Question

1. A compiler generates the object code for individual modules, and a linkage editor is used to combine multiple object modules into a single program binary. How does the linkage editor change the binding of instructions and data to memory addresses? What information needs to be passed from the compiler to the linkage editor to facilitate the memory binding tasks of the linkage editor?

Answer 1

The linkage editor works with object files. This is when the compilation reaches the last stage and all the code has been converted into libraries. At this point, these libraries have to be linked together.

When there are variables and their references used in different program files, the overall execution requires them to be linked to produce the result. The variable/object value and its references used somewhere else is a simple example of binding.

• The linkage editor makes use of data with the object.
• In some cases, the address required can only be known when the object is loaded into memory.
• The linkage editor creates relocation records and processes them.
• The offset and dependency information is used to find the exact address.

When the reference of a relocatable object doesn't match the definition the object is considered unresolved. It can result in errors. The linkage editor requires to restore all the unresolved addresses with the ones that are related to actual variables of the program.

• To do that, the modules keep track of all the instructions that refer to unresolved addresses.
• In the process of linking, all the modules are given an address sequence in the pre-compiled program.
• This program offers unresolved references to some objects.
• The unresolved references are connected to other modules.
• This is done because all the modules are containing instructions to be patched.

The compiler passes the information related to memory to the linkage editor in the form of a script.