ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

Protected Memory System Architecture (PMSA)
B4.1 About the PMSA
The PMSA is based on a Memory Protection Unit (MPU). The PMSA provides a much simpler memory
protection scheme than the MMU based VMSA described in Chapter B3 Virtual Memory System
Architecture (VMSA). The simplification applies to both the hardware and the software. A PMSAv7
processor is identified by the presence of the MPU Type Register, see c0, MPU Type Register (MPUIR) on
page B4-36.
The main simplification is that the MPU does not use translation tables. Instead, System Control
Coprocessor (CP15) registers are used to define protection regions. The protection regions eliminate the
need for:
hardware to perform translation table walks
software to set up and maintain the translation tables.
The use of protection regions has the benefit of making the memory checking fully deterministic. However,
the level of control is region based rather than page based, meaning the control is considerably less
fine-grained than in the VMSA.
A second simplification is that the PMSA does not support virtual to physical address mapping other than
flat address mapping. The physical memory address accessed is the same as the virtual address generated
by the processor.
B4.1.1 Protection regions
In a PMSA implementation, you can use CP15 registers to define protection regions in the physical memory
map. When describing a PMSA implementation, protection regions are often referred to as regions.
This means the PMSA has the following features:
For each defined region, CP15 registers specify:
— the region size
— the base address
— the memory attributes, for example, memory type and access permissions.
Regions of 256 bytes or larger can be split into 8 sub-regions for improved granularity of memory
access control.
The minimum region size supported is IMPLEMENTATION DEFINED.
Memory region control, requiring read and write access to the region configuration registers, is
possible only from privileged modes.
Regions can overlap. If an address is defined in multiple regions, a fixed priority scheme is used to
define the properties of the address being accessed. This scheme gives priority to the region with the
highest region number.
The PMSA can be configured so that an access to an address that is not defined in any region either:
— causes a memory abort
— if it is a privileged access, uses the default memory map.
B4-2 Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. ARM DDI 0406B