Compilers and assemblers generally create each file of object code with the program addresses starting at zero, but few computers let you load your program at location zero. If a program is created from multiple subprograms, all the subprograms have to be loaded at non-overlapping addresses. Relocation is the process of assigning load addresses to the various parts of the program, adjusting the code and data in the program to reflect the assigned addresses. In many systems, relocation happens more than once. It's quite common for a linker to create a program from multiple subprograms, and create one linked output program that starts at zero, with the various subprograms relocated to locations within the big program. Then when the program is loaded, the system picks the actual load address and the linked program is relocated as a whole to the load address.
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