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How Does an RTOS Handle Interrupts and Manage Interrupt Service Routines (ISRs)? | Master RTOS Interview Questions (2026)

On: August 24, 2025

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RTOS Handle Interrupts : When working with embedded systems, the ability to respond quickly to external events is crucial. This is where RTOS interrupts and ISRs (Interrupt Service Routines) come into play. An RTOS (Real-Time Operating System) is specifically designed to handle interrupts efficiently and ensure predictable, low-latency responses to hardware events.

In this article, we’ll explore what interrupts are, how ISRs work, and how an RTOS manages them step by step with examples.

RTOS Handle Interrupts

What Are Interrupts in an RTOS?

An interrupt is a hardware-generated signal that tells the CPU to stop its current task and immediately handle a new event. For example:

A timer interrupt triggers the scheduler to switch tasks.
A UART interrupt signals the arrival of new serial data.
A GPIO interrupt detects a button press.

Without interrupts, the CPU would have to constantly check for events (polling), which is inefficient and wastes processing time.

What Is an Interrupt Service Routine (ISR)?

An ISR (Interrupt Service Routine) is a special function that runs whenever an interrupt occurs. It is executed in interrupt context, which means:

It runs with higher priority than normal tasks.
It should be fast and lightweight.
It cannot perform blocking operations (like waiting for a semaphore indefinitely).

Instead of doing heavy work, an ISR usually just:

Reads or writes data from a hardware register.
Clears the interrupt flag.
Notifies a task to handle detailed processing later.

How RTOS Handles Interrupts (Step by Step)

Let’s break down the lifecycle of RTOS interrupts and ISRs:

Step 1: Interrupt Trigger

A hardware event (like timer overflow or data received) raises an interrupt request.
CPU looks up the ISR address from the Interrupt Vector Table (IVT).
CPU saves the current execution context (registers, program counter, status flags).

Step 2: ISR Execution

ISR immediately takes control.
ISR performs minimal operations:
- Reads sensor/communication data.
- Clears interrupt flags.
- Signals the RTOS if further processing is required.

Step 3: Deferring Work to Tasks

Since ISRs must be short, most RTOSes provide mechanisms to delegate work:
- Semaphores – ISR gives a semaphore to wake a task.
- Queues – ISR sends data to a queue for task processing.
- Event Flags – ISR sets an event flag that tasks monitor.

Step 4: Task Scheduling After ISR

If an ISR unblocks a higher-priority task, the RTOS can immediately switch context to that task.
Otherwise, the interrupted task resumes execution.

This ensures deterministic and low-latency event handling.

RTOS Management of ISRs

Rules for Writing ISRs in RTOS:

Keep them short and fast.
Avoid using blocking calls.
Use ISR-safe RTOS APIs (like xQueueSendFromISR() in FreeRTOS).

Advanced Features in RTOS:

Interrupt Nesting: Higher-priority interrupts can preempt lower-priority ISRs.
Critical Sections: RTOS provides APIs to protect shared data between ISRs and tasks.
Context Switching from ISR: RTOS can trigger a scheduler immediately after ISR execution if needed.

Example: ISR Handling in FreeRTOS

// ISR: UART interrupt handler
void UART_IRQHandler(void)
{
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    char c = UART_ReadChar();  // Read data from UART

    // Send data to queue using ISR-safe function
    xQueueSendFromISR(xUARTQueue, &c, &xHigherPriorityTaskWoken);

    // Trigger context switch if a higher-priority task is waiting
    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
}

// Task: Processes UART data
void vUARTTask(void *pvParameters)
{
    char c;
    for(;;)
    {
        if(xQueueReceive(xUARTQueue, &c, portMAX_DELAY))
        {
            printf("Received: %c\n", c);
        }
    }
}

Explanation:

The ISR only reads data and sends it to a queue.
A dedicated task (vUARTTask) processes the data.
portYIELD_FROM_ISR() ensures that if a higher-priority task is waiting, it runs immediately after ISR execution.

RTOS vs. Linux Interrupt Handling

Feature	RTOS	Linux
Latency	Very low, deterministic	Higher, depends on kernel load
ISR Execution	Must be short and minimal	Often defers to softirqs / workqueues
Blocking Calls	❌ Not allowed in ISR	❌ Not allowed in ISR
Scheduling After ISR	Immediate task preemption possible	Work often deferred, less deterministic
Target Use	Real-time embedded systems	General-purpose systems

Key Takeaways

RTOS interrupts and ISRs provide fast responses to hardware events.
ISRs must be minimal, fast, and non-blocking.
Heavy processing should be deferred to tasks using queues, semaphores, or event flags.
RTOS supports ISR-safe APIs to ensure safe communication between ISRs and tasks.
By enabling deterministic interrupt handling, RTOS ensures reliable real-time performance.

Frequently Asked Questions (FAQ) on RTOS Interrupts and ISRs

1. What is an interrupt in an RTOS?

An interrupt in an RTOS is a hardware signal that tells the CPU to pause its current task and handle a more urgent event through an Interrupt Service Routine (ISR).

2. What is the role of an ISR in RTOS?

An ISR (Interrupt Service Routine) is a special function that runs immediately when an interrupt occurs. It performs minimal work, like reading data or clearing flags, and usually signals a task to handle detailed processing.

3. Why should ISRs be short in an RTOS?

ISRs should be short and fast because they block other interrupts and tasks while running. Long ISRs increase system latency and can break real-time behavior.

4. Can ISRs use RTOS APIs directly?

Not all RTOS APIs are safe to use inside ISRs. Most RTOSes provide ISR-safe functions (e.g., xQueueSendFromISR() in FreeRTOS) that allow safe communication with tasks.

5. How does an RTOS handle context switching after an ISR?

If an ISR unblocks a higher-priority task, the RTOS can immediately perform a context switch after the ISR, ensuring the task runs without delay.

6. What is the difference between interrupts in RTOS and Linux?

RTOS interrupts: Deterministic, low-latency, designed for real-time response.
Linux interrupts: Less deterministic, often deferred using bottom halves, tasklets, or workqueues.

7. Can ISRs nest in an RTOS?

Yes, some RTOSes support nested interrupts, where higher-priority interrupts can preempt lower-priority ISRs. This allows better responsiveness in time-critical systems.

8. How do ISRs communicate with tasks in RTOS?

ISRs typically communicate with tasks using:

Queues (for passing data)
Semaphores (for signaling events)
Event flags (for notifying state changes)

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