Interrupt vector table in Embedded C

Introduction

An interrupt vector table helps the microcontroller know which code to run when something important happens, like a button press or a timer finishing.

When you want your microcontroller to react quickly to events like sensors or buttons.

When you need to handle multiple hardware events without missing any.

When writing embedded programs that must respond to external signals immediately.

Syntax

Embedded C

typedef void (*ISR)(void);

ISR interrupt_vector_table[] __attribute__((section(".isr_vector"))) = {
    Reset_Handler,
    NMI_Handler,
    HardFault_Handler,
    // other interrupt handlers
};

The interrupt vector table is an array of function pointers.

Each entry points to a function called when that interrupt happens.

Examples

This example shows a simple interrupt vector table with two handlers.

Embedded C

void Reset_Handler(void) {
    // code to run on reset
}

void NMI_Handler(void) {
    // code for non-maskable interrupt
}

ISR interrupt_vector_table[] __attribute__((section(".isr_vector"))) = {
    Reset_Handler,
    NMI_Handler
};

Here, a timer interrupt handler is added to the vector table.

Embedded C

void Timer_Handler(void) {
    // code to handle timer interrupt
}

ISR interrupt_vector_table[] __attribute__((section(".isr_vector"))) = {
    Reset_Handler,
    NMI_Handler,
    HardFault_Handler,
    Timer_Handler
};

Sample Program

This program defines three interrupt handlers and an interrupt vector table. It simulates interrupts by calling the handlers directly from the table.

Embedded C

#include <stdio.h>

typedef void (*ISR)(void);

void Reset_Handler(void) {
    printf("Reset interrupt triggered\n");
}

void NMI_Handler(void) {
    printf("NMI interrupt triggered\n");
}

void HardFault_Handler(void) {
    printf("HardFault interrupt triggered\n");
}

ISR interrupt_vector_table[] __attribute__((section(".isr_vector"))) = {
    Reset_Handler,
    NMI_Handler,
    HardFault_Handler
};

int main() {
    // Simulate interrupts by calling handlers directly
    interrupt_vector_table[0](); // Reset
    interrupt_vector_table[1](); // NMI
    interrupt_vector_table[2](); // HardFault
    return 0;
}

Important Notes

The interrupt vector table must be placed at a specific memory location, usually at the start of the program memory.

Each microcontroller has its own rules about the order and number of interrupts in the table.

In real embedded systems, the hardware automatically jumps to the correct handler when an interrupt occurs.

Summary

The interrupt vector table links interrupts to their handler functions.

It is an array of function pointers placed in a special memory section.

Handlers run automatically when their interrupt happens, making your program responsive.

Practice

1.

What is the main purpose of an interrupt vector table in embedded C programming?

easy

A. To store global variables for the program

B. To link interrupt events to their handler functions

C. To manage memory allocation dynamically

D. To perform arithmetic operations automatically

Solution

1. Step 1: Understand the role of interrupt vector table

   The interrupt vector table is a special array that holds addresses of interrupt handler functions.

2. Step 2: Identify its purpose in program flow

   When an interrupt occurs, the processor uses this table to jump to the correct handler automatically.

3. Final Answer:

   To link interrupt events to their handler functions -> Option B

4. Quick Check:

   Interrupt vector table = links interrupts to handlers [OK]

Hint: Interrupt vector table connects interrupts to handlers [OK]

Common Mistakes:

• Thinking it stores variables
• Confusing with memory management
• Assuming it performs calculations

2.

Which of the following is the correct way to declare an interrupt vector table as an array of function pointers in embedded C?

void (*interrupt_vector_table[])(void) = {

easy

A. void interrupt_vector_table(void) = {handler1, handler2};

B. int interrupt_vector_table[] = {handler1, handler2};

C. void (*interrupt_vector_table[])(void) = {handler1, handler2};

D. void *interrupt_vector_table = {handler1, handler2};

Solution

1. Step 1: Recognize function pointer array syntax

   Function pointers to void functions with no parameters are declared as void (*name[])(void).

