Fault handlers (HardFault, MemManage, BusFault, UsageFault) in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a HardFault handler function in ARM Cortex-M.

ARM Architecture

void HardFault_Handler(void) {
    while([1]) {
        // Stay here to indicate fault
    }
}

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

A1

B0

Ctrue

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using while(0) which never loops.

Using boolean keywords that may not be defined in C.

The HardFault handler typically loops infinitely to halt the system when a hard fault occurs. Using while(1) creates an infinite loop.

2fill in blank

medium

Complete the code to enable the MemManage fault handler in the System Control Block (SCB).

ARM Architecture

SCB->SHCSR |= [1];

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

ASCB_SHCSR_MEMFAULTENA_Msk

BSCB_SHCSR_BUSFAULTENA_Msk

CSCB_SHCSR_USGFAULTENA_Msk

DSCB_SHCSR_HARDFAULTENA_Msk

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing MemManage with BusFault or UsageFault enable bits.

Using the HardFault enable bit which is always enabled.

To enable the MemManage fault handler, set the MEMFAULTENA bit in the SHCSR register.

3fill in blank

hard

Fix the error in the UsageFault handler declaration to match ARM Cortex-M conventions.

ARM Architecture

void [1](void) {
    // Handler code
}

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

AUsage_Fault_Handler

BUsageFaultHandler

CUsageFault

DUsageFault_Handler

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the underscore between 'Usage' and 'Fault'.

Using camelCase instead of snake_case.

The correct ARM Cortex-M handler name is UsageFault_Handler with an underscore.

4fill in blank

hard

Fill both blanks to check if a BusFault is active and clear its status in the SCB.

ARM Architecture

if (SCB->CFSR & [1]) {
    SCB->CFSR |= [2];
}

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

ASCB_CFSR_BUSFAULTSR_Msk

BSCB_CFSR_MEMFAULTSR_Msk

CSCB_CFSR_USGFAULTSR_Msk

DSCB_CFSR_BUSFAULTERR_Msk

Attempts:

3 left

💡 Hint

Common Mistakes

Using MemManage fault masks instead of BusFault masks.

Using different masks for checking and clearing.

The BusFault status bits are checked and cleared using SCB_CFSR_BUSFAULTSR_Msk.

5fill in blank

hard

Fill all three blanks to create a dictionary that maps fault names to their active status bits.

ARM Architecture

fault_status = {
    "[1]": SCB->CFSR & [2],
    "[3]": SCB->CFSR & SCB_CFSR_MEMFAULTSR_Msk
}

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

ABusFault

BSCB_CFSR_BUSFAULTSR_Msk

CUsageFault

DSCB_CFSR_USGFAULTSR_Msk

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up fault names and their corresponding masks.

Using incorrect string literals for fault names.

This dictionary maps the fault names 'BusFault' and 'UsageFault' to their respective status bits in the CFSR register.

Practice

1. Which fault handler in ARM architecture is triggered when an illegal memory access occurs?

easy

A. HardFault Handler

B. UsageFault Handler

C. BusFault Handler

D. MemManage Fault Handler

Solution

1. Step 1: Understand the role of MemManage Fault

   The MemManage Fault Handler is designed to catch illegal memory accesses such as accessing protected or invalid memory regions.

2. Step 2: Differentiate from other fault handlers

   UsageFault handles instruction errors, BusFault handles bus errors, and HardFault is a catch-all for severe faults. Illegal memory access specifically triggers MemManage Fault.

3. Final Answer:

   MemManage Fault Handler -> Option D

4. Quick Check:

   Illegal memory access = MemManage Fault Handler [OK]

Hint: Illegal memory access triggers MemManage Fault [OK]

Common Mistakes:

• Confusing BusFault with MemManage Fault
• Thinking UsageFault handles memory errors
• Assuming HardFault handles all faults directly

2. Which of the following is the correct way to declare a HardFault handler function in ARM Cortex-M assembly?

easy

A. void HardFault_Handler(void) __attribute__((interrupt));

B. void HardFault_Handler(void) __attribute__((weak));

C. void HardFault_Handler(void) __attribute__((naked));

D. void HardFault_Handler(void) __attribute__((inline));

Solution

1. Step 1: Understand attribute usage for fault handlers

   HardFault handlers often use the __attribute__((naked)) to write custom assembly without compiler-generated prologue/epilogue.

