Executable code typically contains the addresses of data (eg: fetch,
store) or the address of instructions elsewhere within the particular
hunk of code (eg: branch).
If you include absolute addresses in the code, then the code has to be
mapped into the specified address range, or the address references won't
work. The code is position-dependent.
The image loader then has to fill in the checker-board of the address
space as hunks and pieces are mapped, finding and filling where it can
or must fit.
Or the loader punts, when there are address space conflicts. When two
hunks of code would otherwise end up overlapping.
The alternatives to position-dependence include what is variously known
as a "fix-up" in the image loader or image activator or whatever the
particular platform calls it; where launching the image includes
processing which finds and fixes these addresses when the code is mapped
and for where-ever the code is mapped into the address space.
Alternatively (and when the instruction set permits it) where the
offsets are all base- or segment-relative; where the target data or code
addresses are stored as offsets and the addresses are computed by the
processor at run-time. This code is position-independent.
With position-independent code, the image loader can then load
constituent hunks where-ever it has room in address space. Dynamic
loading (qv: the dlopen() C call) has the same flexibility with these
images, for code that's loaded at run-time.
The difference between position-dependent and position-independent is
minor and not something I've ever seen in the wild outside of a very few
degenerate cases. Where permissible, I tend to prefer to generate the
latter, though it does incur very slight penalties for the fix-ups or
the offset calculations. The overhead is usually lost down in the
noise, however.
This processing is analogous to how virtual memory is implemented. Well
worth your time understanding how that works (in general, or for a
specific box), if you haven't looked into an implementation.
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