Fault handlers (HardFault, MemManage, BusFault, UsageFault) in ARM Architecture - Step-by-Step Execution

Concept Flow - Fault handlers (HardFault, MemManage, BusFault, UsageFault)

CPU Executes Instruction

↓

Fault Occurs?

No→Continue Normal Execution

Yes↓

Identify Fault Type

↓

MemManage

↓

Call Handler

↓

Fault Handler Executes

↓

System Recovery or Reset

The CPU runs instructions and checks for faults. If a fault occurs, it identifies the type and calls the corresponding handler to manage the error.

Execution Sample

ARM Architecture

if (fault == MemManage) {
  MemManage_Handler();
} else if (fault == BusFault) {
  BusFault_Handler();
} else if (fault == UsageFault) {
  UsageFault_Handler();
} else {
  HardFault_Handler();
}

This code checks the fault type and calls the matching fault handler function.

Analysis Table

Step	Fault Detected	Fault Type Check	Handler Called	Action Taken
1	No	N/A	N/A	Continue normal execution
2	Yes	Is MemManage fault?	No	Check next fault type
3	Yes	Is BusFault?	No	Check next fault type
4	Yes	Is UsageFault?	No	Check next fault type
5	Yes	Else HardFault	Yes	Execute HardFault_Handler
6	Yes	Handler executes	HardFault_Handler	System reset or recovery

💡 Fault handler completes or system resets after handling the fault.

State Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
fault	None	Detected as fault	Still fault	Still fault	Identified as HardFault	Handled by HardFault_Handler

Key Insights - 3 Insights

Why does the system call HardFault_Handler if no other fault matches?

Because HardFault is the catch-all fault handler for unexpected or unclassified faults, as shown in execution_table step 5.

Can multiple fault handlers run at the same time?

No, only one fault handler runs per fault occurrence. The system checks fault types in order and calls the first matching handler.

What happens if a fault occurs inside a fault handler?

Typically, the system may reset or enter a safe state to avoid infinite fault loops, as fault handlers are designed to handle faults safely.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the HardFault_Handler called?

A Step 5

B Step 2

C Step 3

D Step 1

💡 Hint

Refer to the 'Handler Called' column in execution_table row for step 5.

According to variable_tracker, what is the fault variable value after step 4?

A "No fault"

B "MemManage fault"

C "Still fault"

D "Handled by HardFault_Handler"

💡 Hint

Check the 'fault' row under 'After Step 4' in variable_tracker.

If the fault was a BusFault, which handler would be called according to the execution flow?

A MemManage_Handler

B BusFault_Handler

C UsageFault_Handler

D HardFault_Handler

💡 Hint

Look at the concept_flow where BusFault leads to BusFault_Handler.

Concept Snapshot

Fault handlers catch CPU errors during execution.
Types: MemManage, BusFault, UsageFault, HardFault.
CPU checks fault type and calls matching handler.
HardFault is the default catch-all handler.
Handlers manage errors or reset system.

Full Transcript

When the CPU runs instructions, it may encounter errors called faults. These faults include Memory Management faults, Bus faults, Usage faults, and Hard faults. The CPU checks which fault occurred and calls the corresponding fault handler function. If none of the specific faults match, the HardFault handler is called as a catch-all. Each handler tries to manage the error safely or reset the system to prevent damage. This process ensures the system can respond to errors in a controlled way.

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