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
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 a memory area has segments assigned that are
600 empty (for example because they are of type bss). In that case, the linker
601 will create an empty output file. This may be suppressed by assigning an empty
602 file 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 cannot help you copying the data from ROM into RAM (this must be
643 done by the startup code of your program), but it has some features that will
644 help you in this process.
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.
688 <sect1>Other MEMORY area attributes<p>
690 There are some other attributes not covered above. Before starting the
691 reference section, I will discuss the remaining things here.
693 You may request symbols definitions also for memory areas. This may be
694 useful for things like a software stack, or an i/o area.
698 STACK: start = $C000, size = $1000, define = yes;
702 This will define some external symbols that may be used in your code:
705 __STACK_START__ This is set to the start of the memory
706 area, $C000 in this example.
707 __STACK_SIZE__ The size of the area, here $1000.
708 __STACK_LAST__ This is NOT the same as START+SIZE.
709 Instead, it it defined as the first
710 address that is not used by data. If we
711 don't define any segments for this area,
712 the value will be the same as START.
713 __STACK_FILEOFFS__ The binary offset in the output file. This
714 is not defined for relocatable output file
718 A memory section may also have a type. Valid types are
721 ro for readonly memory
722 rw for read/write memory.
725 The linker will assure, that no segment marked as read/write or bss is put
726 into a memory area that is marked as readonly.
728 Unused memory in a memory area may be filled. Use the "<tt/fill = yes/"
729 attribute to request this. The default value to fill unused space is zero. If
730 you don't like this, you may specify a byte value that is used to fill these
731 areas with the "<tt/fillval/" attribute. This value is also used to fill unfilled
732 areas generated by the assemblers <tt/.ALIGN/ and <tt/.RES/ directives.
734 The symbol <tt/%S/ may be used to access the default start address (that is,
735 the one defined in the <ref id="FEATURES" name="FEATURES"> section, or the
736 value given on the command line with the <tt><ref id="option-S" name="-S"></tt>
739 To support systems with banked memory, a special attribute named <tt/bank/ is
740 available. The attribute value is an arbitrary 32 bit integer. The assembler
741 has a builtin function named <tt/.BANK/ which may be used with an argument
742 that has a segment reference (for example a symbol). The result of this
743 function is the value of the bank attribute for the run memory area of the
747 <sect1>Other SEGMENT attributes<p>
749 Segments may be aligned to some memory boundary. Specify "<tt/align = num/" to
750 request this feature. Num must be a power of two. To align all segments on a
755 CODE: load = ROM1, type = ro, align = $100;
756 RODATA: load = ROM2, type = ro, align = $100;
757 DATA: load = ROM2, run = RAM2, type = rw, define = yes,
759 BSS: load = RAM2, type = bss, define = yes, align = $100;
763 If an alignment is requested, the linker will add enough space to the output
764 file, so that the new segment starts at an address that is dividable by the
765 given number without a remainder. All addresses are adjusted accordingly. To
766 fill the unused space, bytes of zero are used, or, if the memory area has a
767 "<tt/fillval/" attribute, that value. Alignment is always needed, if you have
768 used the <tt/.ALIGN/ command in the assembler. The alignment of a segment
769 must be equal or greater than the alignment used in the <tt/.ALIGN/ command.
770 The linker will check that, and issue a warning, if the alignment of a segment
771 is lower than the alignment requested in an <tt/.ALIGN/ command of one of the
772 modules making up this segment.
774 For a given segment you may also specify a fixed offset into a memory area or
775 a fixed start address. Use this if you want the code to run at a specific
776 address (a prominent case is the interrupt vector table which must go at
777 address $FFFA). Only one of <tt/ALIGN/ or <tt/OFFSET/ or <tt/START/ may be
778 specified. If the directive creates empty space, it will be filled with zero,
779 of with the value specified with the "<tt/fillval/" attribute if one is given.
780 The linker will warn you if it is not possible to put the code at the
781 specified offset (this may happen if other segments in this area are too
782 large). Here's an example:
786 VECTORS: load = ROM2, type = ro, start = $FFFA;
790 or (for the segment definitions from above)
794 VECTORS: load = ROM2, type = ro, offset = $1FFA;
798 The "<tt/align/", "<tt/start/" and "<tt/offset/" attributes change placement
799 of the segment in the run memory area, because this is what is usually
800 desired. If load and run memory areas are equal (which is the case if only the
801 load memory area has been specified), the attributes will also work. There is
802 also an "<tt/align_load/" attribute that may be used to align the start of the
803 segment in the load memory area, in case different load and run areas have
804 been specified. There are no special attributes to set start or offset for
805 just the load memory area.
