Interrupt vector table in Embedded C - Mini Project: Build & Apply

Interrupt Vector Table Setup in Embedded C

📖 Scenario: You are working on a simple embedded system that uses interrupts to handle events like timers and external signals. To manage these interrupts, you need to create an interrupt vector table that links interrupt numbers to their handler functions.

🎯 Goal: Build a basic interrupt vector table in C that assigns specific interrupt handler functions to interrupt numbers. This will help the microcontroller know which function to run when an interrupt occurs.

📋 What You'll Learn

Create an array called interrupt_vector_table of function pointers

Define three interrupt handler functions: reset_handler, timer_handler, and external_handler

Assign these handlers to the correct positions in the vector table

Print the address of the timer_handler function from the vector table

💡 Why This Matters

🌍 Real World

Microcontrollers use interrupt vector tables to quickly jump to the right code when hardware events happen, like timers or button presses.

💼 Career

Embedded software engineers must understand how to set up and use interrupt vector tables to write efficient and responsive firmware.

1

Define interrupt handler functions

Write three empty interrupt handler functions called reset_handler, timer_handler, and external_handler. Each function should return void and take no parameters.

Embedded C

// Your code here

Hint

Use the syntax void function_name(void) {} to define each handler.

2

Create the interrupt vector table array

Create an array of function pointers called interrupt_vector_table with three elements. Each element should be a pointer to a function returning void and taking no parameters.

Embedded C

void reset_handler(void) {}
void timer_handler(void) {}
void external_handler(void) {}

// Your code here

Hint

Declare interrupt_vector_table as an array of 3 pointers to functions with void return type and no parameters.

3

Assign handlers to the vector table

Assign reset_handler to interrupt_vector_table[0], timer_handler to interrupt_vector_table[1], and external_handler to interrupt_vector_table[2].

Embedded C

void reset_handler(void) {}
void timer_handler(void) {}
void external_handler(void) {}

void (*interrupt_vector_table[3])(void);

// Your code here

Hint

Use the assignment operator = to link each handler function to the correct index in the array.

4

Print the address of the timer handler

Write a printf statement to print the address stored in interrupt_vector_table[1]. Use %p format specifier and cast the function pointer to void *.

Embedded C

void reset_handler(void) {}
void timer_handler(void) {}
void external_handler(void) {}

void (*interrupt_vector_table[3])(void);

interrupt_vector_table[0] = reset_handler;
interrupt_vector_table[1] = timer_handler;
interrupt_vector_table[2] = external_handler;

// Your code here

Hint

Use printf("Timer handler address: %p\n", (void *)interrupt_vector_table[1]); inside main().

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