Stack frame setup in ARM Architecture - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the Purpose of Stack Frames

What is the main purpose of setting up a stack frame during a function call in ARM architecture?

A To allocate space for local variables and save the return address and previous frame pointer

B To increase the CPU clock speed temporarily during function execution

C To directly access hardware peripherals without using registers

D To clear all registers before executing the function

💡 Hint

Think about what information a function needs to keep track of when it calls another function.

Setting up a stack frame allows the function to reserve space for its local variables and save important information like the return address and the previous frame pointer. This helps in returning control correctly after the function finishes.

📋 Factual

intermediate

Identifying Stack Frame Components

Which register in ARM architecture typically holds the frame pointer used in stack frame setup?

A R14

B R13

C R7

D R15

💡 Hint

It is a general-purpose register often used as a frame pointer in ARM calling conventions.

In many ARM calling conventions, R7 is used as the frame pointer to reference the current stack frame, while R13 is the stack pointer, R14 is the link register, and R15 is the program counter.

🔍 Analysis

advanced

Analyzing Stack Frame Setup Instructions

Consider the following ARM assembly instructions executed at the start of a function:

push {r4, r5, r6, lr}
add r7, sp, #0
sub sp, sp, #16

What is the purpose of these instructions in the context of stack frame setup?

A They save registers, set the frame pointer, and allocate space for local variables

B They clear the stack and reset all registers to zero

C They load function arguments into registers and jump to the function

D They restore saved registers and return from the function

💡 Hint

Look at what each instruction does: pushing registers, setting a pointer, and adjusting the stack pointer.

The push instruction saves registers and the link register to the stack. The add sets the frame pointer (R7) to the current stack pointer. The sub reserves space on the stack for local variables by moving the stack pointer down.

❓ Comparison

advanced

Comparing Stack Pointer and Frame Pointer Roles

Which statement best describes the difference between the stack pointer (SP) and the frame pointer (FP) in ARM stack frame setup?

A SP and FP are the same register with different names

B SP is used only for function arguments; FP stores return addresses

C SP is a general-purpose register; FP is used to access hardware registers

D SP points to the top of the stack and changes during function execution; FP points to a fixed location in the stack frame

💡 Hint

Think about which pointer moves as local variables are pushed or popped and which remains stable during a function.

The stack pointer (SP) moves as data is pushed or popped from the stack, reflecting the current top of the stack. The frame pointer (FP) remains fixed during the function to provide a stable reference point for accessing local variables and parameters.

❓ Reasoning

expert

Determining Stack Frame Size from Assembly

Given the following ARM assembly snippet at function start:

push {r4, r5, r6, lr}
sub sp, sp, #32

What is the total size in bytes of the stack frame allocated for this function, including saved registers and local variables?

A 32 bytes

B 48 bytes

C 16 bytes

D 64 bytes

💡 Hint

Each register pushed uses 4 bytes. Add the space reserved by sub sp, sp, #32.

The push saves 4 registers (r4, r5, r6, lr), each 4 bytes, totaling 16 bytes. The sub sp, sp, #32 reserves 32 bytes for local variables. Total stack frame size is 16 + 32 = 48 bytes.

Practice

1. What is the primary purpose of setting up a stack frame in ARM architecture during a function call?

easy

A. To save the return address and local variables safely

B. To increase the CPU clock speed

C. To clear all registers before execution

D. To allocate memory for global variables

Solution

1. Step 1: Understand stack frame role

   A stack frame stores important data like return address and local variables for each function call.

2. Step 2: Identify the main purpose

   This setup ensures the function can return correctly and use local data safely.

3. Final Answer:

   To save the return address and local variables safely -> Option A

4. Quick Check:

   Stack frame purpose = Save return address and locals [OK]

Hint: Stack frame saves return address and locals [OK]

Common Mistakes:

• Confusing stack frame with CPU speed control
• Thinking it clears all registers
• Mixing local and global variable storage

2. Which ARM instruction is commonly used to save the link register (return address) onto the stack during stack frame setup?

easy

A. MOV LR, SP

B. POP {LR}

C. PUSH {LR}

D. ADD SP, SP, #4

Solution

1. Step 1: Identify instruction to save LR

   The link register (LR) holds the return address and is saved on the stack using PUSH.

