Stack pointer (R13/SP) in ARM Architecture - Mini Project: Build & Apply

Understanding the Stack Pointer (R13/SP) in ARM Architecture

📖 Scenario: Imagine you are learning how a computer keeps track of temporary data during program execution. In ARM processors, the stack pointer (register R13 or SP) helps manage this temporary data by pointing to the top of a special memory area called the stack.

🎯 Goal: You will build a simple conceptual model of the stack pointer (R13/SP) behavior in ARM architecture. This model will help you understand how the stack pointer moves when data is added or removed from the stack.

📋 What You'll Learn

Create a variable representing the stack pointer (SP) with an initial memory address value.

Define a constant for the size of one stack element (e.g., 4 bytes).

Write code to simulate pushing data onto the stack by decreasing the SP.

Write code to simulate popping data from the stack by increasing the SP.

💡 Why This Matters

🌍 Real World

Understanding the stack pointer is essential for debugging, writing low-level code, and working with function calls and local variables in ARM-based systems.

💼 Career

Knowledge of the stack pointer is important for embedded systems developers, firmware engineers, and anyone working with ARM assembly or low-level programming.

1

Initialize the Stack Pointer (SP)

Create a variable called SP and set it to the initial memory address 0x1000 representing the top of the stack.

ARM Architecture

# Create a variable called SP and set it to 0x1000
# Your code here

Hint

The stack pointer holds the address of the current top of the stack. Start with 0x1000 as the initial address.

2

Define Stack Element Size

Create a constant called STACK_ELEMENT_SIZE and set it to 4 bytes, representing the size of one stack element.

ARM Architecture

SP = 0x1000
# Define STACK_ELEMENT_SIZE as 4
# Your code here

Hint

Each item pushed or popped from the stack usually occupies 4 bytes in ARM architecture.

3

Simulate Pushing Data onto the Stack

Write code to simulate pushing data onto the stack by decreasing the SP by STACK_ELEMENT_SIZE. Use the expression SP -= STACK_ELEMENT_SIZE.

ARM Architecture

SP = 0x1000
STACK_ELEMENT_SIZE = 4
# Decrease SP by STACK_ELEMENT_SIZE to simulate push
# Your code here

Hint

When pushing data, the stack pointer moves down in memory by the size of one element.

4

Simulate Popping Data from the Stack

Write code to simulate popping data from the stack by increasing the SP by STACK_ELEMENT_SIZE. Use the expression SP += STACK_ELEMENT_SIZE.

ARM Architecture

SP = 0x1000
STACK_ELEMENT_SIZE = 4
SP -= STACK_ELEMENT_SIZE
# Increase SP by STACK_ELEMENT_SIZE to simulate pop
# Your code here

Hint

When popping data, the stack pointer moves up in memory by the size of one element.

Practice

1. What is the primary role of the stack pointer (R13/SP) in ARM processors?

easy

A. It tracks the top of the stack during program execution.

B. It stores the current instruction being executed.

C. It holds the address of the program's entry point.

D. It manages the processor's clock speed.

Solution

1. Step 1: Understand the stack pointer's function

   The stack pointer (R13/SP) is a special register that points to the top of the stack in memory.

2. Step 2: Identify its role in program execution

   It helps manage data pushed onto or popped from the stack, such as function calls and local variables.

3. Final Answer:

   It tracks the top of the stack during program execution. -> Option A

4. Quick Check:

   Stack pointer = top of stack tracker [OK]

Hint: Stack pointer always points to the current top of the stack [OK]

Common Mistakes:

• Confusing stack pointer with program counter
• Thinking it stores instructions
• Assuming it controls processor speed

2. Which of the following is the register number for the stack pointer register in ARM assembly?

easy

A. R14

B. R13

C. SP

D. PC

Solution

1. Step 1: Recall ARM register naming

   In ARM architecture, R13 is the register used as the stack pointer.

2. Step 2: Match register names to functions

   R14 is the link register, PC is the program counter, and SP is an alias for R13.