2. Step 2: Compare other attributes

   __attribute__((interrupt)) is not standard for ARM Cortex-M, __attribute__((weak)) allows override but is not for declaration style, __attribute__((inline)) is unrelated.

3. Final Answer:

   void HardFault_Handler(void) __attribute__((naked)); -> Option C

4. Quick Check:

   HardFault handler uses naked attribute [OK]

Hint: HardFault handlers use naked attribute for assembly code [OK]

Common Mistakes:

• Using interrupt attribute which is not standard here
• Confusing weak with naked attribute
• Assuming inline is correct for handlers

3. Given the following ARM Cortex-M code snippet, which fault handler will be triggered if an undefined instruction is executed?

__attribute__((naked)) void UsageFault_Handler(void) {
  while(1) {}
}

void main(void) {
  __asm volatile ("udf #0");
}

medium

A. UsageFault

B. BusFault

C. MemManage Fault

D. HardFault

Solution

1. Step 1: Identify the fault caused by undefined instruction

   The assembly instruction "udf #0" is an undefined instruction that triggers UsageFault in ARM Cortex-M processors.

2. Step 2: Match the fault handler to the fault type

   UsageFault_Handler is designed to catch undefined instructions and other usage errors, so it will be triggered here.

3. Final Answer:

   UsageFault -> Option A

4. Quick Check:

   Undefined instruction = UsageFault [OK]

Hint: Undefined instructions trigger UsageFault [OK]

Common Mistakes:

• Thinking undefined instruction triggers HardFault
• Confusing BusFault with UsageFault
• Assuming MemManage handles instruction faults

4. In an ARM Cortex-M system, a BusFault handler is not executing as expected after a bus error. Which of the following is the most likely cause?

medium

A. BusFault is not enabled in the System Control Block (SCB)

B. MemManage Fault handler is overriding BusFault

C. UsageFault handler is masking BusFault

D. HardFault handler is disabled

Solution

1. Step 1: Check fault enabling in SCB

   BusFault must be enabled in the System Control Block (SCB) to be active. If not enabled, BusFault exceptions won't trigger the handler.

2. Step 2: Understand fault priority and masking

   MemManage and UsageFault handlers do not override or mask BusFault. HardFault cannot be disabled as it is a core fault handler.

3. Final Answer:

   BusFault is not enabled in the System Control Block (SCB) -> Option A

4. Quick Check:

   BusFault handler requires SCB enable [OK]

Hint: Enable BusFault in SCB to activate its handler [OK]

Common Mistakes:

• Assuming HardFault can be disabled
• Confusing fault masking between handlers
• Ignoring SCB configuration for faults

5. You are designing a fault handler system for an embedded ARM Cortex-M device. You want to catch all memory protection faults and bus errors but handle usage faults separately. Which configuration should you apply?

hard

A. Disable MemManage; enable BusFault and UsageFault handlers only

B. Enable MemManage and BusFault handlers; keep UsageFault enabled but with separate handler

C. Enable only HardFault handler to catch all faults

D. Enable UsageFault and MemManage; disable BusFault handler

Solution

1. Step 1: Identify handlers for memory protection and bus errors

   MemManage Fault handler catches memory protection faults, BusFault handler catches bus errors.

2. Step 2: Keep UsageFault separate

   UsageFault handler should remain enabled and separate to handle usage errors like undefined instructions.

3. Step 3: Choose configuration matching requirements

   Enabling MemManage and BusFault handlers while keeping UsageFault enabled separately matches the design goal.

4. Final Answer:

   Enable MemManage and BusFault handlers; keep UsageFault enabled but with separate handler -> Option B

5. Quick Check:

   MemManage + BusFault enabled; UsageFault separate [OK]

Hint: Enable MemManage and BusFault; keep UsageFault separate [OK]

Common Mistakes:

• Disabling MemManage when memory faults needed
• Relying only on HardFault for all errors
• Disabling BusFault when bus errors must be caught