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Memory Protection Unit
ARM DDI 0363E Copyright © 2009 ARM Limited. All rights reserved. 7-3
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This section describes:
• Memory regions
• Overlapping regions on page 7-4
• Background regions on page 7-6
• TCM regions on page 7-6.
7.1.1 Memory regions
Before the MPU is enabled, you must program at least one valid protection region. If you do not
do this, the processor will enter a state that only reset can recover.
When the MPU is disabled, no access permission checks are performed, and memory attributes
are assigned according to the default memory map. See Table 7-1 on page 7-2.
For more information on how to enable or disable the MPU, see MPU interaction with memory
system on page 7-11.
Depending on the implementation, the MPU has a maximum of eight or 12 regions. Using CP15
register c6 you can specify the following for each region:
• region base address
• region size
• subregion enables
• region attributes
• region access permissions
• region enable.
Region base address
The base address defines the start of the memory region. You must align this to a region-sized
boundary. For example, if a region size of 8KB is programmed for a given region, the base
address must be a multiple of 8KB.
Note
If the region is not aligned correctly, this results in Unpredictable behavior.
Region size
The region size is specified as a 5-bit value, encoding a range of values from 32 bytes, a
cache-line length, to 4GB. Table 4-32 on page 4-51 shows the encoding.
Subregions
Each region can be split into eight equal sized non-overlapping subregions. An access to a
memory address in a disabled subregion does not use the attributes and permissions defined for
that region. Instead, it uses the attributes and permissions of a lower priority region or generates
a background fault if no other regions overlap at that address. This enables increased protection
and memory attribute granularity.
All region sizes between 256 bytes and 4GB support eight subregions. Region sizes below 256
bytes do not support subregions, and the subregion disable field is SBZ/UNP for regions of less
than 256 bytes in size.