Bus fault and memory protection in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the type of fault caused by an invalid memory access.

ARM Architecture

if (fault_status == [1]) {
    // Handle bus fault
}

Drag options to blanks, or click blank then click option'

A0x05

B0x01

C0x0B

D0x0F

Attempts:

3 left

💡 Hint

Common Mistakes

Using a status code that corresponds to a different fault type.

The value 0x05 corresponds to a bus fault status indicating an invalid memory access on ARM processors.

2fill in blank

medium

Complete the code to enable memory protection unit (MPU) for a specific region.

ARM Architecture

MPU->RNR = [1]; // Select region number
MPU->RBAR = region_base_address;
MPU->RASR = region_attributes;

Drag options to blanks, or click blank then click option'

A3

B1

C2

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using a region number outside the valid range.

Region number 0 is often the first MPU region to configure.

3fill in blank

hard

Fix the error in the code that checks if a bus fault occurred by reading the correct fault status register.

ARM Architecture

if (SCB->[1] & (1 << 1)) {
    // Bus fault occurred
}

Drag options to blanks, or click blank then click option'

ADFSR

BHFSR

CCFSR

DMMFAR

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong fault status register leading to incorrect fault detection.

The CFSR (Configurable Fault Status Register) bit 1 indicates if a Bus Fault occurred.

4fill in blank

hard

Fill both blanks to configure an MPU region with read-only access and enable it.

ARM Architecture

MPU->RASR = ([1] << 2) | ([2] << 0);

Drag options to blanks, or click blank then click option'

A0x06

B0x03

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing access permission bits or forgetting to enable the region.

0x06 sets the access permission bits for read-only, and 1 enables the region.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps region numbers to their base addresses if the region is enabled.

ARM Architecture

region_map = { [1]: [2] for [1] in range(8) if is_region_enabled([1]) }

Drag options to blanks, or click blank then click option'

Ai

Bget_region_base(i)

Dregion_num

Attempts:

3 left

💡 Hint

Common Mistakes

Using inconsistent variable names or incorrect function calls.

Using i as the loop variable and key, and get_region_base(i) to get the base address for enabled regions.

Practice

1. What is the primary purpose of a bus fault in ARM architecture?

easy

A. To increase the clock speed of the processor

B. To detect errors during memory or device access

C. To speed up data transfer between CPU and memory

D. To manage power consumption of the system

Solution

1. Step 1: Understand bus fault function

   A bus fault occurs when there is an error accessing memory or devices, such as invalid addresses or hardware failures.

2. Step 2: Identify the main role of bus faults

   Bus faults help detect these errors to prevent system crashes or data corruption.

3. Final Answer:

   To detect errors during memory or device access -> Option B

4. Quick Check:

   Bus fault = error detection [OK]

Hint: Bus faults catch access errors, not speed or power [OK]

Common Mistakes:

• Confusing bus faults with performance features
• Thinking bus faults manage power or speed
• Assuming bus faults fix errors automatically

2. Which of the following is the correct way to enable the Memory Protection Unit (MPU) in ARM Cortex-M processors?

easy

A. Clear the ENABLE bit in the MPU Control Register

B. Write 0x00 to the MPU Region Number Register

C. Set the ENABLE bit in the MPU Control Register

D. Disable all interrupts before configuring MPU

Solution

1. Step 1: Identify MPU enabling method

   The MPU is enabled by setting the ENABLE bit in the MPU Control Register, which activates memory protection.

2. Step 2: Evaluate other options

   Writing 0x00 to the Region Number Register selects a region but does not enable MPU. Clearing ENABLE disables MPU. Disabling interrupts is good practice but not how MPU is enabled.

3. Final Answer:

   Set the ENABLE bit in the MPU Control Register -> Option C

4. Quick Check:

   MPU enable = set ENABLE bit [OK]

Hint: MPU enabled by setting ENABLE bit, not clearing it [OK]

Common Mistakes:

• Confusing region selection with enabling MPU
• Thinking disabling interrupts enables MPU
• Clearing ENABLE bit instead of setting it

3. Consider this scenario: An ARM Cortex-M processor tries to access an invalid memory address outside the allowed MPU region. What is the expected system behavior?

medium

A. The processor ignores the access and continues execution

B. The system resets immediately without exception

C. The MPU automatically corrects the address

D. A MemManage fault exception is triggered to handle the error

Solution

1. Step 1: Understand MPU role in access control

   The MPU restricts access to certain memory regions. Access outside these regions is illegal.

2. Step 2: Identify system response to illegal access

   When illegal access occurs, the processor triggers a MemManage fault exception to handle the error safely.

3. Final Answer:

   A MemManage fault exception is triggered to handle the error -> Option D

4. Quick Check:

   Invalid MPU access = MemManage fault exception [OK]

Hint: MPU violation triggers MemManage fault exception [OK]

Common Mistakes:

• Assuming MPU fixes addresses automatically
• Thinking system resets immediately without exception
• Believing processor ignores illegal access

4. You configured an MPU region but the system still allows access to that memory area, causing a security risk. What is the most likely cause?

medium

A. MPU is not enabled after configuration

B. Bus fault handler is missing

C. Memory region size is too large

D. Interrupts are disabled globally

Solution

1. Step 1: Check MPU activation status

   Even if regions are configured, the MPU must be enabled to enforce protection.

2. Step 2: Analyze other options

   Missing bus fault handler causes faults to be unhandled but does not disable protection. Region size affects coverage but not enforcement. Interrupts disabled do not affect MPU enforcement.

3. Final Answer:

   MPU is not enabled after configuration -> Option A

4. Quick Check:

   MPU config needs enable bit set [OK]

Hint: MPU config needs enable bit set to work [OK]

Common Mistakes:

• Forgetting to enable MPU after setup
• Confusing bus fault handler with MPU enforcement
• Assuming interrupts affect MPU protection

5. An embedded system uses MPU to protect three memory regions: code, data, and peripherals. The code region is set as read-only, data as read-write, and peripherals as no-execute. Which of the following statements is true about this configuration?

hard

A. The MPU prevents code from being modified and peripherals from running code

B. The MPU allows peripherals to execute code but prevents data writes

C. The MPU disables all access to the data region

D. The MPU allows code to be written but not executed

Solution

1. Step 1: Analyze code region protection

   Setting code region as read-only prevents modification, ensuring code integrity.

2. Step 2: Analyze peripherals region protection

   Setting peripherals as no-execute stops code execution from peripheral memory, enhancing security.

3. Step 3: Confirm data region access

   Data region set as read-write allows normal data operations.

4. Final Answer:

   The MPU prevents code from being modified and peripherals from running code -> Option A

5. Quick Check:

   Read-only code + no-exec peripherals = protection [OK]

Hint: Read-only code and no-exec peripherals protect system [OK]

Common Mistakes:

• Thinking peripherals can execute code
• Assuming data region is blocked
• Believing code region is writable