MPU region configuration in ARM Architecture

Introduction

The MPU (Memory Protection Unit) helps protect memory areas by controlling access rights. Configuring MPU regions lets you decide which parts of memory can be read, written, or executed.

To prevent accidental overwriting of important data in memory.

To protect code sections from being modified by mistake.

To isolate different parts of a program for security.

To control access permissions for peripherals or memory areas.

To catch bugs early by restricting illegal memory access.

Core Concept

ARM Architecture

MPU->RNR = region_number;
MPU->RBAR = base_address;
MPU->RASR = attributes_and_size;

MPU->RNR selects which region to configure.

MPU->RBAR sets the start address of the region.

Key Points

Configures region 0 starting at address 0x20000000 with specific size and permissions.

ARM Architecture

MPU->RNR = 0;
MPU->RBAR = 0x20000000;
MPU->RASR = (0x3 << 24) | (0xE << 1) | 1;

Configures region 1 for flash memory with execute permissions.

ARM Architecture

MPU->RNR = 1;
MPU->RBAR = 0x08000000;
MPU->RASR = (0x5 << 24) | (0x7 << 1) | 1;

Detailed Explanation

This example disables the MPU, sets region 0 to cover SRAM starting at 0x20000000 with read/write access, then enables the MPU again.

ARM Architecture

/* Example: Configure MPU region 0 for SRAM with read/write permissions */
MPU->CTRL = 0; // Disable MPU before configuration
MPU->RNR = 0; // Select region 0
MPU->RBAR = 0x20000000; // Base address of SRAM
MPU->RASR = (0x3 << 24) | (0xE << 1) | 1; // Enable region, size 32KB, full access
MPU->CTRL = 1; // Enable MPU

Important Notes

Always disable the MPU before changing region settings to avoid errors.

Region sizes must be powers of two and aligned to their size.

MPU settings take effect immediately after enabling the MPU.

Summary

MPU regions control memory access rights to protect memory.

Each region has a number, base address, size, and access permissions.

Proper configuration helps improve system safety and security.

Practice

1. What is the primary purpose of configuring MPU regions in an ARM system?

easy

A. To increase CPU clock speed

B. To handle input/output device communication

C. To manage power consumption

D. To control memory access rights and protect memory

Solution

1. Step 1: Understand MPU function

   The Memory Protection Unit (MPU) is designed to control access to different memory areas.

2. Step 2: Identify the main goal of MPU regions

   MPU regions define access permissions to protect memory from unauthorized access.

3. Final Answer:

   To control memory access rights and protect memory -> Option D

4. Quick Check:

   MPU protects memory = B [OK]

Hint: MPU is about memory protection, not speed or power [OK]

Common Mistakes:

• Confusing MPU with CPU speed control
• Thinking MPU manages power or I/O devices
• Assuming MPU handles software tasks

2. Which of the following is the correct way to specify the size of an MPU region?

easy

A. Using a power-of-two size like 256, 512, 1024 bytes

B. Using the exact byte size like 1024 bytes

C. Using any arbitrary size value

D. Using the number of memory pages

Solution

1. Step 1: Recall MPU size rules

   MPU regions must have sizes that are powers of two (e.g., 256, 512, 1024 bytes).

2. Step 2: Eliminate incorrect options

   Exact byte sizes not power of two or arbitrary sizes are invalid; number of pages is unrelated.

3. Final Answer:

   Using a power-of-two size like 256, 512, 1024 bytes -> Option A

4. Quick Check:

   MPU size = power of two [OK]

Hint: MPU sizes must be powers of two, not arbitrary [OK]

Common Mistakes:

• Using sizes not in powers of two
• Confusing size with memory pages
• Specifying exact byte sizes without power-of-two check

3. Given an MPU region configured with base address 0x20000000 and size 512 bytes, which of the following addresses is NOT inside this region?

medium

A. 0x20000200

B. 0x200001FF

C. 0x20000000

D. 0x20000100

Solution

1. Step 1: Calculate region address range

   Base address is 0x20000000, size is 512 bytes, so region covers 0x20000000 to 0x200001FF (512 bytes).

2. Step 2: Check each address

   Addresses 0x20000100, 0x200001FF, and 0x20000000 fall inside the range; 0x20000200 is outside.

3. Final Answer:

   0x20000200 -> Option A

4. Quick Check:

   Address outside 0x20000000-0x200001FF = D [OK]

Hint: Region ends at base + size - 1 address [OK]

Common Mistakes:

• Off-by-one errors in address range
• Misunderstanding base address as inclusive/exclusive
• Confusing hex address increments

4. A developer configures an MPU region with base address 0x10000000 and size 300 bytes. What is the problem with this configuration?

medium

A. Base address must be zero

B. Size must be a power of two, 300 is invalid

C. Base address must be aligned to size, 0x10000000 is not aligned

D. No problem, configuration is valid

Solution

1. Step 1: Check size validity

   MPU region sizes must be powers of two; 300 is not a power of two.

2. Step 2: Verify base address alignment

   Base address must be aligned to the size, but since size is invalid, alignment check is secondary.

3. Final Answer:

   Size must be a power of two, 300 is invalid -> Option B

4. Quick Check:

   MPU size power-of-two rule violated = A [OK]

Hint: Always check size is power of two before alignment [OK]

Common Mistakes:

• Ignoring size power-of-two requirement
• Assuming any base address is valid
• Confusing alignment rules with size validity

5. You want to configure two MPU regions: one for a 1 KB read-only memory area starting at 0x08000000, and another for a 2 KB read-write memory area starting at 0x08000400. Which of the following configurations correctly sets the base addresses and sizes without overlap?

hard

A. Region1: base=0x08000000, size=1024 bytes; Region2: base=0x08000400, size=2048 bytes

B. Region1: base=0x08000000, size=2048 bytes; Region2: base=0x08000400, size=1024 bytes

C. Region1: base=0x08000000, size=1024 bytes; Region2: base=0x08000800, size=2048 bytes

D. Region1: base=0x08000000, size=512 bytes; Region2: base=0x08000200, size=2048 bytes

Solution

1. Step 1: Calculate address ranges for each region

   Region1: base 0x08000000, size 1024 bytes covers 0x08000000 to 0x080003FF.
   Region2: base 0x08000400, size 2048 bytes covers 0x08000400 to 0x08000BFF.

2. Step 2: Check for overlap and alignment

   Region1: base=0x08000000, size=1024 bytes; Region2: base=0x08000400, size=2048 bytes -- invalid MPU configuration because Region2 base address 0x08000400 is not aligned to its size 2048 bytes (0x08000400 mod 2048 = 1024 ≠ 0).
   Region1: base=0x08000000, size=1024 bytes; Region2: base=0x08000800, size=2048 bytes sets Region2 base at 0x08000800, which is after Region1 ends, no overlap.
   Region1: base=0x08000000, size=2048 bytes; Region2: base=0x08000400, size=1024 bytes swaps sizes causing overlap.
   Region1: base=0x08000000, size=512 bytes; Region2: base=0x08000200, size=2048 bytes sizes and bases cause overlap and incorrect sizes.

3. Final Answer:

   Region1: base=0x08000000, size=1024 bytes; Region2: base=0x08000800, size=2048 bytes -> Option C

4. Quick Check:

   No overlap and correct alignment = C [OK]

Hint: Ensure regions do not overlap by checking address ranges [OK]

Common Mistakes:

• Overlapping regions due to incorrect base or size
• Ignoring alignment to power-of-two sizes
• Swapping sizes and bases incorrectly