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ARM Architectureknowledge~5 mins

Interrupt enable and disable in ARM Architecture - Time & Space Complexity

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Time Complexity: Interrupt enable and disable
O(n)
Understanding Time Complexity

When enabling or disabling interrupts in ARM architecture, it is important to understand how the time taken changes as the number of instructions or operations grows.

We want to know how the execution time scales when these interrupt control instructions are used repeatedly.

Scenario Under Consideration

Analyze the time complexity of the following ARM instructions that enable and disable interrupts.


    CPSID i      // Disable IRQ interrupts
    CPSIE i      // Enable IRQ interrupts
    NOP          // No operation (wait)
    CPSID f      // Disable FIQ interrupts
    CPSIE f      // Enable FIQ interrupts

This code snippet shows simple instructions to turn interrupts on or off in ARM processors.

Identify Repeating Operations

Look for instructions that repeat or cause delays.

  • Primary operation: Each CPSID or CPSIE instruction executes once per call.
  • How many times: These instructions run a fixed number of times, not inside loops.
How Execution Grows With Input

Since each interrupt enable or disable instruction runs once, the total time grows directly with how many times you call them.

Input Size (n)Approx. Operations
1010 instructions
100100 instructions
10001000 instructions

Pattern observation: The time increases in a straight line as you add more enable/disable calls.

Final Time Complexity

Time Complexity: O(n)

This means the time to enable or disable interrupts grows directly with how many times you do it.

Common Mistake

[X] Wrong: "Enabling or disabling interrupts takes constant time no matter how many times it is done."

[OK] Correct: Each instruction runs once per call, so if you do it multiple times, total time adds up linearly.

Interview Connect

Understanding how simple instructions like interrupt control scale with usage helps you reason about system responsiveness and performance in real devices.

Self-Check

"What if the interrupt enable/disable instructions were placed inside a loop that runs n times? How would the time complexity change?"