idea of programming modules... don't need to recompile. just relink.
abstraction... know we are going to jump somewhere. only replace the
peice we need.
use a linker.
- put code and data modules symbollically into memory
- determine addr. of data and instruction labels
- patch internal and external references.
faster to patch code than to recompile entire thing.
like an editor: find old addresses, replace with new ones.
