Fault analysis and debugging in ARM Architecture - Step-by-Step Execution

Concept Flow - Fault analysis and debugging

Start Program Execution

↓

Fault Occurs?

No→Continue Execution

Yes↓

Fault Detected by CPU

↓

Save Fault Info in Registers

↓

Enter Debug Mode or Exception Handler

↓

Analyze Fault Info

↓

Identify Fault Cause

↓

Apply Fix or Debug

↓

Resume or Halt Execution

This flow shows how an ARM CPU detects a fault during execution, saves fault details, enters debug mode, and how a developer analyzes and fixes the fault.

Execution Sample

ARM Architecture

MOV R0, #0
LDR R1, [R0]
STR R2, [R1]

This code tries to load from address 0 and then store to an address from that load, which may cause a fault if address 0 is invalid.

Analysis Table

Step	Instruction	Action	Fault Detected	Fault Info Saved	Next Step
1	MOV R0, #0	R0 set to 0	No	N/A	Execute next
2	LDR R1, [R0]	Load from address 0	Yes	Fault status register updated	Enter fault handler
3	Fault Handler	Save PC, LR, fault status	N/A	Registers hold fault info	Analyze fault
4	Analyze Fault	Check fault status and address	N/A	Identify invalid memory access	Debug or fix
5	Fix Code	Change address or handle error	N/A	Fault resolved	Resume execution
6	Resume Execution	Continue program	No	N/A	Normal operation

💡 Execution stops or enters handler when fault detected at step 2 due to invalid memory access

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 5	Final
R0	undefined	0	0	0	0	0
R1	undefined	undefined	fault	fault	fault	valid after fix
Fault Status Register	0	0	updated	saved	saved	cleared
Program Counter (PC)	start	next	fault address	handler	fixed code	next instruction

Key Insights - 3 Insights

Why does the CPU stop normal execution and enter a fault handler?

When the CPU detects an invalid operation like accessing a forbidden memory address (step 2 in execution_table), it saves fault info and enters the fault handler to prevent damage or undefined behavior.

What information is saved when a fault occurs?

The CPU saves the program counter, link register, and fault status registers (step 3) so the debugger can understand what caused the fault.

How does fixing the code help resume normal execution?

By correcting the invalid memory access or handling the error (step 5), the fault condition is cleared, allowing the CPU to continue running safely (step 6).

Visual Quiz - 3 Questions

Look at the execution_table at step 2. What causes the fault to be detected?

A Loading from an invalid memory address

B Storing to a valid memory address

C Moving a value into a register

D Executing a no-operation instruction

💡 Hint

Check the 'Action' and 'Fault Detected' columns at step 2 in execution_table

At which step does the CPU save fault information into registers?

A Step 1

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the 'Fault Info Saved' column in execution_table

If the invalid memory address was corrected earlier, how would the execution_table change?

A Fault Handler would be entered at step 3

B Fault Detected would be 'No' at step 2

C Fault Status Register would update at step 2

D Program would halt immediately

💡 Hint

Refer to the 'Fault Detected' column at step 2 and consider what happens if no fault occurs

Concept Snapshot

Fault analysis in ARM involves detecting errors during execution,
saving fault details in special registers,
entering a fault handler for debugging,
and fixing the cause to resume normal operation.
Common faults include invalid memory access or illegal instructions.
Debugging uses saved info to identify and correct issues.

Full Transcript

Fault analysis and debugging in ARM architecture starts when the CPU detects a fault such as an invalid memory access. The CPU stops normal execution and saves fault information like the program counter and fault status registers. It then enters a fault handler mode where the fault details are analyzed. Developers use this information to identify the cause of the fault, such as accessing an invalid address. After fixing the code or handling the error, the CPU can resume normal execution. This process helps prevent crashes and undefined behavior by managing faults systematically.

Practice

1. What is the primary purpose of fault analysis in ARM systems?

easy

A. To find why the system fails or behaves incorrectly

B. To improve the speed of the processor

C. To add new features to the ARM system

D. To reduce the power consumption of the device

Solution

1. Step 1: Understand the role of fault analysis

   Fault analysis is used to identify the reasons behind system failures or incorrect behavior.

