Chapter 13 - Memory Protection Units

Abstract

Some embedded systems use a multitasking operating or control system and must ensure that a running task does not disrupt the operation of other tasks. The shielding of system resources and other tasks from unwanted access is called protection and is the subject of this chapter. ARM provides several processors that are equipped with hardware that actively protects system resources, either through a memory protection unit (MPU), or through a memory management unit (MMU). There are two methods to handle memory protection. The first method is known as unprotected and uses voluntarily enforced software control routines to manage rules for task interaction. The second method is known as protected and it uses hardware and software to enforce rules for task interaction. In a protected system, the hardware protects areas of memory by generating an abort when access permission is violated and software responds to handle the abort routines and manage control to memory-based resources. An ARM MPU uses regions as the primary construct for system protection. A region is a set of attributes associated with an area of memory. Regions can overlap, allowing the use of a background region to shield a dormant task's memory areas from unwanted access by the current running task. Several steps are required to initialize the MPU. The first step sets the size and location of the instruction and data regions using CP15:c6. The second step sets the access permission for each region using CP15:c5. The third step sets the cache and write- buffer attributes for each region using CP15:c2 for cache and CP15:c3 for the write buffer. The last step enables active regions using CP15:c6 and the caches, write buffer, and MPU using CP15:c1.