Because of swapping and compaction, a process may occupy different main memory locations during its lifetime.
Therefore, physical memory references by a process cannot be fixed.
Physical address versus logical address:
- Physical address is an absolute address in main memory.
- Logical address is a reference to a memory location independent of the physical organization of memory.
- Relative address is a logical address in which the address is expressed as a location relative to some known point in the program (e.g. the beginning).