What is a fixup?

An object file .o / .obj may refer to some external it will output placeholder code like

“Put address <some_symbol> to register 5”.

And a note that says “Fill in that <some_symnol> with the actual address of symbol ‘foo’”.

Fixup refers to the process of using the above 2 information to update <some_symbol> to the actual correct address.

This information can be stored in the assembler.