Chapter 1: Linking and loading

1  The basic job of any linker or loader is simple: it binds more abstract names to more concrete names, which permits programmers to write code using the more abstract names.

2  Overlays, a technique that let programmers arrange for different parts of a program to share the same memory, with each overlay loaded on demand when another part of the program called into it.

3  With the advent of hardware relocation and virtual memory, linkers and loaders actually got less complex, since each program could again have an entire address space.  Programs could be linked to be loaded at fixed addresses, with hardware rather than software relocation taking care of any load-time relocation. But computers with hardware relocation invariably run more than one program, frequently multiple copies of the same program. When a computer runs multiple instances of one program, some parts of the program are the same among all running instance (the executable code, in particular), while other parts are unique to each instance. If the parts that don't change can be separated out from the parts that do change, the operating system can use a single copy of the unchanging part, saving considerable storage. Compilers and assemblers were modified to create object code in multiple sections, with one section for read only code and another section for writable data, the linker had to be able to combine all of sections of each type so that the linked program would have all the code in one place and all of the data in another. This didn't delay address binding any more than it already was, since addresses were still assigned at link time, but more work was deferred to the linker to assign addresses for all the sections. ----- Lacrimosa: That's why the assembled  code is devided into "data" and "code" sections, or more sections. So, I get the original answer. :)