3. Final Answer:

   R13 -> Option B

4. Quick Check:

   Stack pointer register = R13 [OK]

Hint: Remember R13 is SP, R14 is LR, PC is R15 [OK]

Common Mistakes:

• Confusing R13 with R14 (link register)
• Thinking SP is the register number
• Confusing it with PC (R15)

3. Consider this ARM assembly snippet:

SUB SP, SP, #4
STR R0, [SP]
ADD SP, SP, #4

What does this code do to the stack pointer?

medium

A. Moves SP down by 4, stores R0, then moves SP back up by 4

B. Moves SP up by 4, stores R0, then moves SP down by 4

C. Stores R0 without changing SP

D. Causes a runtime error due to invalid SP manipulation

Solution

1. Step 1: Analyze the SUB instruction

   SUB SP, SP, #4 decreases SP by 4, moving the stack pointer down to allocate space.

2. Step 2: Understand STR and ADD instructions

   STR R0, [SP] stores the value of R0 at the new top of the stack. ADD SP, SP, #4 restores SP back up by 4.

3. Final Answer:

   Moves SP down by 4, stores R0, then moves SP back up by 4 -> Option A

4. Quick Check:

   SUB decreases SP, ADD restores SP [OK]

Hint: SUB decreases SP (push), ADD increases SP (pop) [OK]

Common Mistakes:

• Mixing up SUB and ADD effects on SP
• Assuming SP does not change
• Thinking this causes an error

4. In ARM assembly, a programmer writes:

ADD SP, SP, #8
LDR R1, [SP, #-4]

What is the problem with this code?

medium

A. There is no problem; code works as intended.

B. The immediate value #8 is invalid for ADD instruction.

C. LDR cannot use negative offsets in ARM assembly.

D. SP is moved up before loading, causing incorrect address access.

Solution

1. Step 1: Examine ADD SP, SP, #8

   This moves SP up by 8, effectively popping 8 bytes off the stack.

2. Step 2: Analyze LDR with negative offset

   LDR R1, [SP, #-4] tries to load from SP minus 4 bytes, but since SP moved up, this address may not point to the intended data.

3. Step 3: Identify the problem

   Moving SP up before loading causes the load to access an incorrect or invalid memory location.

4. Final Answer:

   SP is moved up before loading, causing incorrect address access. -> Option D

5. Quick Check:

   SP must point correctly before LDR [OK]

Hint: Adjust SP after loading, not before, to avoid wrong addresses [OK]

Common Mistakes:

• Assuming negative offsets are invalid
• Ignoring SP movement effect on address
• Believing immediate #8 is illegal

5. You want to save registers R0 to R3 on the stack before a function call and restore them after. Which ARM assembly sequence correctly uses the stack pointer (R13/SP) to do this?

hard

A. SUB SP, SP, #4
STR R0, [SP]
STR R1, [SP, #4]
STR R2, [SP, #8]
STR R3, [SP, #12]
...function call...
LDMIA SP!, {R0-R3}

B. ADD SP, SP, #16
STR R0, [SP]
STR R1, [SP, #4]
STR R2, [SP, #8]
STR R3, [SP, #12]
...function call...
LDMFD SP!, {R0-R3}

C. SUB SP, SP, #16
STR R0, [SP]
STR R1, [SP, #4]
STR R2, [SP, #8]
STR R3, [SP, #12]
...function call...
LDMIA SP!, {R0-R3}

D. SUB SP, SP, #16
STR R0, [SP, #12]
STR R1, [SP, #8]
STR R2, [SP, #4]
STR R3, [SP]
...function call...
LDMIA SP!, {R0-R3}

Solution

1. Step 1: Allocate stack space for 4 registers

   SUB SP, SP, #16 reserves 16 bytes (4 registers x 4 bytes each) by moving SP down.

2. Step 2: Store registers in ascending order

   STR instructions store R0 at [SP], R1 at [SP+4], R2 at [SP+8], and R3 at [SP+12], matching stack layout.

3. Step 3: Restore registers after function call

   LDMIA SP!, {R0-R3} loads multiple registers and increments SP, restoring original SP value.

4. Final Answer:

   The sequence using SUB SP, SP, #16, ordered STRs, and LDMIA SP!, {R0-R3} -> Option C

5. Quick Check:

   SUB allocates, STR stores, LDMIA restores [OK]

Hint: Subtract before storing, add after loading with LDMIA SP! [OK]

Common Mistakes:

• Using ADD instead of SUB to allocate stack space
• Incorrect offsets for storing registers
• Reversing order of stored registers
• Using wrong load multiple instruction