embOS MPU on CortexM

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With embOS-MPU different memory regions with separate access rights and memory attributes can be defined. A memory region can be added with OS_MPU_AddRegion().


OS_MPU_AddRegion() has the following prototype:

void OS_MPU_AddRegion(OS_TASK* pTask,
                      OS_U32   BaseAddr,
                      OS_U32   Size,
                      OS_U32   Permissions,
                      OS_U32   Attributes);

ARMv7-M Memory Attributes

embOS-MPU includes defines for the permissions (e.g. OS_MPU_READONLY), but not for the attributes since they are core specific.

The Cortex-M memory attributes include the following bits:

Bufferable Write to memory can be carried out by a write buffer while the processor continues on next instruction execution.
Cacheable Data obtained from memory read can be copied to a memory cache so that next time it is accessed the value can be obtained from the cache to speed up the program execution.
Sharable Data in this memory region could be shared by multiple bus masters. Memory system needs to ensure coherency of data between different bus masters in shareable memory region.
TEX Type Extension field

These bits are implemented in the Cortex-M MPU Region Base Attribute and Size Register (0xE000EDA0):

31:29       Reserved
28          XN            R/W         — Instruction Access Disable (1 = disable instruction fetch from this region; an attempt to do so will result in a memory management fault)
27          Reserved
26:24       AP            R/W         — Data Access Permission field
23:22       Reserved
21:19       TEX           R/W         — Type Extension field
18          S             R/W         — Shareable
17          C             R/W         — Cacheable
16          B             R/W         — Bufferable
15:8        SRD           R/W         — Subregion disable
7:6         Reserved
5:1         REGION SIZE   R/W         — MPU Protection Region size
0           ENABLE        R/W         — Region enable

Possible values are:

TEX C B S Memory Type Shareability Other attributes
000 0 0 0 Strongly ordered Shareable[1] -
1 0 Device
1 0 0 Normal Not shareable Outer and inner write-through. No write allocate.
1 Shareable
1 0 Not shareable Outer and inner write-back. No write allocate.
1 Shareable
001 0 0 0 Not shareable Outer and inner noncacheable.
1 Shareable
1 0 Reserved encoding -
1 0 0 Implementation defined
1 0 Normal Not shareable Outer and inner write-back. Write and read allocate.
1 Shareable
010 0 0 0 Device Not shareable[1] Nonshared device.
1 0 Reserved encoding -
1 0 0
1 0
1BB A A 0 Normal Not shareable Cached memory, BB = outer policy, AA = inner policy:
Encoding Cache Policy
00 Non-cacheable
01 Write back, write and read allocate
10 Write through, no write allocate
11 Write back, no write allocate
1 Shareable
  1. 1.0 1.1 The value of the S-bit is ignored in this encoding.

With embOS-MPU Cortex-M the Attributes parameter is shifted by 16 bits and set in the Cortex-M Region Base Attribute and Size Register.


Setting the memory attribute of a region to Write back, no write allocate:

#define TEX_100    (4u << 3)
#define CACHEABLE  (1u << 1)
#define BUFFERABLE (1u << 0)

OS_MPU_AddRegion(&HPTask, 0x00, 0x2000, OS_MPU_READONLY, TEX_100 | BUFFERABLE | CACHEABLE);