Interrupt vector table in Embedded C - Step-by-Step Execution

Concept Flow - Interrupt vector table

CPU Reset

↓

Fetch Interrupt Vector

↓

Jump to ISR Address

↓

Execute ISR

↓

Return to Main Program

When an interrupt occurs, the CPU looks up the interrupt vector table to find the address of the correct Interrupt Service Routine (ISR) and jumps there to handle the interrupt.

Execution Sample

Embedded C

void ISR_Timer(void) {
    // Timer interrupt code
}

void (* const interrupt_vector_table[])(void) = {
    ISR_Timer
};

Defines an interrupt vector table with one entry pointing to the Timer ISR function.

Execution Table

Step	Event	Action	Vector Table Lookup	ISR Called	Result
1	CPU Reset	Fetch reset vector	Address of Reset ISR	Reset ISR	Start main program
2	Timer Interrupt	Fetch timer vector	Address of ISR_Timer	ISR_Timer	Execute timer ISR code
3	ISR_Timer completes	Return from ISR	N/A	Main program resumes	Continue normal execution
4	No more interrupts	Idle	N/A	N/A	Program runs main loop

💡 No interrupts pending, CPU runs main program continuously

Variable Tracker

Variable	Start	After Interrupt	After ISR	Final
Program Counter (PC)	Reset vector address	ISR_Timer address	Return address after ISR	Main program address
Interrupt Flag	0 (no interrupt)	1 (interrupt triggered)	0 (interrupt handled)	0 (idle)

Key Moments - 3 Insights

Why does the CPU jump to a specific address when an interrupt occurs?

Because the CPU uses the interrupt vector table to find the address of the ISR to handle that interrupt, as shown in execution_table step 2.

What happens to the program counter during an interrupt?

The program counter saves the current address and loads the ISR address from the vector table, as tracked in variable_tracker under Program Counter.

How does the CPU know when to return to the main program?

After the ISR finishes, the CPU uses the saved return address to resume the main program, shown in execution_table step 3.

Visual Quiz - 3 Questions

Look at the execution_table, what ISR is called at step 2?

A Reset ISR

B Main program

C ISR_Timer

D No ISR

💡 Hint

Check the 'ISR Called' column at step 2 in execution_table

According to variable_tracker, what happens to the Interrupt Flag after the ISR completes?

A It stays 1

B It resets to 0

C It becomes 2

D It is undefined

💡 Hint

Look at the Interrupt Flag values after ISR in variable_tracker

If a new interrupt occurs, how would the execution_table change?

A A new step with fetching vector and calling ISR would be added

B The CPU would ignore the interrupt

C The program counter would not change

D The main program would stop permanently

💡 Hint

Refer to the flow in execution_table steps 2 and 3 for interrupt handling

Concept Snapshot

Interrupt vector table holds addresses of ISRs.
CPU uses it to jump to correct ISR on interrupt.
Program counter changes to ISR address.
After ISR, CPU returns to main program.
Each interrupt type has a vector entry.

Full Transcript

An interrupt vector table is a list of addresses where the CPU can find the code to handle different interrupts. When the CPU resets, it starts at the reset vector. When an interrupt happens, the CPU looks up the vector table to find the ISR address and jumps there. The program counter changes to this ISR address. After the ISR runs, the CPU returns to where it left off in the main program. The interrupt flag signals when an interrupt is active and resets after handling. This process ensures the CPU responds quickly and correctly to hardware events.