2. Step 2: Compare with other options

   Improving speed, adding features, or reducing power are not the main goals of fault analysis.

3. Final Answer:

   To find why the system fails or behaves incorrectly -> Option A

4. Quick Check:

   Fault analysis = Find system failure reasons [OK]

Hint: Fault analysis finds failure causes, not performance tweaks [OK]

Common Mistakes:

• Confusing fault analysis with performance optimization
• Thinking fault analysis adds features
• Assuming it reduces power consumption

2. Which ARM register is commonly used to hold the address of the faulting instruction during debugging?

easy

A. Program Counter (PC)

B. Stack Pointer (SP)

C. Link Register (LR)

D. Fault Address Register (FAR)

Solution

1. Step 1: Identify the register related to fault address

   The Program Counter (PC) holds the address of the current instruction, including the faulting instruction during debugging.

2. Step 2: Differentiate from other registers

   FAR holds the fault address for data faults, but the PC points to the faulting instruction address; SP manages stack, LR stores return address.

3. Final Answer:

   Program Counter (PC) -> Option A

4. Quick Check:

   Faulting instruction address = PC register [OK]

Hint: PC holds faulting instruction address, not FAR or SP [OK]

Common Mistakes:

• Confusing PC with fault address register
• Mixing up SP and LR roles
• Assuming PC always shows fault location

3. Given the following ARM fault status register value: 0x05, what type of fault does it indicate?

medium

A. Alignment fault

B. Data abort fault

C. Prefetch abort fault

D. Instruction cache maintenance fault

Solution

1. Step 1: Understand fault status codes

   In ARM, fault status register value 0x05 typically indicates a prefetch abort fault.

2. Step 2: Match code to fault type

   Alignment faults usually have different codes; data abort and instruction cache faults differ from 0x05.

3. Final Answer:

   Prefetch abort fault -> Option C

4. Quick Check:

   0x05 = Prefetch abort fault [OK]

Hint: 0x05 in fault status means prefetch abort [OK]

Common Mistakes:

• Mixing prefetch abort with data abort
• Assuming 0x05 means alignment fault
• Confusing instruction cache fault with prefetch abort

4. You see an ARM system crashing due to a data abort fault. Which of the following is the most likely cause?

medium

A. Incorrect clock configuration

B. Executing an undefined instruction

C. Stack overflow due to recursion

D. Accessing an invalid memory address

Solution

1. Step 1: Understand data abort faults

   Data abort faults occur when the processor tries to access invalid or protected memory addresses.

2. Step 2: Eliminate other options

   Undefined instructions cause undefined instruction faults, stack overflow causes different errors, and clock issues don't cause data aborts.

3. Final Answer:

   Accessing an invalid memory address -> Option D

4. Quick Check:

   Data abort = invalid memory access [OK]

Hint: Data abort means invalid memory access [OK]

Common Mistakes:

• Confusing data abort with undefined instruction fault
• Blaming stack overflow for data abort
• Assuming clock issues cause data abort

5. You want to improve system reliability by automatically logging faults in an ARM system. Which combination of features is best suited for this?

hard

A. Disable fault detection to avoid system crashes

B. Use Fault Status Registers and configure exception handlers to log faults

C. Use only the Program Counter to track faults

D. Increase CPU clock speed and disable interrupts

Solution

1. Step 1: Identify features for fault logging

   Fault Status Registers provide fault details; exception handlers can capture and log faults automatically.

2. Step 2: Evaluate other options

   Increasing clock speed or disabling interrupts does not help fault logging; using only PC is insufficient; disabling fault detection reduces reliability.

3. Final Answer:

   Use Fault Status Registers and configure exception handlers to log faults -> Option B

4. Quick Check:

   Fault registers + handlers = reliable fault logging [OK]

Hint: Combine fault registers with handlers for logging [OK]

Common Mistakes:

• Thinking faster CPU fixes faults
• Ignoring fault registers for logging
• Disabling fault detection to avoid crashes