Definition
Logical vs. Physical Address
In memory management, the operating system distinguishes between two types of addresses:
- Physical Address (Absolute Address): The actual location in the physical random-access memory chips.
- Logical Address: A reference to a memory location that is independent of the current assignment of data to physical memory.
Relocation
Since processes may be moved between different memory locations (due to compaction or swapping), the addresses used within the compiled code cannot be physical. Instead, processes use relative addresses—logical addresses expressed as a displacement from some known point (usually the start of the process).
Address Translation
The hardware, specifically the Memory Management Unit, translates logical addresses to physical addresses at runtime using base and bound registers.
- Base Register: Stores the physical starting address of the process.
- Bound Register: Stores the length of the process’s allocated memory.
The physical address is calculated as:
Significance
- Protection: The MMU verifies that every logical address is within the bound register before performing the translation.
- Efficiency: Allows the OS to load a program anywhere in RAM without needing to modify the program’s internal address references.