Interrupt vector table in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an interrupt vector table in embedded systems?

It is a special table in memory that holds addresses of interrupt service routines (ISRs). When an interrupt happens, the processor uses this table to find and run the correct ISR.

beginner

How does the processor use the interrupt vector table?

When an interrupt occurs, the processor looks up the interrupt number in the vector table to get the address of the ISR, then jumps to that address to handle the interrupt.

intermediate

Why is the interrupt vector table usually placed at a fixed memory location?

Because the processor expects the table at a known address to quickly find ISRs without searching, making interrupt handling fast and reliable.

intermediate

In C, how do you typically define an interrupt vector table?

You create an array of function pointers where each pointer points to an ISR. This array is placed at the fixed memory address the processor expects.

advanced

What happens if an interrupt occurs but its vector table entry points to a null or wrong address?

The processor may jump to an invalid location causing a crash or undefined behavior, so every interrupt vector must point to a valid ISR or a default handler.

What does the interrupt vector table store?

A Data values for interrupts

B Memory addresses of variables

C Processor status flags

D Addresses of interrupt service routines

The interrupt vector table stores addresses of ISRs to handle interrupts.

Where is the interrupt vector table usually located?

A At a fixed memory address

B Random memory location

C Inside the CPU registers

D In external storage

It is placed at a fixed memory address so the processor can find it quickly.

What is the role of the interrupt vector table when an interrupt occurs?

A To disable all interrupts

B To find the ISR address and jump to it

C To reset the processor

D To store interrupt data

The processor uses the table to find the ISR address and execute it.

In embedded C, how is the interrupt vector table commonly implemented?

A As an array of function pointers

B As a linked list

C As a string array

D As a set of variables

An array of function pointers points to ISRs for each interrupt.

What can happen if an interrupt vector points to an invalid address?

A Processor resets automatically

B Interrupt is ignored safely

C Processor may crash or behave unpredictably

D Interrupt is handled by default

Jumping to an invalid address can cause a crash or undefined behavior.

Explain what an interrupt vector table is and why it is important in embedded systems.

Think about how the processor finds the right code to run when an interrupt happens.

Describe how you would implement an interrupt vector table in embedded C.

Consider how C handles functions and memory layout.

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