Interrupt enable and disable in ARM Architecture - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of enabling interrupts in ARM architecture?

Enabling interrupts allows the processor to respond to external or internal events by temporarily pausing the current task and running a special function called an interrupt handler.

beginner

How does disabling interrupts affect the processor's behavior?

Disabling interrupts prevents the processor from responding to interrupt requests, ensuring that the current code runs without interruption until interrupts are enabled again.

intermediate

Name the ARM processor register commonly used to enable or disable interrupts.

The CPSR (Current Program Status Register) contains bits that control interrupt enable and disable states, such as the I bit for IRQ and the F bit for FIQ interrupts.

intermediate

What is the difference between IRQ and FIQ interrupts in ARM?

IRQ (Interrupt Request) is a normal interrupt, while FIQ (Fast Interrupt Request) is a higher priority interrupt designed for faster response with dedicated registers.

beginner

Why might a programmer disable interrupts temporarily in an ARM system?

To protect critical sections of code from being interrupted, ensuring data integrity and preventing race conditions during sensitive operations.

Which register bit in ARM controls the enabling of IRQ interrupts?

A I bit in CPSR

B F bit in CPSR

C T bit in CPSR

D M bit in CPSR

The I bit in the CPSR register is used to enable or disable IRQ interrupts.

What happens when interrupts are disabled in an ARM processor?

A Processor speeds up execution

B Processor ignores interrupt requests

C Processor resets

D Processor enters low power mode

Disabling interrupts causes the processor to ignore interrupt requests until they are enabled again.

Which interrupt type in ARM has higher priority and faster response?

A FIQ

B NMI

C IRQ

D SWI

FIQ (Fast Interrupt Request) has higher priority and is designed for faster response than IRQ.

Why is it important to disable interrupts during critical code sections?

A To enable debugging

B To increase power consumption

C To slow down the processor

D To prevent data corruption

Disabling interrupts prevents other code from interrupting critical sections, avoiding data corruption.

Which of the following is NOT a reason to disable interrupts?

A Protect critical code

B Prevent race conditions

C Improve multitasking

D Ensure data integrity

Disabling interrupts actually reduces multitasking ability by preventing the processor from handling other tasks.

Explain how enabling and disabling interrupts affects the flow of program execution in ARM processors.

Think about what happens when an interrupt occurs and how the processor decides to pause or continue.

Describe the difference between IRQ and FIQ interrupts and why a system might use both.

Consider which interrupt is faster and how that helps in real-time systems.

Practice

1. What is the main purpose of disabling interrupts in ARM architecture?

easy

A. To prevent the processor from responding to events temporarily

B. To speed up the processor permanently

C. To reset the processor

D. To enable all hardware devices

Solution

1. Step 1: Understand what interrupts do

   Interrupts allow the processor to respond to events like input or timers.

2. Step 2: Understand disabling interrupts

   Disabling interrupts stops the processor from responding to these events temporarily, usually to protect critical code.

3. Final Answer:

   To prevent the processor from responding to events temporarily -> Option A

4. Quick Check:

   Disabling interrupts = stop event response temporarily [OK]

Hint: Disabling interrupts means stopping event responses temporarily [OK]

Common Mistakes:

• Thinking disabling interrupts speeds up the processor
• Confusing disabling interrupts with resetting the processor
• Believing disabling interrupts enables hardware devices

2. Which ARM assembly instruction is used to enable interrupts?

easy

A. CPSID i

B. MOV R0, #1

C. NOP

D. CPSIE i

Solution

1. Step 1: Recall interrupt enable instruction

   The instruction to enable interrupts in ARM is CPSIE i.

2. Step 2: Differentiate from disable instruction

   CPSID i disables interrupts, so it is not the correct answer.

3. Final Answer:

   CPSIE i -> Option D

4. Quick Check:

   Enable interrupts = CPSIE i [OK]

Hint: Enable interrupts with CPSIE i, disable with CPSID i [OK]

Common Mistakes:

• Confusing CPSIE i with CPSID i
• Choosing unrelated instructions like NOP or MOV
• Assuming CPSID i enables interrupts

3. What will happen if the following ARM instructions are executed in order?

CPSID i
CPSIE i

medium

A. Interrupts remain enabled

B. Interrupts remain disabled

C. Interrupts are first disabled then enabled

D. Processor resets

Solution

1. Step 1: Analyze first instruction CPSID i

   This disables interrupts, so after this instruction interrupts are off.

2. Step 2: Analyze second instruction CPSIE i

   This enables interrupts, so after this instruction interrupts are on again.

3. Final Answer:

   Interrupts are first disabled then enabled -> Option C

4. Quick Check:

   CPSID i then CPSIE i = disable then enable interrupts [OK]

Hint: CPSID disables, CPSIE enables interrupts in sequence [OK]

Common Mistakes:

• Assuming interrupts stay disabled after CPSIE i
• Thinking processor resets after these instructions
• Confusing the order of enable and disable

4. Identify the error in this ARM assembly snippet meant to disable interrupts:

CPSIE i

medium

A. No error, it disables interrupts correctly

B. It enables interrupts instead of disabling them

C. It resets the processor

D. Syntax error: missing register

Solution

1. Step 1: Understand the instruction CPSIE i

   CPSIE i is used to enable interrupts, not disable them.

2. Step 2: Compare with correct disable instruction

   The correct instruction to disable interrupts is CPSID i.

3. Final Answer:

   It enables interrupts instead of disabling them -> Option B

4. Quick Check:

   CPSIE i enables, not disables interrupts [OK]

Hint: CPSIE i enables interrupts; use CPSID i to disable [OK]

Common Mistakes:

• Confusing CPSIE i as disabling interrupts
• Thinking CPSIE i causes syntax error
• Believing CPSIE i resets processor

5. You want to protect a critical section of code from interrupts in ARM. Which sequence correctly disables interrupts before the section and enables them after?

hard

A. CPSID i before, CPSIE i after

B. CPSIE i before, CPSID i after

C. NOP before, CPSID i after

D. MOV R0, #0 before, MOV R0, #1 after

Solution

1. Step 1: Disable interrupts before critical section

   Use CPSID i to disable interrupts and protect the code.

2. Step 2: Enable interrupts after critical section

   Use CPSIE i to re-enable interrupts after the critical code finishes.

3. Final Answer:

   CPSID i before, CPSIE i after -> Option A

4. Quick Check:

   Disable then enable interrupts around critical code [OK]

Hint: Disable interrupts before, enable after critical code [OK]

Common Mistakes:

• Reversing enable and disable order
• Using unrelated instructions like NOP or MOV
• Not re-enabling interrupts after critical section