Why is it not possible to enforce memory protection at compile time?

 Every process should be protected against unwanted interference by other process, whether intentional or accidental. Therefore programs in other process must not be able to reference memory locations in a process for reading or writing purposes without permission.

      Satisfaction of relocation requirements add to the difficulty in satisfying the protection requirement. For the reason that the location of the program in main memory is unpredictable, it is unfeasible to check absolute addresses at compile time to assure protection. The majority programming languages permit the dynamic calculation at the run time. Therefore all memory    references generated by a process should check at run time to ensure that they refer only to the memory space allocated to that process. It is able to be seen that the mechanism that supports relocation also supports protection.