807 To suppress the warning, the linker issues if it encounters a segment that is
808 not found in any of the input files, use "<tt/optional=yes/" as additional
809 segment attribute. Be careful when using this attribute, because a missing
810 segment may be a sign of a problem, and if you're suppressing the warning,
811 there is no one left to tell you about it.
813 <sect1>The FILES section<p>
815 The <tt/FILES/ section is used to support other formats than straight binary
816 (which is the default, so binary output files do not need an explicit entry
817 in the <tt/FILES/ section).
819 The <tt/FILES/ section lists output files and as only attribute the format of
820 each output file. Assigning binary format to the default output file would
829 The only other available output format is the o65 format specified by Andre
830 Fachat (see the <htmlurl url="http://www.6502.org/users/andre/o65/fileformat.html"
831 name="6502 binary relocation format specification">). It is defined like this:
839 The necessary o65 attributes are defined in a special section labeled
844 <sect1>The FORMAT section<p>
846 The <tt/FORMAT/ section is used to describe file formats. The default (binary)
847 format has currently no attributes, so, while it may be listed in this
848 section, the attribute list is empty. The second supported format,
849 <htmlurl url="http://www.6502.org/users/andre/o65/fileformat.html" name="o65">,
850 has several attributes that may be defined here.
854 o65: os = lunix, version = 0, type = small,
855 import = LUNIXKERNEL,
862 <sect1>The FEATURES section<label id="FEATURES"><p>
864 In addition to the <tt/MEMORY/ and <tt/SEGMENTS/ sections described above, the
865 linker has features that may be enabled by an additional section labeled
869 <sect2>The CONDES feature<p>
871 <tt/CONDES/ is used to tell the linker to emit module constructor/destructor
876 CONDES: segment = RODATA,
878 label = __CONSTRUCTOR_TABLE__,
879 count = __CONSTRUCTOR_COUNT__;
883 The <tt/CONDES/ feature has several attributes:
887 <tag><tt>segment</tt></tag>
889 This attribute tells the linker into which segment the table should be
890 placed. If the segment does not exist, it is created.
893 <tag><tt>type</tt></tag>
895 Describes the type of the routines to place in the table. Type may be one of
896 the predefined types <tt/constructor/, <tt/destructor/, <tt/interruptor/, or
897 a numeric value between 0 and 6.
900 <tag><tt>label</tt></tag>
902 This specifies the label to use for the table. The label points to the start
903 of the table in memory and may be used from within user written code.
906 <tag><tt>count</tt></tag>
908 This is an optional attribute. If specified, an additional symbol is defined
909 by the linker using the given name. The value of this symbol is the number
910 of entries (<em/not/ bytes) in the table. While this attribute is optional,
911 it is often useful to define it.
914 <tag><tt>order</tt></tag>
916 Optional attribute that takes one of the keywords <tt/increasing/ or
917 <tt/decreasing/ as an argument. Specifies the sorting order of the entries
918 within the table. The default is <tt/increasing/, which means that the
919 entries are sorted with increasing priority (the first entry has the lowest
920 priority). "Priority" is the priority specified when declaring a symbol as
921 <tt/.CONDES/ with the assembler, higher values mean higher priority. You may
922 change this behaviour by specifying <tt/decreasing/ as the argument, the
923 order of entries is reversed in this case.
925 Please note that the order of entries with equal priority is undefined.
929 Without specifying the <tt/CONDES/ feature, the linker will not create any
930 tables, even if there are <tt/condes/ entries in the object files.
932 For more information see the <tt/.CONDES/ command in the <htmlurl
933 url="ca65.html" name="ca65 manual">.
936 <sect2>The STARTADDRESS feature<p>
938 <tt/STARTADDRESS/ is used to set the default value for the start address,
939 which can be referenced by the <tt/%S/ symbol. The builtin default for the
940 linker is $200.
944 # Default start address is $1000
945 STARTADDRESS: default = $1000;
949 Please note that order is important: The default start address must be defined
950 <em/before/ the <tt/%S/ symbol is used in the config file. This does usually
951 mean, that the <tt/FEATURES/ section has to go to the top of the config file.
