Stack frame setup in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push the link register onto the stack at the start of a function.

ARM Architecture

push [1]

Drag options to blanks, or click blank then click option'

A{r0}

B{pc}

C{sp}

D{lr}

Attempts:

3 left

💡 Hint

Common Mistakes

Using {pc} instead of {lr} will push the program counter, which is not typical for function prologue.

Pushing {sp} or {r0} is incorrect for saving return address.

In ARM assembly, the link register (lr) holds the return address. Pushing {lr} saves it on the stack for later return.

2fill in blank

medium

Complete the code to allocate 16 bytes on the stack for local variables.

ARM Architecture

sub sp, sp, [1]

Drag options to blanks, or click blank then click option'

A#4

B#8

C#16

D#32

Attempts:

3 left

💡 Hint

Common Mistakes

Using #8 only allocates 8 bytes, which may be insufficient.

Using #32 allocates too much space unnecessarily.

Subtracting 16 bytes from the stack pointer reserves space for local variables in the stack frame.

3fill in blank

hard

Fix the error in the instruction that restores the stack pointer after a function call.

ARM Architecture

add sp, sp, [1]

Drag options to blanks, or click blank then click option'

A#16

B#12

C#8

D#4

Attempts:

3 left

💡 Hint

Common Mistakes

Adding back #8 or #12 bytes will not fully restore the stack pointer.

Adding #4 bytes is too little and causes stack corruption.

The stack pointer must be incremented by the same amount it was decremented to restore the stack frame correctly. If 16 bytes were allocated, add back 16 bytes.

4fill in blank

hard

Fill both blanks to correctly save and restore the frame pointer (r7) in the function prologue and epilogue.

ARM Architecture

push [1]
...
pop [2]

Drag options to blanks, or click blank then click option'

A{r7}

B{lr}

C{r6}

D{sp}

Attempts:

3 left

💡 Hint

Common Mistakes

Pushing {lr} instead of {r7} will save the return address, not the frame pointer.

Using different registers for push and pop causes errors.

The frame pointer register (r7) is saved at the start and restored at the end of the function to maintain the stack frame.

5fill in blank

hard

Fill all three blanks to create a stack frame setup that saves lr and r7, allocates 12 bytes, and restores them properly.

ARM Architecture

push [1]
sub sp, sp, [2]
...
add sp, sp, [3]

Drag options to blanks, or click blank then click option'

A{lr, r7}

B#12

D{r7}

Attempts:

3 left

💡 Hint

Common Mistakes

Using {r7} alone in push does not save lr.

Allocating different amounts in sub and add causes stack imbalance.

The push instruction saves both lr and r7 registers. Then 12 bytes are allocated on the stack. Finally, 12 bytes are added back to restore the stack pointer.

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