Linker

There is no reason why assembler cannot generate an executable program as object code file but it is almost never done. The object module cannot be run by itself but requires another step called linking. This model allows breaking big program into smaller parts and each gets written on its own separately.

To process each modules into an executable, linker first builds an index called symbol table, with an entry for every named item in every object module. Once the symbol table is complete, the linker builds an image of how the executable will be arranged in memory when the OS loads it. The image is written to the disk as the executable.

Object modules are allowed to refer to symbols in other object modules using the EXTERN keyword. In the declaring module, the label must be defined using the GLOBAL keyword.

During assembly, these external references are left as holes in the calling module to be filled later. As the linker builds the executable image, it learns where these symbols are in the final image and update them in the reference holes.

Debugging information is a step backward: portions of the soruce code, which was all stripped out early in the assembly process, are put back into the object module by the assembler. These portions of the source code are mostly names of data items and procedures.

Linker also makes the executable relocatable (all address are referenced relative to the start of the executable. This enable the executable to be loaded in any physcial address.