Interrupt enable and disable in ARM Architecture - Mini Project: Build & Apply

Interrupt Enable and Disable

📖 Scenario: You are learning how to control interrupts in an ARM processor. Interrupts allow the processor to pause its current task and respond to important events. Sometimes, you need to enable interrupts to allow these responses, and other times you need to disable them to prevent interruptions during critical operations.

🎯 Goal: Build a simple step-by-step understanding of how to enable and disable interrupts using ARM assembly instructions and registers.

📋 What You'll Learn

Create a variable representing the Current Program Status Register (CPSR) with an initial value.

Create a mask variable to represent the interrupt disable bits.

Write code to disable interrupts by setting the appropriate bits in CPSR.

Write code to enable interrupts by clearing the appropriate bits in CPSR.

💡 Why This Matters

🌍 Real World

Controlling interrupts is essential in embedded systems to manage when the processor should respond to external events or continue critical tasks without interruption.

💼 Career

Embedded systems engineers and firmware developers often need to enable or disable interrupts to ensure system stability and responsiveness.

1

Create CPSR register variable

Create a variable called CPSR and set it to the hexadecimal value 0x00000000 to represent the initial status register.

ARM Architecture

# Create a variable called CPSR and set it to 0x00000000
# Your code here

Hint

The CPSR register holds flags including interrupt enable/disable bits. Start with zero.

2

Create interrupt mask variable

Create a variable called INTERRUPT_MASK and set it to the hexadecimal value 0xC0 which represents the bits to disable IRQ and FIQ interrupts.

ARM Architecture

CPSR = 0x00000000
# Create a variable called INTERRUPT_MASK and set it to 0xC0
# Your code here

Hint

The bits 6 and 7 in CPSR control IRQ and FIQ interrupts. Setting these bits disables interrupts.

3

Disable interrupts by setting bits

Write code to disable interrupts by setting the bits in CPSR using the INTERRUPT_MASK. Use bitwise OR operation: CPSR = CPSR | INTERRUPT_MASK.

ARM Architecture

CPSR = 0x00000000
INTERRUPT_MASK = 0xC0
# Disable interrupts by setting bits in CPSR
# Your code here

Hint

Use the OR operator to set bits without changing other bits.

4

Enable interrupts by clearing bits

Write code to enable interrupts by clearing the bits in CPSR using the INTERRUPT_MASK. Use bitwise AND with the inverse mask: CPSR = CPSR & (~INTERRUPT_MASK).

ARM Architecture

CPSR = 0x00000000
INTERRUPT_MASK = 0xC0
CPSR = CPSR | INTERRUPT_MASK
# Enable interrupts by clearing bits in CPSR
# Your code here

Hint

Use AND with the inverse mask to clear bits and enable interrupts.

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