Vector table relocation in ARM Architecture - Mini Project: Build & Apply

Vector Table Relocation in ARM Architecture

📖 Scenario: You are working on an embedded ARM system. The system uses a vector table to handle interrupts and exceptions. By default, this vector table is located at address 0x00000000 in memory. However, for your application, you need to move (relocate) the vector table to a new memory address, 0x20000000, to support your custom interrupt handlers.

🎯 Goal: Build a step-by-step understanding of how to relocate the vector table in ARM architecture by setting up the initial vector table address, configuring the relocation address, applying the relocation, and completing the setup for the system to use the new vector table location.

📋 What You'll Learn

Create a variable representing the original vector table address at 0x00000000

Create a variable for the new vector table address at 0x20000000

Write the instruction to relocate the vector table to the new address

Complete the setup by enabling the vector table relocation in the system control register

💡 Why This Matters

🌍 Real World

Embedded systems often relocate the vector table to RAM to allow dynamic changes to interrupt handlers or to support bootloaders.

💼 Career

Understanding vector table relocation is essential for embedded firmware engineers working with ARM Cortex-M microcontrollers.

1

Set the original vector table address

Create a variable called original_vector_table and set it to the address 0x00000000 representing the default vector table location.

ARM Architecture

# Your code here

Hint

The vector table starts at address 0x00000000 by default in ARM systems.

2

Set the new vector table address

Create a variable called new_vector_table and set it to the address 0x20000000 where you want to relocate the vector table.

ARM Architecture

original_vector_table = 0x00000000
# Your code here

Hint

The new vector table address is typically in RAM or a different memory region.

3

Relocate the vector table

Write a statement that assigns the new_vector_table address to the system register VTOR (Vector Table Offset Register) to relocate the vector table.

ARM Architecture

original_vector_table = 0x00000000
new_vector_table = 0x20000000
# Your code here

Hint

The VTOR register holds the base address of the vector table in ARM Cortex-M processors.

4

Enable vector table relocation

Add a statement to enable vector table relocation by setting the VTOR register properly and ensuring the system control register SCB_VTOR is configured to use the new vector table address.

ARM Architecture

original_vector_table = 0x00000000
new_vector_table = 0x20000000
VTOR = new_vector_table
# Your code here

Hint

The system control block's VTOR register must be set to the new vector table address to complete relocation.

Practice

1. What is the primary purpose of relocating the vector table in ARM architecture?

easy

A. To increase the processor clock speed

B. To change the base address where interrupt handlers are located

C. To enable floating-point operations

D. To disable interrupts permanently

Solution

1. Step 1: Understand the vector table role

   The vector table contains addresses of interrupt handlers used by the processor.

2. Step 2: Purpose of relocation

   Relocation changes the base address so the processor knows where to find these handlers in memory.

3. Final Answer:

   To change the base address where interrupt handlers are located -> Option B

4. Quick Check:

   Vector table relocation = changing interrupt handler base address [OK]

Hint: Vector table relocation changes interrupt handler address base [OK]

Common Mistakes:

• Confusing relocation with clock speed changes
• Thinking relocation disables interrupts
• Assuming relocation enables floating-point

2. Which ARM register is used to relocate the vector table base address?

easy

A. VTOR (Vector Table Offset Register)

B. SP (Stack Pointer)

C. LR (Link Register)

D. PC (Program Counter)

Solution

1. Step 1: Identify the register for vector table base

   The VTOR register holds the base address of the vector table in ARM processors.

2. Step 2: Confirm other registers' roles

   PC controls instruction flow, LR stores return address, SP manages stack; none relocate vector table.

3. Final Answer:

   VTOR (Vector Table Offset Register) -> Option A

4. Quick Check:

   VTOR = vector table base register [OK]

Hint: VTOR register sets vector table base address [OK]

Common Mistakes:

• Confusing PC with VTOR
• Using LR or SP for vector table relocation
• Not knowing VTOR purpose

3. Given the following code snippet in ARM assembly, what is the effect of the instruction?
MOV R0, #0x20000000
STR R0, [VTOR]

medium

A. Enables interrupts globally

B. Resets the processor

C. Sets the vector table base address to 0x20000000

D. Clears the vector table

Solution

1. Step 1: Analyze MOV instruction

   MOV R0, #0x20000000 loads the address 0x20000000 into register R0.

2. Step 2: Analyze STR instruction

   STR R0, [VTOR] stores the value in R0 into the VTOR register, relocating the vector table base.

3. Final Answer:

   Sets the vector table base address to 0x20000000 -> Option C

4. Quick Check:

   STR to VTOR = relocate vector table base [OK]

Hint: STR to VTOR sets new vector table base address [OK]

Common Mistakes:

• Thinking it resets the processor
• Assuming it enables interrupts
• Believing it clears the vector table

4. A developer tries to relocate the vector table by writing to VTOR but the interrupts still point to the old addresses. What is the most likely cause?

medium

A. The stack pointer is incorrectly set

B. The processor clock is too slow

C. The interrupt handlers are missing

D. VTOR register is write-protected or not enabled for writing

Solution

1. Step 1: Understand VTOR write requirements

   VTOR may be write-protected or require special permissions to update its value.

2. Step 2: Check other options

   Clock speed or stack pointer issues do not affect vector table relocation; missing handlers cause faults but not relocation failure.

3. Final Answer:

   VTOR register is write-protected or not enabled for writing -> Option D

4. Quick Check:

   VTOR write protection blocks relocation [OK]

Hint: Check VTOR write permissions if relocation fails [OK]

Common Mistakes:

• Blaming clock speed for relocation issues
• Confusing stack pointer with vector table base
• Assuming missing handlers prevent relocation

5. In a system with multiple memory regions, how can vector table relocation improve system flexibility?

hard

A. By allowing the vector table to be moved to RAM for dynamic interrupt handler updates

B. By increasing the processor's clock frequency automatically

C. By disabling unused interrupts permanently

D. By encrypting the interrupt handlers for security

Solution

1. Step 1: Understand relocation benefits

   Relocating the vector table to RAM allows changing interrupt handlers at runtime, improving flexibility.

2. Step 2: Evaluate other options

   Clock frequency changes, disabling interrupts, or encryption are unrelated to vector table relocation.

3. Final Answer:

   By allowing the vector table to be moved to RAM for dynamic interrupt handler updates -> Option A

4. Quick Check:

   Relocation enables dynamic interrupt handler updates [OK]

Hint: Relocate vector table to RAM for flexible interrupt updates [OK]

Common Mistakes:

• Thinking relocation changes clock speed
• Assuming relocation disables interrupts
• Believing relocation encrypts handlers