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# 4.4. Format Specific Linking Notes

Some formats require special information to be able to generate actual binaries. If the specific format you are interested in is not listed in this section, then there is nothing special you need to know about to create a final binary.

## 4.4.1. OS9 Modules

OS9 modules need to embed several items into the module header. These items are the type of module, the langauge of the module, the module attributes, the module revision number, the data size (bss), and the execution offset. These are all either calculated or default to reasonable values.

The data size is calcuated as the sum of all sections named "bss" or ".bss" in all object files that are linked together.

The execution offset is calculated from the address of the special symbol "__start" which must be an exported (external) symbol in one of the objects to be linked.

The type defaults to "Prgrm" or "Program module". The language defaults to "Objct" or "6809 object code". Attributes default to enabling the re-entrant flag. And finally, the revision defaults to zero.

The embedded module name is the output filename. If the output filename includes more than just the filename, this will probably not be what you want.

The type, language, attributes, revision, and module name can all be overridden by providing a special section in exactly one of the object files to be linked. This section is called "__os9" (note the two underscores). To override the type, language, attributes, or revision values, define a non-exported symbol in this section called "type", "lang", "attr", or "rev" respectively. Any other symbols defined are ignored. To override the module name, include as the only actual code in the section a NUL terminated string (the FCN directive is useful for this). If there is no code in the section or it beings with a NUL, the default name will be used. Any of the preceeding that are not defined in the special section will retain their default values.

The built-in link script for OS9 modules will place the following sections, in order, in the module: "code", ".text", "data", ".data". It will merge all sections with the name "bss" or ".bss" into the "data" section. All other section names are ignored. What this means is that you must define your data variables in the a section called "bss" or ".bss" even though you will be refencing them all as offsets from U. This does have the unpleasant side effect that all BSS references will end up being 16 bit offsets because the assembler cannot know what the offset will be once the linker is finished its work. Thus, if the tightest possible code is required, having LWASM directly output the module is a better choice.

While the built-in link script is probably sufficient for most purposes, you can provide your own script. If you provide a custom link script, you must start your code and data sections at location 000D to accommodate the module header. Otherwise, you will have an incorrect location for the execution offset. You must use the ENTRY directive in the script to define the entry point for the module.

It should also be obvious from the above that you cannot mix the bss (rmb) definitions with the module code when linking separately. Those familiar with typical module creation will probably find this an unpleasant difference but it is unavoidable.

It should also be noted that direct page references should also be avoided because you cannot know ahead of time whether the linker is going to end up putting a particular variable in the first 256 bytes of the module's data space. If, however, you know for certain you will have less than 256 bytes of defined data space across all of the object files that will be linked, you can instead use forced DP addressing for your data addresses instead of the ,u notation. When linking with 3rd party libraries, this practice should be avoided. Also, when creating libraries, always use the offset from U technique.

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