Recursive function in assembly in ARM Architecture - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the base case in ARM recursive functions

In a recursive ARM assembly function calculating factorial, what is the purpose of the base case?

A To increase the stack pointer to allocate more space

B To jump to the start of the program

C To call the function again with the same input

D To stop the recursion by returning a known value when input is 0 or 1

💡 Hint

Think about how recursion ends to avoid infinite loops.

The base case stops recursion by returning a fixed value when the input reaches 0 or 1, preventing infinite calls.

🔍 Analysis

intermediate

Stack usage in ARM recursive calls

What happens to the stack pointer (SP) during each recursive call in an ARM assembly function?

A SP remains unchanged

B SP increases to allocate more space

C SP decreases to allocate space for return address and local variables

D SP is reset to zero

💡 Hint

Consider how the stack grows when functions call themselves.

Each recursive call pushes data onto the stack, so SP decreases to allocate space.

📋 Factual

advanced

Register preservation in ARM recursion

Which registers must be preserved (saved and restored) by a recursive ARM assembly function to maintain correct execution?

A Callee-saved registers like R4-R11 and LR

B Only R0 and R1

C All registers including R0-R3

D No registers need preservation

💡 Hint

Think about which registers the function must keep unchanged across calls.

Callee-saved registers (R4-R11 and LR) must be preserved to avoid corrupting data during recursion.

🚀 Application

advanced

Output of ARM recursive factorial function

Given an ARM assembly recursive factorial function called with input 4, what is the returned result?

ARM Architecture

Input: R0 = 4
Function: factorial(n) returns n! recursively

A 24

B 10

C 16

D 120

💡 Hint

Recall the factorial values for small numbers.

Factorial of 4 is 4 × 3 × 2 × 1 = 24.

❓ Reasoning

expert

Identifying error in ARM recursive function code snippet

What error will occur if the link register (LR) is not saved before a recursive call in ARM assembly?

ARM Architecture

Recursive call without pushing LR onto stack

A Stack overflow immediately

B Return address overwritten causing incorrect return

C No error, function works fine

D Processor halts due to invalid instruction

💡 Hint

Consider what happens to LR during nested calls.

Not saving LR causes it to be overwritten on recursive calls, so the function returns to wrong address.

Practice

1. What is the main purpose of a recursive function in ARM assembly?

easy

A. To execute instructions only once

B. To call itself to solve smaller parts of a problem

C. To avoid using the stack

D. To run faster than iterative loops

Solution

1. Step 1: Understand recursion concept

   A recursive function calls itself to break down a problem into smaller parts.

2. Step 2: Apply to ARM assembly context

   In ARM assembly, recursion is used to solve problems by repeated self-calls with updated parameters.

3. Final Answer:

   To call itself to solve smaller parts of a problem -> Option B

4. Quick Check:

   Recursion means self-calling function [OK]

Hint: Recursion means function calls itself repeatedly [OK]

Common Mistakes:

• Thinking recursion avoids stack usage
• Confusing recursion with iteration
• Assuming recursion always runs faster

2. Which ARM assembly instruction is commonly used to return from a recursive function call?

easy

A. BL

B. MOV

C. BX LR

D. PUSH

Solution

1. Step 1: Identify return instruction

   In ARM assembly, BX LR is used to branch back to the address stored in the link register, effectively returning from a function.

2. Step 2: Confirm usage in recursion

   Recursive functions use BX LR to return after completing each call.

3. Final Answer:

   BX LR -> Option C

4. Quick Check:

   Return uses BX LR instruction [OK]

Hint: Return from function with BX LR instruction [OK]

Common Mistakes:

• Using BL to return instead of call
• Confusing PUSH with return
• Using MOV incorrectly for return

3. Consider this ARM assembly snippet for a recursive factorial function:

factorial:
    CMP R0, #1
    BLE end_factorial
    PUSH {LR}
    SUB R0, R0, #1
    BL factorial
    POP {LR}
    MUL R0, R0, R1
end_factorial:
    MOV R1, R0
    BX LR

What will be the value in R1 after calling factorial with R0 = 3?

medium

A. 0

B. 3

C. 1

D. 6

Solution

1. Step 1: Trace the recursive factorial

   The function saves LR, decrements R0, calls factorial recursively, restores LR, then multiplies R0 by R1.

2. Step 2: Calculate factorial for R0=3

   Call factorial(3): R0=3, not base case, PUSH LR, SUB R0=2, BL factorial(2). factorial(2): PUSH LR, SUB R0=1, BL factorial(1). factorial(1): base case, sets R1=1, returns. factorial(2): POP LR, MUL R0=1 * R1=1 =1, MOV R1=1, return. factorial(3): POP LR, MUL R0=2 * R1=1=2, MOV R1=2, return.

3. Step 3: Correction

   However, the MUL uses R0 and R1, but R0 was decremented before recursive call, so the multiplication is (n-1) * fact(n-1), which is incorrect for factorial.

4. Step 4: Final value

   Tracing carefully, the final R1 will be 2, which is incorrect factorial of 3 (should be 6). But the code as given will produce 2.

5. Final Answer:

   2 -> Option D

Hint: SUB decrements R0 before call; MUL uses decremented R0 causing incorrect factorial [Check carefully]

Common Mistakes:

• Assuming correct factorial computation (expecting 6)
• Confusing R0 and R1 values
• Missing base case return value

4. Identify the error in this recursive ARM assembly function snippet:

factorial:
    CMP R0, #1
    BLE end_factorial
    SUB R0, R0, #1
    BL factorial
    MUL R0, R0, R1
end_factorial:
    MOV R1, R0
    BX LR

What is the main problem?

medium

A. Missing PUSH and POP of LR causing return address loss

B. Incorrect base case comparison

C. Wrong multiplication order

D. Using MOV instead of MOVS

Solution

1. Step 1: Check stack usage for recursion

   Recursive calls must save LR on stack before calling to preserve return address.

2. Step 2: Identify missing PUSH/POP

   This code lacks PUSH {LR} before BL and POP {LR} after, risking corrupted return address.

3. Final Answer:

   Missing PUSH and POP of LR causing return address loss -> Option A

4. Quick Check:

   Save LR on stack in recursion [OK]

Hint: Always save LR on stack before recursive call [OK]

Common Mistakes:

• Ignoring stack save for LR
• Assuming base case is wrong
• Confusing MOV with MOVS

5. You want to write a recursive ARM assembly function to compute the nth Fibonacci number. Which of these is essential to correctly implement recursion here?

hard

A. Handle base cases to stop recursion and save LR on stack before recursive calls

B. Use only registers without stack to speed up recursion

C. Avoid base cases to allow infinite recursion for all n

D. Use unconditional jumps instead of BL for recursive calls

Solution

1. Step 1: Importance of base cases

   Base cases stop recursion to prevent infinite loops, essential for Fibonacci calculation.

2. Step 2: Save LR on stack before recursive calls

   Saving LR preserves return addresses during multiple recursive calls, avoiding crashes.

3. Final Answer:

   Handle base cases to stop recursion and save LR on stack before recursive calls -> Option A

4. Quick Check:

   Base cases + LR save = correct recursion [OK]

Hint: Always include base cases and save LR in recursion [OK]

Common Mistakes:

• Skipping base cases causing infinite recursion
• Not saving LR causing return errors
• Using jumps instead of BL for calls