Vector table structure in ARM Architecture

Introduction

The vector table structure helps the processor know where to jump when an interrupt or exception happens. It organizes addresses for handling different events.

When setting up an embedded system to handle interrupts safely.

When programming a microcontroller to respond to hardware signals.

When debugging why an interrupt is not handled correctly.

When customizing exception handling for specific system needs.

Core Concept

ARM Architecture

Vector Table:
  - Entry 0: Initial Stack Pointer
  - Entry 1: Reset Handler Address
  - Entry 2: NMI Handler Address
  - Entry 3: HardFault Handler Address
  - ...
  - Other exception and interrupt handler addresses

The vector table is usually placed at a fixed memory address, often at the start of the memory.

Each entry is a 32-bit address pointing to the start of a handler function.

Key Points

This shows the first four entries in a vector table with addresses for stack and handlers.

ARM Architecture

0x00000000: 0x20002000  ; Initial Stack Pointer
0x00000004: 0x00001000  ; Reset Handler
0x00000008: 0x00001010  ; NMI Handler
0x0000000C: 0x00001020  ; HardFault Handler

This example shows how to define a vector table in C, placing it in a special section.

ARM Architecture

Vector Table in C:
void (* const vector_table[])(void) __attribute__((section(".isr_vector"))) = {
  (void (*)(void))((unsigned long)&_estack),
  Reset_Handler,
  NMI_Handler,
  HardFault_Handler,
  // Other handlers
};

Detailed Explanation

This program defines a simple vector table with initial stack pointer and three handlers. The vector table is placed in a special memory section so the processor can find it at startup.

ARM Architecture

# Example vector table entries in C
#include <stdint.h>

extern uint32_t _estack;

void Reset_Handler(void) {
  // Reset code here
}

void NMI_Handler(void) {
  // NMI code here
}

void HardFault_Handler(void) {
  // HardFault code here
}

void (* const vector_table[])(void) __attribute__((section(".isr_vector"))) = {
  (void (*)(void))&_estack,  // Initial Stack Pointer
  Reset_Handler,             // Reset Handler
  NMI_Handler,               // NMI Handler
  HardFault_Handler          // HardFault Handler
};

int main() {
  // Main program
  return 0;
}

Important Notes

The first entry is not a function but the initial stack pointer value.

Each following entry is the address of a function that handles a specific interrupt or exception.

The vector table must be aligned and placed at the correct memory address for the processor to use it properly.

Summary

The vector table organizes addresses for interrupt and exception handlers.

The first entry is the initial stack pointer, followed by handler addresses.

It is essential for the processor to know where to jump when events occur.

Practice

1. What is the first entry in the ARM vector table?

easy

A. Program counter start address

B. Reset handler address

C. Initial stack pointer address

D. Interrupt service routine address

Solution

1. Step 1: Understand vector table structure

   The ARM vector table begins with the initial stack pointer value to set up the stack.

2. Step 2: Identify the first entry

   The first entry is the initial stack pointer address, followed by the reset handler address.

3. Final Answer:

   Initial stack pointer address -> Option C

4. Quick Check:

   First entry = Initial stack pointer address [OK]

Hint: Remember: stack pointer comes before reset handler [OK]

Common Mistakes:

• Confusing reset handler as first entry
• Thinking program counter is first
• Assuming interrupt routine is first

2. Which of the following correctly describes the second entry in the ARM vector table?

easy

A. Reset handler address

B. Initial stack pointer address

C. Undefined instruction handler

D. Non-maskable interrupt handler

Solution

1. Step 1: Recall vector table order

   The first entry is the initial stack pointer, so the second entry is the reset handler.

2. Step 2: Confirm second entry role

   The reset handler is the address where the processor starts executing after reset.

3. Final Answer:

   Reset handler address -> Option A

4. Quick Check:

   Second entry = Reset handler address [OK]

Hint: Second entry always points to reset handler [OK]

Common Mistakes:

• Mixing up with interrupt handlers
• Choosing stack pointer again
• Confusing with exception handlers

3. Given the following vector table snippet:

0x00000000: 0x20001000  (Initial SP)
0x00000004: 0x00001005  (Reset Handler)
0x00000008: 0x00001010  (NMI Handler)
0x0000000C: 0x00001020  (HardFault Handler)

Which address will the processor jump to on a HardFault exception?

medium

A. 0x20001000

B. 0x00001005

C. 0x00001010

D. 0x00001020

Solution

1. Step 1: Locate HardFault entry in vector table

   The HardFault handler address is at offset 0x0C in the vector table.

2. Step 2: Identify the address at 0x0000000C

   The address at 0x0000000C is 0x00001020, which is the HardFault handler.

3. Final Answer:

   0x00001020 -> Option D

4. Quick Check:

   HardFault handler = 0x00001020 [OK]

Hint: Match exception to its vector table offset [OK]

Common Mistakes:

• Choosing reset handler address
• Confusing NMI and HardFault handlers
• Selecting initial stack pointer

4. A developer sets the vector table's first entry to 0x00000000 instead of the initial stack pointer. What issue will most likely occur?

medium

A. Stack pointer will be invalid causing a crash

B. Processor will reset normally

C. Reset handler will not execute

D. Interrupts will be ignored

Solution

1. Step 1: Understand role of first vector entry

   The first entry sets the initial stack pointer address for the processor.

2. Step 2: Analyze effect of setting it to zero

   Setting it to 0x00000000 means the stack pointer points to an invalid memory location, causing a crash.

3. Final Answer:

   Stack pointer will be invalid causing a crash -> Option A

4. Quick Check:

   Invalid stack pointer = Crash [OK]

Hint: First vector must be valid stack pointer [OK]

Common Mistakes:

• Thinking reset handler won't run
• Assuming interrupts are disabled
• Believing processor resets normally

5. You want to add a custom interrupt handler for a new peripheral. Where in the ARM vector table should you place this handler's address?

hard

A. At the first entry replacing the initial stack pointer

B. At the specific interrupt vector slot assigned to that peripheral

C. At the second entry replacing the reset handler

D. At the end of the vector table after all default handlers

Solution

1. Step 1: Understand vector table layout

   The vector table has fixed slots for each exception and interrupt type.

2. Step 2: Identify correct slot for custom interrupt

   Custom peripheral interrupts must be placed at their assigned interrupt vector slot, not at stack pointer or reset handler entries.

3. Final Answer:

   At the specific interrupt vector slot assigned to that peripheral -> Option B

4. Quick Check:

   Custom handler = Assigned interrupt vector slot [OK]

Hint: Place handlers in their assigned vector slots only [OK]

Common Mistakes:

• Replacing stack pointer or reset handler
• Appending handler at vector table end
• Ignoring fixed vector slot assignments