Stack pointer (R13/SP) in ARM Architecture - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Role of the Stack Pointer in ARM Architecture

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

A It points to the top of the stack, managing function call data and local variables.

B It stores the current instruction address being executed.

C It holds the status flags of the processor.

D It is used to store general-purpose data temporarily.

💡 Hint

Think about what the stack pointer tracks during function calls.

The stack pointer (R13/SP) always points to the top of the stack in memory. It is used to manage data related to function calls, such as return addresses and local variables.

📋 Factual

intermediate

Default Register Number for Stack Pointer

In ARM architecture, which register number corresponds to the stack pointer?

A R15

B R12

C R14

D R13

💡 Hint

Remember the special purpose registers R13, R14, and R15.

R13 is the dedicated register for the stack pointer (SP) in ARM architecture.

🔍 Analysis

advanced

Effect of Stack Pointer Increment

What happens if the stack pointer (R13/SP) is incremented by 4 in an ARM processor using a full descending stack?

A The stack pointer remains unchanged because it only decrements in a full descending stack.

B The stack pointer moves to the next lower memory address, pushing a new item onto the stack.

C The stack pointer moves to the next higher memory address, effectively popping one 4-byte item from the stack.

D The stack pointer causes a processor exception due to invalid operation.

💡 Hint

Consider how a full descending stack grows and shrinks in memory.

In a full descending stack, the stack grows downward (towards lower addresses). Incrementing the stack pointer moves it upward, effectively removing the top item (popping).

❓ Comparison

advanced

Stack Pointer Behavior in Different Stack Models

Which statement correctly compares the stack pointer behavior in full descending and empty ascending stack models?

A In full descending, SP points to the next free location; in empty ascending, SP points to the last used location.

B In full descending, SP points to the last used location; in empty ascending, SP points to the next free location.

C Both models have SP pointing to the last used location in memory.

D Both models have SP pointing to the next free location in memory.

💡 Hint

Think about whether the stack grows up or down and where SP points in each model.

In a full descending stack, the stack grows downward and SP points to the last used item. In an empty ascending stack, the stack grows upward and SP points to the next free location.

❓ Reasoning

expert

Stack Pointer and Interrupt Handling

During an interrupt in ARM architecture, why is it important to save the current stack pointer (R13/SP) value before switching to the interrupt service routine (ISR)?

A To preserve the current function's local variables and return address for correct resumption after ISR.

B To reset the stack pointer to zero for the ISR to start fresh.

C To increase the stack size dynamically during the ISR execution.

D To disable the stack pointer temporarily to prevent stack overflow.

💡 Hint

Consider what happens to the program flow and data during an interrupt.

Saving the current stack pointer preserves the state of the interrupted program, including local variables and return address, so it can resume correctly after the ISR finishes.

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