Memory Mapping and Protection

• With relocation and limit registers, each logical address must be less than the limit register;
• The MMU maps the logical address dynamically by adding the value in the relocation register. This mapped address is sent to memory (see Fig. 4).

  Figure 4: Hardware support for relocation and limit registers.

  
• When the CPU scheduler selects a process for execution, the dispatcher loads the relocation and limit registers with the correct values as part of the context switch.
• The relocation-register scheme provides an effective way to allow the OS size to change dynamically.
• For example, the OS contains code and buffer space for device drivers.
  • If a device driver (or other OS service) is not commonly used, we do not want to keep the code and data in memory.
  • Such code is sometimes called transient OS code; it comes and goes as needed.
  • Thus, using this code changes the size of the OS during program execution.