Stack frame setup in ARM Architecture - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a stack frame in ARM architecture?

A stack frame is a section of the stack that holds data for a single function call, including local variables, return address, and saved registers.

beginner

Why do ARM functions set up a stack frame?

To organize local data and save the return address and registers so the function can return correctly and maintain data integrity.

intermediate

Which register typically points to the start of the stack frame in ARM?

The frame pointer register (usually R7 or FP) points to the base of the current stack frame.

intermediate

What is the first step in setting up a stack frame in ARM assembly?

The first step is to push the previous frame pointer and the link register (LR) onto the stack to save them.

beginner

How does the stack grow in ARM architecture during stack frame setup?

The stack grows downward, meaning the stack pointer decreases as data is pushed onto the stack.

What does the stack frame store during a function call in ARM?

A Local variables, return address, and saved registers

B Only the return address

C Only global variables

D Only the function parameters

The stack frame stores local variables, the return address, and saved registers to maintain function state.

Which register usually holds the frame pointer in ARM stack frame setup?

A LR (Link Register)

B R0

C R7

D SP (Stack Pointer)

R7 is commonly used as the frame pointer to mark the base of the current stack frame.

During stack frame setup, what happens to the stack pointer (SP)?

A It decreases as data is pushed onto the stack

B It increases as data is pushed onto the stack

C It stays the same

D It points to the heap

The stack grows downward in ARM, so the stack pointer decreases when data is pushed.

What is the purpose of saving the link register (LR) during stack frame setup?

A To hold function parameters

B To save the return address for the function

C To store local variables

D To point to the stack base

The link register holds the return address, so saving it preserves where to return after the function ends.

Which of the following is NOT typically stored in a stack frame?

A Saved registers

B Local variables

C Return address

D Global variables

Global variables are stored in a fixed memory area, not in the stack frame.

Explain the steps involved in setting up a stack frame in ARM architecture.

Think about what needs to be saved and how the stack pointer moves.

Describe why the stack grows downward in ARM and how this affects stack frame setup.

Consider the direction of stack growth and its practical reasons.

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