MC9S12XE-Family Reference Manual , Rev. 1.19
Freescale Semiconductor
227
Chapter 4
Memory Protection Unit (S12XMPUV1)
4.1
Introduction
The MPU module provides basic functionality required to protect memory mapped resources from
undesired accesses. Multiple address range comparators compare memory accesses against eight memory
protection descriptors located in the MPU module to determine if each access is valid or not. The
comparison is sensitive to which bus master generates the access and the type of the access.
The MPU module can be used to isolate memory ranges accessible by different bus masters. It can be also
be used by an operating system or software kernel to isolate the regions of memory “legally” available to
specific software tasks, with the kernel re-configuring the task specific memory protection descriptors in
supervisor state during task-switching.
4.1.1
Preface
The following terms and abbreviations are used in the document.
Table 4-2. Terminology
4.1.2
Overview
The MPU module monitors the bus activity of each bus master. The data describing each access is fed into
multiple address range comparators. The output of the comparators is used to determine if a particular
Table 4-1. Revision History
Revision
Number
Revision Date
Sections
Affected
Description of Changes
V01.04
14 Sep 2005
- Added note to only use the CPU to clear the AE flag.
- Added disclaimer to avoid changing descriptors while they are in use
because of other bus-masters doing accesses.
V01.05
14 Mar 2006
- Clarified that interrupt generation is independent of AEF bit state.
- Corrected preliminary statement about execution of violating accesses.
V01.06
09 Oct 2006
- Made Revision History entries public.
Term
Meaning
MCU
Micro-Controller Unit
MPU
Memory Protection Unit
CPU
S12X Central Processing Unit (see S12XCPU Reference Manual)
XGATE
XGATE Co-processor (see XGATE chapter)
supervisor state
refers to the supervisor state of the S12XCPU (see S12XCPU Reference Manual)
user state
refers to the user state of the S12XCPU (see S12XCPU Reference Manual)
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