Not worth much if the '51 works as standard, single-purpose, "program once, use forever" embedded chip, but if you want to create something like OS, where you allocate memory for an application/module, then load it into that memory from some external sources, and the memory allocation address isn't know earlier, you have two options - either first (just before, during or just after loading into the memory) pass the program through some preprocessor that would adjust all addresses - slow, bulky, error-prone solution, or just write the programs/modules so they can be relocated - using only relative addressesing modes - sjmp, cjne, djnz, @A+PC etc, so no matter where the program is located in the memory, it behaves just the same. Of course if the allocation is more "stiff", i.e. your OS allocates whole 2K pages and loads the modules at the beginning of given page, your program can use AJMP but not LJMP etc.