAVE segment contains the original values of the zeropage locations used
1073 by the ZEROPAGE segment. It is placed in its own segment because it must not be
1078 <sect>Bugs/Feedback<p>
1080 If you have problems using the linker, if you find any bugs, or if you're
1081 doing something interesting with it, I would be glad to hear from you. Feel
1082 free to contact me by email (<htmlurl url="mailto:uz@cc65.org"
1083 name="uz@cc65.org">).
1089 ld65 (and all cc65 binutils) are (C) Copyright 1998-2005 Ullrich von
1090 Bassewitz. For usage of the binaries and/or sources the following
1091 conditions do apply:
1093 This software is provided 'as-is', without any expressed or implied
1094 warranty. In no event will the authors be held liable for any damages
1095 arising from the use of this software.
1097 Permission is granted to anyone to use this software for any purpose,
1098 including commercial applications, and to alter it and redistribute it
1099 freely, subject to the following restrictions:
1102 <item> The origin of this software must not be misrepresented; you must not
1103 claim that you wrote the original software. If you use this software
1104 in a product, an acknowledgment in the product documentation would be
1105 appreciated but is not required.
1106 <item> Altered source versions must be plainly marked as such, and must not
1107 be misrepresented as being the original software.
1108 <item> This notice may not be removed or altered from any source