embOS MPU on CortexM
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()
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:
Settings | Resulting attributes | ||||||
---|---|---|---|---|---|---|---|
TEX | C | B | S | Memory Type | Shareability | Other attributes | |
000 | 0 | 0 | 0 | Strongly ordered | Shareable[1] | - | |
1 | |||||||
1 | 0 | Device | |||||
1 | |||||||
1 | 0 | 0 | Normal | Not shareable | Outer write through, no write allocate Inner write through, no write allocate | ||
1 | Shareable | ||||||
1 | 0 | Not shareable | Outer write back, no write allocate Inner write back, no write allocate | ||||
1 | Shareable | ||||||
001 | 0 | 0 | 0 | Not shareable | Outer noncacheable Inner noncacheable | ||
1 | Shareable | ||||||
1 | 0 | Reserved encoding | |||||
1 | |||||||
1 | 0 | 0 | Implementation defined encoding | ||||
1 | |||||||
1 | 0 | Normal | Not shareable | Outer write back, write and read allocate Inner write back, write and read allocate | |||
1 | Shareable | ||||||
010 | 0 | 0 | 0 | Device | Not shareable[1] | - | |
1 | |||||||
1 | 0 | Reserved encoding | |||||
1 | |||||||
1 | 0 | 0 | |||||
1 | |||||||
1 | 0 | ||||||
1 | |||||||
100 | 0 | 0 | 0 | Normal | Not shareable | Outer noncacheable Inner noncacheable | |
1 | Shareable | ||||||
1 | 0 | Not shareable | Outer noncacheable Inner write back, write and read allocate | ||||
1 | Shareable | ||||||
1 | 0 | 0 | Not shareable | Outer noncacheable Inner write through, no write allocate | |||
1 | Shareable | ||||||
1 | 0 | Not shareable | Outer noncacheable Inner write back, no write allocate | ||||
1 | Shareable | ||||||
101 | 0 | 0 | 0 | Not shareable | Outer write back, write and read allocate Inner noncacheable | ||
1 | Shareable | ||||||
1 | 0 | Not shareable | Outer write back, write and read allocate Inner write back, write and read allocate | ||||
1 | Shareable | ||||||
1 | 0 | 0 | Not shareable | Outer write back, write and read allocate Inner write through, no write allocate | |||
1 | Shareable | ||||||
1 | 0 | Not shareable | Outer write back, write and read allocate Inner write back, no write allocate | ||||
1 | Shareable | ||||||
110 | 0 | 0 | 0 | Not shareable | Outer write through, no write allocate Inner noncacheable | ||
1 | Shareable | ||||||
1 | 0 | Not shareable | Outer write through, no write allocate Inner write back, write and read allocate | ||||
1 | Shareable | ||||||
1 | 0 | 0 | Not shareable | Outer write through, no write allocate Inner write through, no write allocate | |||
1 | Shareable | ||||||
1 | 0 | Not shareable | Outer write through, no write allocate Inner write back, no write allocate | ||||
1 | Shareable | ||||||
111 | 0 | 0 | 0 | Not shareable | Outer write back, no write allocate Inner noncacheable | ||
1 | Shareable | ||||||
1 | 0 | Not shareable | Outer write back, no write allocate Inner write back, write and read allocate | ||||
1 | Shareable | ||||||
1 | 0 | 0 | Not shareable | Outer write back, no write allocate Inner write through, no write allocate | |||
1 | Shareable | ||||||
1 | 0 | Not shareable | Outer write back, no write allocate Inner write back, no write allocate | ||||
1 | Shareable |
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.
Example
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);