Vector table relocation in ARM Architecture - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the vector table in ARM architecture?

The vector table is a list of addresses that point to the start of exception or interrupt handlers. It helps the processor know where to jump when an event occurs.

intermediate

Why do we relocate the vector table in ARM systems?

Relocation allows the vector table to be moved from its default location (usually at address 0x00000000) to another memory area, like RAM, enabling dynamic changes to interrupt handlers or supporting different boot modes.

intermediate

How is vector table relocation typically done in ARM Cortex-M processors?

By changing the Vector Table Offset Register (VTOR) to point to the new vector table address in memory, usually in RAM or flash.

intermediate

What is the benefit of relocating the vector table to RAM?

Relocating to RAM allows the system to modify interrupt vectors at runtime, which is useful for bootloaders, operating systems, or applications that need to change interrupt behavior dynamically.

beginner

What could happen if the vector table is not correctly relocated or set?

The processor might jump to wrong addresses on interrupts or exceptions, causing system crashes, unpredictable behavior, or failure to handle events properly.

Where is the default location of the vector table in ARM Cortex-M processors?

A 0x08000000

B 0x20000000

C 0x00000000

D 0x40000000

The vector table is by default located at address 0x00000000 in ARM Cortex-M processors.

Which register is used to relocate the vector table in ARM Cortex-M?

A VTOR

B PC

C SP

D LR

The Vector Table Offset Register (VTOR) holds the address of the vector table and is used to relocate it.

What is a key advantage of relocating the vector table to RAM?

A Faster interrupt handling

B Ability to modify interrupt vectors at runtime

C Lower power consumption

D Increased flash memory size

Relocating to RAM allows the system to change interrupt vectors dynamically during operation.

If the vector table is not correctly set, what is a likely outcome?

A System boots faster

B Memory usage decreases

C Interrupts are ignored

D Processor jumps to wrong addresses causing crashes

Incorrect vector table location causes the processor to jump to wrong addresses on interrupts, leading to crashes.

Vector table relocation is important in which of these scenarios?

A Bootloader that needs to update interrupt handlers

B Read-only memory devices

C Simple calculator application

D Static firmware with no interrupts

Bootloaders often relocate the vector table to update interrupt handlers dynamically.

Explain what vector table relocation is and why it is used in ARM processors.

Think about how the processor knows where to jump on interrupts and why changing that location might be helpful.

Describe the risks or problems that can occur if the vector table is not properly relocated or configured.

Consider what happens when the processor tries to handle an interrupt but the address is wrong.

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