Preserving callee-saved registers in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to save a callee-saved register before using it.

ARM Architecture

push [1]

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

Ar0

Br1

Cr4

Dr2

Attempts:

3 left

💡 Hint

Common Mistakes

Saving caller-saved registers like r0 instead of callee-saved.

Not saving any register before use.

In ARM architecture, registers r4 to r11 are callee-saved. Saving r4 with push r4 preserves its value across function calls.

2fill in blank

medium

Complete the code to restore a callee-saved register after use.

ARM Architecture

pop [1]

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

Ar5

Br0

Cr3

Dr1

Attempts:

3 left

💡 Hint

Common Mistakes

Restoring caller-saved registers instead of callee-saved.

Forgetting to restore the register.

Register r5 is callee-saved in ARM. Restoring it with pop r5 returns its original value after the function finishes.

3fill in blank

hard

Fix the error in the code to properly preserve callee-saved registers.

ARM Architecture

push [1]
// function code
pop r4

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

Ar5

Br1

Cr0

Dr2

Attempts:

3 left

💡 Hint

Common Mistakes

Pushing one register and popping a different one.

Using caller-saved registers for preservation.

The code pushes r5 but pops r4, which is incorrect. Both push and pop must use the same register to preserve its value properly.

4fill in blank

hard

Fill both blanks to save and restore two callee-saved registers correctly.

ARM Architecture

push [1], [2]
// function code
pop r7, r8

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

Ar7

Br8

Cr4

Dr0

Attempts:

3 left

💡 Hint

Common Mistakes

Saving registers that are not callee-saved.

Mismatch between push and pop registers.

Registers r7 and r8 are callee-saved. Saving them with push r7, r8 and restoring with pop r7, r8 preserves their values across the function.

5fill in blank

hard

Fill all three blanks to create a dictionary mapping callee-saved registers to their save instructions.

ARM Architecture

registers = [1]: '[2]', [3]: 'push [3]'

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

Ar4

Br5

Cpush r4

Dr7

Attempts:

3 left

💡 Hint

Common Mistakes

Using caller-saved registers as keys.

Incorrect push instruction strings.

This dictionary maps r4 to its save instruction push r4 and r7 to push r7. Both are callee-saved registers.

Practice

1. What is the main purpose of preserving callee-saved registers in ARM architecture?

easy

A. To speed up the execution of the program

B. To allow the caller to modify registers freely

C. To keep the values intact across function calls

D. To reduce the number of registers used

Solution

1. Step 1: Understand the role of callee-saved registers

   Callee-saved registers must keep their values unchanged after a function call to avoid breaking the caller's data.

2. Step 2: Identify the purpose of preserving these registers

   Preserving means saving and restoring their values so the caller sees no change.

3. Final Answer:

   To keep the values intact across function calls -> Option C

4. Quick Check:

   Preserving registers = keeping values intact [OK]

Hint: Remember: callee saves to protect caller's data [OK]

Common Mistakes:

• Thinking callee-saved registers speed up execution
• Confusing callee-saved with caller-saved registers
• Believing registers can be freely modified without saving

2. Which ARM assembly instruction is commonly used to save callee-saved registers at the start of a function?

easy

A. MOV

B. ADD

C. CMP

D. PUSH

Solution

1. Step 1: Identify instruction to save registers

   The PUSH instruction stores registers onto the stack, preserving their values.

2. Step 2: Confirm PUSH is used at function start

   Functions use PUSH to save callee-saved registers before modifying them.

3. Final Answer:

   PUSH -> Option D

4. Quick Check:

   Save registers = PUSH [OK]

Hint: PUSH saves registers on stack at function start [OK]

Common Mistakes:

• Using MOV which copies but doesn't save to stack
• Using ADD which changes values, not saves
• Using CMP which compares values, not saves

3. Consider this ARM function snippet:

func:
  PUSH {r4, r5}
  MOV r4, #10
  MOV r5, #20
  POP {r4, r5}
  BX lr

What will be the value of registers r4 and r5 after returning to the caller?

medium

A. r4 and r5 unchanged from caller

B. r4 and r5 contain random values

C. r4 = 0, r5 = 0

D. r4 = 10, r5 = 20

Solution

1. Step 1: Analyze PUSH and POP effect

   PUSH saves original r4 and r5 values on stack; POP restores them before return.

2. Step 2: Understand MOV instructions inside function

   Inside function, r4 and r5 are changed, but POP restores original values before returning.

3. Final Answer:

   r4 and r5 unchanged from caller -> Option A

4. Quick Check:

   Saved and restored registers = unchanged after call [OK]

Hint: PUSH then POP restores original register values [OK]

Common Mistakes:

• Assuming MOV changes persist after POP
• Ignoring that POP restores saved registers
• Thinking registers reset to zero automatically

4. A function modifies callee-saved registers but forgets to restore them before returning. What problem can this cause?

medium

A. The program may crash or behave unpredictably

B. The function will run faster

C. The caller's registers remain unchanged

D. The stack pointer will not be affected

Solution

1. Step 1: Understand callee-saved register role

   These registers must be restored to keep caller's data intact.

2. Step 2: Consequence of not restoring

   If not restored, caller sees wrong values, causing crashes or wrong behavior.

3. Final Answer:

   The program may crash or behave unpredictably -> Option A

4. Quick Check:

   Not restoring callee-saved = crashes/unpredictable [OK]

Hint: Not restoring callee-saved causes crashes [OK]

Common Mistakes:

• Thinking it improves speed
• Believing caller's registers stay safe without restore
• Ignoring stack pointer effects

5. You have a function that uses registers r4, r5, and r6 (all callee-saved). Which sequence correctly preserves and restores these registers?

hard

A. MOV r4, #0
MOV r5, #0
MOV r6, #0
...function body...
BX lr

B. PUSH {r4, r5, r6}
...function body...
POP {r4, r5, r6}

C. PUSH {r4}
PUSH {r5}
PUSH {r6}
...function body...
POP {r4}
POP {r5}
POP {r6}

D. POP {r4, r5, r6}
...function body...
PUSH {r4, r5, r6}

Solution

1. Step 1: Identify correct saving method

   Registers must be pushed together at function start to save their original values.

2. Step 2: Identify correct restoring method

   Registers must be popped in the same order before returning to restore original values.

3. Step 3: Check options for correct sequence

   PUSH {r4, r5, r6} ...function body... POP {r4, r5, r6} pushes all three registers at once and pops them before return, which is correct.

4. Final Answer:

   PUSH {r4, r5, r6} ...function body... POP {r4, r5, r6} -> Option B

5. Quick Check:

   Push all then pop all = correct preservation [OK]

Hint: Push all callee-saved at start, pop all before return [OK]

Common Mistakes:

• Pushing and popping registers separately in wrong order
• Using POP before PUSH
• Overwriting registers without saving