955 <sect1>The SYMBOLS section<label id="SYMBOLS"><p>
957 The configuration file may also be used to define symbols used in the link
958 stage or to force symbols imports. This is done in the SYMBOLS section. The
959 symbol name is followed by a colon and symbol attributes.
961 The following symbol attributes are supported:
965 <tag><tt>addrsize</tt></tag>
967 The <tt/addrsize/ attribute specifies the address size of the symbol and
970 <item><tt/zp/, <tt/zeropage/ or <tt/direct/
971 <item><tt/abs/, <tt/absolute/ or <tt/near/
973 <item><tt/long/ or <tt/dword/.
976 Without this attribute, the default address size is <tt/abs/.
978 <tag><tt>type</tt></tag>
980 This attribute is mandatory. Its value is one of <tt/export/, <tt/import/ or
981 <tt/weak/. <tt/export/ means that the symbol is defined and exported from
982 the linker config. <tt/import/ means that an import is generated for this
983 symbol, eventually forcing a module that exports this symbol to be included
984 in the output. <tt/weak/ is similar as <tt/export/. However, the symbol is
985 only defined if it is not defined elsewhere.
987 <tag><tt>value</tt></tag>
989 This must only be given for symbols of type <tt/export/ or <tt/weak/. It
990 defines the value of the symbol and may be an expression.
994 The following example defines the stack size for an application, but allows
995 the programmer to override the value by specifying <tt/--define
996 __STACKSIZE__=xxx/ on the command line.
1000 # Define the stack size for the application
1001 __STACKSIZE__: type = weak, value = $800;
1007 <sect1>Builtin configurations<p>
1009 The builtin configurations are part of the linker source. They can be retrieved
1010 with <tt/--dump-config/ and don't have a special format. So if you need a
1011 special configuration, it's a good idea to start with the builtin configuration
1012 for your system. In a first step, just replace <tt/-t target/ by <tt/-C
1013 configfile/. Then go on and modify the config file to suit your needs.
1017 <sect1>Secondary configurations<p>
1019 Several machine specific binary packages are distributed together with secondary
1020 configurations (in the cfg directory). These configurations can be used with
1021 <tt/-C configfile/ too.
1025 <sect>Special segments<p>
1027 The builtin config files do contain segments that have a special meaning for
1028 the compiler and the libraries that come with it. If you replace the builtin
1029 config files, you will need the following information.
1033 The INIT segment is used for initialization code that may be reused once
1034 execution reaches main() - provided that the program runs in RAM. You
1035 may for example add the INIT segment to the heap in really memory
1036 constrained systems.
1040 For the LOWCODE segment, it is guaranteed that it won't be banked out, so it
1041 is reachable at any time by interrupt handlers or similar.
1045 This segment contains the startup code which initializes the C software stack
1046 and the libraries. It is placed in its own segment because it needs to be
1047 loaded at the lowest possible program address on several platforms.
1051 The ZPSAVE segment contains the original values of the zeropage locations used
1052 by the ZEROPAGE segment. It is placed in its own segment because it must not be
1057 <sect>Bugs/Feedback<p>
1059 If you have problems using the linker, if you find any bugs, or if you're
1060 doing something interesting with it, I would be glad to hear from you. Feel
1061 free to contact me by email (<htmlurl url="mailto:uz@cc65.org"
1062 name="uz@cc65.org">).
1068 ld65 (and all cc65 binutils) are (C) Copyright 1998-2005 Ullrich von
1069 Bassewitz. For usage of the binaries and/or sources the following
1070 conditions do apply:
1072 This software is provided 'as-is', without any expressed or implied
1073 warranty. In no event will the authors be held liable for any damages
1074 arising from the use of this software.
1076 Permission is granted to anyone to use this software for any purpose,
1077 including commercial applications, and to alter it and redistribute it
1078 freely, subject to the following restrictions:
1081 <item> The origin of this software must not be misrepresented; you must not
1082 claim that you wrote the original software. If you use this software
1083 in a product, an acknowledgment in the product documentation would be
1084 appreciated but is not required.
1085 <item> Altered source versions must be plainly marked as such, and must not
1086 be misrepresented as being the original software.
1087 <item> This notice may not be removed or altered from any source