Mapping Virtual Address to Physical Address

The address translation can be done in the three ways or at three different stages –

Compile Time

In some cases it is pre-known at the compile time itself about the final address (physical) of the memory for a running process.

This helps the system overall as –

• It is faster to load an existing executable format process

Note – There are chances for the program itself to crash as the defined address space (physical) may already be preoccupied by another process. In this case crash may happen and the system itself or with user intervention has to re-compile the whole program.

Load Time

The above crashing issue can be solved here, or in cases where it is not known that where the process memory will finally be residing. Here the relocatable address will be generated.

Loader does –

• Job of the loader is to translate the above relocatable address to an absolute address
• Adds : Base address to all, logical addresses, in turn to generate absolute address.

Note – Reloading of the whole process is necessary if in case the base address itself has changed in the mean time,

Execution Time

We all know that at the time of execution of a process we may need dynamic memory to save intermediate results or remove the results, i.e. allocation and de-allocation is inadverent at execution time.

Adding addresses as follows –

• CPU generated logical address
  • 245
• MMU generated relocation register, which is also called as base register
  • 8000
• So finally physical memory location will be
  • 8000 + 245 = 8245