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.