2. Step 2: Match correct instruction

   PUSH {LR} stores LR on the stack; POP {LR} restores it. MOV and ADD do not save LR on stack.

3. Final Answer:

   PUSH {LR} -> Option C

4. Quick Check:

   Save LR with PUSH {LR} [OK]

Hint: Use PUSH to save LR on stack [OK]

Common Mistakes:

• Using POP instead of PUSH to save
• Confusing MOV with stack operations
• Adding to SP instead of saving LR

3. Consider this ARM assembly snippet for function entry:

SUB SP, SP, #16
PUSH {LR}

What is the effect on the stack pointer (SP) after these instructions?

medium

A. SP increases by 16 bytes

B. SP decreases by 16 bytes, then by 4 bytes

C. SP decreases by 20 bytes

D. SP remains unchanged

Solution

1. Step 1: Analyze SUB SP, SP, #16

   This subtracts 16 from SP, moving it down by 16 bytes to allocate space.

2. Step 2: Analyze PUSH {LR}

   PUSH stores LR on stack, which decreases SP by 4 bytes (ARM uses 4-byte words).

3. Final Answer:

   SP decreases by 16 bytes, then by 4 bytes -> Option B

4. Quick Check:

   SP moves down 16 + 4 bytes = 20 bytes total [OK]

Hint: SUB reduces SP, PUSH reduces SP by 4 bytes [OK]

Common Mistakes:

• Adding to SP instead of subtracting
• Ignoring PUSH effect on SP
• Assuming SP stays same after PUSH

4. In this ARM function prologue, what is the error?

SUB SP, SP, #8
POP {LR}

medium

A. LR should not be used in function prologue

B. SUB should add to SP, not subtract

C. SP should not be changed in prologue

D. POP should be PUSH to save LR first

Solution

1. Step 1: Understand stack frame setup order

   Function prologue saves LR by PUSH before changing SP to allocate space.

2. Step 2: Identify incorrect instruction

   POP removes data from stack; here it tries to restore LR before saving it, which is wrong.

3. Final Answer:

   POP should be PUSH to save LR first -> Option D

4. Quick Check:

   Save LR with PUSH before POP [OK]

Hint: Save LR with PUSH before POP [OK]

Common Mistakes:

• Using POP instead of PUSH to save LR
• Changing SP incorrectly
• Ignoring function prologue conventions

5. You want to create a stack frame that saves LR and allocates 24 bytes for local variables. Which sequence correctly sets up the stack frame in ARM assembly?

hard

A. SUB SP, SP, #24
PUSH {LR}

B. PUSH {LR}
SUB SP, SP, #24

C. PUSH {LR, R4}
SUB SP, SP, #20

D. SUB SP, SP, #28
PUSH {LR}

Solution

1. Step 1: Save LR before allocating locals

   Correct order is to allocate space first by subtracting from SP, then save LR with PUSH.

2. Step 2: Check options for correct order and size

   SUB SP, SP, #24 PUSH {LR} subtracts 24 bytes first, then saves LR, which is the correct order to allocate space and save registers.

3. Step 3: Re-examine options carefully

   SUB SP, SP, #24 PUSH {LR}: SUB SP, SP, #24 then PUSH {LR} - correct order and size.
   PUSH {LR} SUB SP, SP, #24: PUSH {LR} then SUB SP, SP, #24 - wrong order.
   PUSH {LR, R4} SUB SP, SP, #20: PUSH {LR, R4} then SUB SP, SP, #20 - wrong registers and size.
   SUB SP, SP, #28 PUSH {LR}: SUB SP, SP, #28 then PUSH {LR} - wrong size.

4. Final Answer:

   SUB SP, SP, #24 PUSH {LR} -> Option A

5. Quick Check:

   Allocate locals first, then save LR [OK]

Hint: SUB SP first to allocate locals, then PUSH LR [OK]

Common Mistakes:

• Allocating space after saving LR
• Using wrong register sets in PUSH
• Mismatching allocated bytes and saved registers