2. Step 2: Match declaration with initialization

   void (*interrupt_vector_table[])(void) = {handler1, handler2}; correctly declares an array of such pointers and initializes it with handler functions.

3. Final Answer:

   void (*interrupt_vector_table[])(void) = {handler1, handler2}; -> Option C

4. Quick Check:

   Function pointer array syntax = void (*interrupt_vector_table[])(void) = {handler1, handler2}; [OK]

Hint: Function pointer arrays use void (*name[])(void) syntax [OK]

Common Mistakes:

• Using int type instead of function pointers
• Declaring as a single function, not array
• Using void* without function pointer cast

3.

Given the following code snippet, what will be the output when the interrupt at index 1 occurs?

void handler0(void) { printf("Handler 0 called\n"); }
void handler1(void) { printf("Handler 1 called\n"); }
void (*interrupt_vector_table[])(void) = {handler0, handler1};

// Simulate interrupt 1
interrupt_vector_table[1]();

medium

A. Handler 1 called

B. Handler 0 called

C. No output, runtime error

D. Compilation error

Solution

1. Step 1: Identify the function called by interrupt_vector_table[1]

   Index 1 points to handler1, which prints "Handler 1 called".

2. Step 2: Understand the output of the function call

   Calling interrupt_vector_table[1]() executes handler1, printing the message.

3. Final Answer:

   Handler 1 called -> Option A

4. Quick Check:

   interrupt_vector_table[1]() calls handler1 [OK]

Hint: Index 1 calls handler1, prints its message [OK]

Common Mistakes:

• Confusing index 0 and 1
• Expecting no output or error
• Misreading function pointer call syntax

4.

Consider this interrupt vector table declaration:

void handlerA(void) { /* ... */ }
void handlerB(void) { /* ... */ }

void (*interrupt_vector_table[])(void) = {handlerA, handlerB};

// Somewhere else in code
interrupt_vector_table[2]();

What is the problem here?

medium

A. Index 2 is out of bounds, causing undefined behavior

B. handlerB is not a valid function pointer

C. interrupt_vector_table must be declared as int array

D. No problem, code runs fine

Solution

1. Step 1: Check the size of interrupt_vector_table

   The array has 2 elements at indices 0 and 1 only.

2. Step 2: Analyze the call to interrupt_vector_table[2]()

   Index 2 is outside the array bounds, leading to undefined behavior or crash.

3. Final Answer:

   Index 2 is out of bounds, causing undefined behavior -> Option A

4. Quick Check:

   Accessing beyond array size = out of bounds error [OK]

Hint: Array indices start at 0; index 2 is invalid here [OK]

Common Mistakes:

• Assuming array auto-expands
• Thinking handlerB is invalid
• Ignoring out-of-bounds errors

5.

You want to add a new interrupt handler handler2 to an existing vector table with 2 handlers. Which is the correct way to update the vector table to include handler2 at index 2?

void handler0(void) { /* ... */ }
void handler1(void) { /* ... */ }
void handler2(void) { /* ... */ }

// Existing vector table
void (*interrupt_vector_table[])(void) = {handler0, handler1};

// How to add handler2 correctly?

hard

A. No need to add; interrupts beyond index 1 are ignored

B. Assign handler2 directly: interrupt_vector_table[2] = handler2;

C. Use a pointer cast to add handler2 at index 2

D. Redefine the vector table with all three handlers: {handler0, handler1, handler2}

Solution

1. Step 1: Understand array size fixed at declaration

   Arrays in C have fixed size; you cannot add elements dynamically.

2. Step 2: Update vector table by redefining with all handlers

   To include handler2, redefine the array with three elements: {handler0, handler1, handler2}.

3. Final Answer:

   Redefine the vector table with all three handlers: {handler0, handler1, handler2} -> Option D

4. Quick Check:

   Arrays fixed size; redefine to add elements [OK]

Hint: Arrays fixed size; redefine to add handlers [OK]

Common Mistakes:

• Trying to assign beyond array size
• Using pointer casts unsafely
• Assuming interrupts beyond defined handlers work