how does relocation work?
what is a relocating loader
Relocation refers to the process of adjusting the physical addresses of program code and data when a program is loaded into memory. It ensures that the program can execute correctly regardless of where it is loaded in memory. Relocation is necessary because program code and data have fixed addresses during development but can be loaded into different memory locations during execution.
When a program is compiled, the compiler typically generates object code with relative addresses. These addresses are relative to the start of the program's memory space. However, the actual memory address where the program will be loaded is not known at compile time.
To handle this, a relocating loader is used. A relocating loader is a tool or a part of the operating system responsible for loading an executable program into memory and adjusting the program's addresses based on the actual memory location where it is loaded.
Here's a simplified explanation of how a relocating loader works:
1. The loader reads the executable file and determines the size of the program code and data sections.
2. It allocates memory to load the program into.
3. The loader loads the program code and data into the allocated memory.
4. It then analyzes the program's object code and identifies all the addresses that need to be adjusted.
5. The loader calculates the difference between the actual address where the program is loaded and the expected base address.
6. Finally, it updates the program's absolute addresses by adding the calculated difference, effectively relocating the program in memory.
By performing the relocation process, the loader ensures that the program can run correctly regardless of where it is loaded in memory. This is particularly important when multiple programs are simultaneously running or when the computer's memory is fragmented.