Exception priority levels in ARM Architecture - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are exception priority levels in ARM architecture?

Exception priority levels determine the order in which exceptions are handled. Higher priority exceptions can interrupt lower priority ones.

beginner

Name the main types of exceptions in ARM Cortex-M processors.

The main types are: Reset, NMI (Non-Maskable Interrupt), HardFault, and other configurable interrupts with different priority levels.

intermediate

How does ARM architecture assign priority to exceptions?

ARM uses a priority number where lower numbers mean higher priority. The processor always handles the exception with the highest priority first.

intermediate

What happens if two exceptions with the same priority occur simultaneously in ARM?

The exception with the lower exception number (vector number) is handled first, ensuring a consistent order.

beginner

Why is understanding exception priority important in embedded systems?

It helps design reliable systems by ensuring critical tasks interrupt less important ones, improving responsiveness and safety.

In ARM architecture, which exception has the highest priority?

A Reset

B NMI

C HardFault

D SysTick

Reset exception has the highest priority and occurs when the system starts or resets.

What does a lower priority number mean in ARM exception handling?

A Equal priority

B Lower priority

C No priority

D Higher priority

In ARM, lower priority numbers indicate higher priority exceptions.

If two exceptions have the same priority, how does ARM decide which to handle first?

A Randomly

B By exception number (lower first)

C By time of occurrence

D By software setting

ARM handles the exception with the lower exception number first when priorities are equal.

Which exception cannot be masked or disabled in ARM Cortex-M?

A PendSV

B SysTick

C NMI

D UsageFault

NMI (Non-Maskable Interrupt) cannot be disabled or masked.

Why are exception priority levels important in real-time systems?

A To ensure critical tasks run first

B To avoid exceptions

C To save memory

D To slow down processing

Priority levels ensure that critical tasks interrupt less important ones for timely response.

Explain how ARM architecture uses exception priority levels to manage multiple interrupts.

Think about how the processor decides which interrupt to handle first.

Describe why understanding exception priority levels is crucial when designing embedded systems.

Consider the impact on system safety and performance.

Practice

1. In ARM architecture, what does a lower exception priority number indicate?

easy

A. No priority assigned

B. Lower priority for handling the exception

C. Higher priority for handling the exception

D. Equal priority to all exceptions

Solution

1. Step 1: Understand priority numbering in ARM

   In ARM processors, exception priority levels use numbers where lower numbers mean higher priority.

2. Step 2: Interpret the meaning of lower numbers

   A lower number means the exception is handled before others with higher numbers.

3. Final Answer:

   Higher priority for handling the exception -> Option C

4. Quick Check:

   Lower number = Higher priority [OK]

Hint: Lower number means higher priority in ARM exceptions [OK]

Common Mistakes:

• Thinking higher numbers mean higher priority
• Assuming all exceptions have equal priority
• Confusing priority with exception type

2. Which of the following correctly represents the priority order in ARM exception levels from highest to lowest?

easy

A. 3, 2, 1, 0

B. 0, 1, 2, 3

C. 1, 0, 3, 2

D. 2, 3, 0, 1

Solution

1. Step 1: Recall ARM priority numbering

   Lower numbers indicate higher priority, so priority order starts from 0 upwards.

2. Step 2: Arrange numbers from lowest to highest

   The correct order from highest to lowest priority is 0, 1, 2, 3.

3. Final Answer:

   0, 1, 2, 3 -> Option B

4. Quick Check:

   Priority order = 0 to 3 [OK]

Hint: Priority order counts up from 0 as highest [OK]

Common Mistakes:

• Reversing the order thinking higher numbers are higher priority
• Mixing up the sequence of numbers
• Assuming priority starts at 1 instead of 0

3. Given these exception priorities in ARM: IRQ = 2, FIQ = 0, SVC = 1, which exception will be handled first?

medium

A. FIQ

B. IRQ

C. SVC

D. All handled simultaneously

Solution

1. Step 1: Identify priority numbers for each exception

   FIQ has priority 0, SVC has 1, IRQ has 2.

2. Step 2: Determine which priority is highest

   Lower number means higher priority, so FIQ with 0 is handled first.

3. Final Answer:

   FIQ -> Option A

4. Quick Check:

   Lowest number (0) = FIQ first [OK]

Hint: Exception with priority 0 handled first [OK]

Common Mistakes:

• Choosing IRQ because it sounds more common
• Assuming SVC is highest priority by name
• Thinking all exceptions run at the same time

4. If an ARM system incorrectly assigns a higher number to a critical exception, what is the likely problem?

medium

A. The critical exception may be delayed or ignored

B. The system will crash immediately

C. All exceptions will have equal priority

D. The critical exception will run first anyway

Solution

1. Step 1: Understand priority impact on handling

   Higher numbers mean lower priority, so a critical exception with a high number is handled later.

2. Step 2: Predict system behavior

   This delay can cause the critical exception to be delayed or missed, affecting system reliability.

3. Final Answer:

   The critical exception may be delayed or ignored -> Option A

4. Quick Check:

   Wrong priority number delays critical exception [OK]

Hint: Critical exceptions need lowest priority number [OK]

Common Mistakes:

• Assuming system crashes immediately
• Thinking priority numbers do not affect handling
• Believing all exceptions run equally regardless of priority

5. You have three exceptions with priorities: Timer = 1, UART = 3, Reset = 0. If Timer, UART, and Reset exceptions occur simultaneously, which will ARM handle first and why?

hard

A. UART, because it has the highest number

B. Timer, because it occurs first

C. Timer, because it has a medium priority

D. Reset, because it has the lowest number

Solution

1. Step 1: Identify priorities and exceptions involved

   Reset has priority 0, Timer 1, UART 3. Timer, UART, and Reset occur simultaneously.

2. Step 2: Determine which exception ARM handles first

   ARM handles the exception with the lowest priority number first, which is Reset with 0.

3. Step 3: Understand simultaneous exceptions handling

   Since Reset has the highest priority, it preempts others.

4. Final Answer:

   Reset, because it has the lowest number -> Option D

5. Quick Check:

   Lowest number = highest priority = Reset first [OK]

Hint: Lowest priority number exception handled first always [OK]

Common Mistakes:

• Overlooking Reset's highest priority
• Picking Timer due to medium priority
• Choosing UART because it occurs simultaneously