Protection Unit
ARM DDI 0144B
Copyright 1999, 2000 ARM Limited. All rights reserved.
3-7
3.4
Overlapping regions
The protection unit can be programmed with two or more overlapping regions. When
overlapping regions are programmed, a fixed priority scheme is applied to determine
the region attributes that are applied to the memory access.
Attributes for region 7 take highest priority. Attributes for region 0 take lowest priority.
For example:
Data region 2 is programmed to be 4KB in size, starting from address
0x3000
with
Dap[1:0]=10 (Privileged mode full access, User mode read only).
Data region 1 is programmed to be 16KB in size, starting from address
0x0
with
Dap[1:0]=01 (Privileged mode access only).
When the processor performs a data load from address
0x3010
while in User mode, the
address falls into both region 1 and region 2, as shown in Figure 3-2 on page 3-7.
Because there is a clash, the attributes associated with region 2 are applied. In this case,
the load does not abort.
Figure 3-2 Overlapping memory regions
3.4.1
Background regions
Overlapping regions increase the flexibility of mapping the eight regions onto physical
memory devices in the system. You can also use the overlapping properties to specify a
background region. For example, there might be a number of physical memory areas
sparsely distributed across the 4GB address space. If a programming error occurs
therefore, it might be possible for the processor to issue an address that does not fall into
any defined region.
0x4000
0x3000
0x3010
0x0
Region 2
Region 1