NVIC (Nested Vectored Interrupt Controller) in ARM Architecture - Step-by-Step Execution

Concept Flow - NVIC (Nested Vectored Interrupt Controller)

Interrupt Occurs

↓

Is Interrupt Enabled?

No→Ignore Interrupt

Yes↓

NVIC Checks Priority

↓

Save Current Context

↓

Jump to Interrupt Handler

↓

Execute Handler

↓

Restore Context

↓

Return to Main Program

When an interrupt occurs, NVIC checks if it is enabled and its priority, then saves the current state, runs the interrupt handler, and finally restores the state to continue the main program.

Execution Sample

ARM Architecture

Interrupt occurs
NVIC checks priority and enable
If enabled, save context
Jump to handler
Execute handler
Restore context and return

This sequence shows how NVIC handles an interrupt from detection to returning to the main program.

Analysis Table

Step	Action	Condition/Check	Result/Output
1	Interrupt signal detected	Interrupt occurs	NVIC starts processing
2	Check if interrupt is enabled	Interrupt enabled?	Yes -> proceed; No -> ignore
3	Check interrupt priority	Priority compared to current	If higher priority, proceed
4	Save current CPU context	Context saved	Registers and state saved
5	Jump to interrupt handler	Handler address fetched	Program counter set to handler
6	Execute interrupt handler	Handler code runs	Interrupt serviced
7	Restore CPU context	Context restored	Registers and state restored
8	Return to main program	Return from interrupt	Program resumes normal execution
9	End	No more interrupts	NVIC waits for next interrupt

💡 Interrupt handled and CPU context restored, returning to main program execution

State Tracker

Variable	Start	After Step 4	After Step 7	Final
Interrupt Enabled Flag	Depends on config	Checked	Checked	No change
Interrupt Priority	Set by user	Checked	Checked	No change
CPU Context (Registers)	Main program state	Saved	Restored	Main program state restored
Program Counter (PC)	Main program address	Set to handler	Set back to main	Main program address

Key Insights - 3 Insights

Why does NVIC save the CPU context before running the interrupt handler?

NVIC saves the CPU context (Step 4 in execution_table) so that after the interrupt handler finishes, the main program can resume exactly where it left off without losing any data.

What happens if the interrupt is not enabled when it occurs?

If the interrupt is not enabled (Step 2 in execution_table), NVIC ignores it and does not run the handler, so the main program continues uninterrupted.

How does NVIC decide which interrupt to handle first if multiple occur?

NVIC compares the priority of interrupts (Step 3 in execution_table) and handles the one with the highest priority first.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does NVIC save the CPU context?

A Step 4

B Step 6

C Step 2

D Step 8

💡 Hint

Check the 'Action' column for 'Save current CPU context' in execution_table.

According to variable_tracker, what happens to the Program Counter after Step 5?

A It is reset to zero

B It remains at the main program address

C It is set to the interrupt handler address

D It is saved and not changed

💡 Hint

Look at the 'Program Counter (PC)' row in variable_tracker after Step 4 and Step 7.

If an interrupt is disabled, what does NVIC do according to the execution_table?

A Runs the interrupt handler anyway

B Ignores the interrupt

C Saves context but skips handler

D Restarts the CPU

💡 Hint

See Step 2 in execution_table under 'Condition/Check' and 'Result/Output'.

Concept Snapshot

NVIC handles interrupts by checking if they are enabled and their priority.
If enabled and highest priority, it saves CPU context, runs the interrupt handler, then restores context.
This allows the main program to pause and resume seamlessly.
NVIC supports nested interrupts by priority.
It is essential for responsive embedded systems.

Full Transcript

The Nested Vectored Interrupt Controller (NVIC) manages interrupts in ARM processors. When an interrupt occurs, NVIC first checks if the interrupt is enabled and compares its priority with others. If the interrupt is enabled and has sufficient priority, NVIC saves the current CPU context, including registers and program counter. Then it jumps to the interrupt handler to execute the interrupt service routine. After the handler finishes, NVIC restores the saved CPU context so the main program can continue exactly where it left off. If the interrupt is not enabled, NVIC ignores it. NVIC allows multiple interrupts to be prioritized and nested, ensuring critical tasks are handled promptly without losing program state.