If-else implementation in assembly in ARM Architecture - Mini Project: Build & Apply

If-else Implementation in Assembly

📖 Scenario: You are learning how to write simple decision-making code using ARM assembly language. This is useful when you want your program to choose between two actions based on a condition, like checking if a number is positive or negative.

🎯 Goal: Build a small ARM assembly program that uses an if-else structure to check if a number is zero or not, and sets a register to different values accordingly.

📋 What You'll Learn

Create a register with a specific value to test

Set up a label for branching

Use a conditional branch instruction to implement if-else logic

Set a register to one value if the condition is true, another if false

💡 Why This Matters

🌍 Real World

Assembly language is used in low-level programming for embedded systems, device drivers, and performance-critical code where direct control of hardware is needed.

💼 Career

Understanding if-else in assembly helps in debugging, optimizing code, and working with hardware-level programming in fields like embedded systems engineering and systems programming.

1

DATA SETUP: Initialize register with test value

Load the value 5 into register r0 to use as the test number.

ARM Architecture

# Load the value 5 into r0
# Your code here

Hint

Use the MOV instruction to put the number 5 into r0.

2

CONFIGURATION: Prepare a label for branching

Create a label called else_part that will mark where the else code starts.

ARM Architecture

MOV r0, #5
# Your code here

Hint

Labels end with a colon : and mark a place in the code.

3

CORE LOGIC: Implement if-else using conditional branch

Compare r0 with zero using CMP r0, #0. Then use the conditional branch BEQ else_part to jump to else_part if r0 is zero. If not zero, set r1 to 1. Then use B end_part to skip over the else part.

ARM Architecture

MOV r0, #5
# Your code here
else_part:

Hint

Use CMP to compare, BEQ to branch if equal, MOV to set r1, and B end_part to skip the else.

4

COMPLETION: Complete else part to set r1 to zero

At label else_part, set r1 to 0. Then create a label end_part: to complete the if-else structure.

ARM Architecture

MOV r0, #5
CMP r0, #0
BEQ else_part
MOV r1, #1
B end_part
else_part:
# Your code here

Hint

After the label else_part, use MOV r1, #0 then add end_part:.

Practice

1. In ARM assembly, which instruction is primarily used to compare two values before an if-else branch?

easy

A. MOV

B. CMP

C. ADD

D. STR

Solution

1. Step 1: Understand the purpose of CMP

   The CMP instruction compares two registers by subtracting one from the other and setting condition flags without storing the result.

2. Step 2: Identify its role in if-else

   These condition flags are then used by branch instructions to decide which code path to take, implementing if-else logic.

3. Final Answer:

   CMP -> Option B

4. Quick Check:

   Compare = CMP [OK]

Hint: CMP sets flags for branching decisions in if-else [OK]

Common Mistakes:

• Confusing CMP with MOV which moves data
• Using ADD instead of CMP for comparison
• Thinking STR stores comparison results

2. Which of the following is the correct syntax for a conditional branch in ARM assembly that jumps if two values are equal?

easy

A. BNE label

B. BL label

C. BEQ label

D. BGT label

Solution

1. Step 1: Understand branch conditions

   BEQ means 'Branch if Equal', which jumps if the zero flag is set after a CMP instruction.

2. Step 2: Match condition to syntax

   BNE means 'Branch if Not Equal', BGT means 'Branch if Greater Than', and BL is 'Branch with Link' (function call), so only BEQ matches the condition for equality.

3. Final Answer:

   BEQ label -> Option C

4. Quick Check:

   Equal branch = BEQ [OK]

Hint: BEQ branches when values are equal after CMP [OK]

Common Mistakes:

• Using BNE for equality
• Confusing BL with conditional branch
• Using BGT for equality check

3. Given the ARM assembly snippet:

CMP R0, #10
BEQ equal_label
MOV R1, #0
B end_label
equal_label:
MOV R1, #1
end_label:

What value will R1 hold if R0 equals 10?

medium

A. 0

B. 10

C. Undefined

D. 1

Solution

1. Step 1: Analyze CMP and BEQ

   The CMP compares R0 with 10. If equal, BEQ jumps to equal_label.

2. Step 2: Follow the branch logic

   If R0 is 10, execution jumps to equal_label where R1 is set to 1. Otherwise, R1 is set to 0 before jumping to end_label.

3. Final Answer:

   1 -> Option D

4. Quick Check:

   R0=10 triggers BEQ, sets R1=1 [OK]

Hint: If equal, BEQ jumps to set R1=1 [OK]

Common Mistakes:

• Assuming R1 stays 0
• Ignoring the branch to equal_label
• Confusing MOV targets

4. Identify the error in this ARM assembly if-else snippet:

CMP R2, #5
BNE else_label
MOV R3, #1
else_label:
MOV R3, #0

medium

A. Missing branch to skip else after if block

B. Incorrect comparison instruction

C. Wrong register used in MOV

D. No labels defined

Solution

1. Step 1: Understand the flow

   If R2 equals 5, BNE is not taken, so MOV R3, #1 executes, then execution continues to else_label and MOV R3, #0 runs.

2. Step 2: Identify missing branch

   There is no branch to skip the else block after the if block, so both blocks run sequentially, overwriting R3.

3. Final Answer:

   Missing branch to skip else after if block -> Option A

4. Quick Check:

   Missing jump after if causes both blocks to run [OK]

Hint: Add branch after if block to skip else [OK]

Common Mistakes:

• Assuming else block is skipped automatically
• Using wrong branch instruction
• Ignoring label definitions

5. How would you implement a nested if-else in ARM assembly to check if R4 is greater than 10, then if R5 equals 20, setting R6 to 1; else set R6 to 0?

hard

A. Use CMP and BGT for R4, then CMP and BEQ for R5 with nested labels

B. Use only one CMP for both conditions combined

C. Use MOV instructions without branches

D. Use STR to store comparison results

Solution

1. Step 1: Implement outer if with CMP and BGT

   Compare R4 with 10 using CMP, then branch with BGT to inner if block if true.

2. Step 2: Implement inner if with CMP and BEQ

   Inside the inner block, compare R5 with 20 and branch with BEQ to set R6 to 1; else set R6 to 0.

3. Step 3: Use labels to separate blocks

   Use labels to mark inner if, else, and end blocks to control flow clearly.

4. Final Answer:

   Use CMP and BGT for R4, then CMP and BEQ for R5 with nested labels -> Option A

5. Quick Check:

   Nested if uses CMP + BGT then CMP + BEQ [OK]

Hint: Use nested CMP and conditional branches with labels [OK]

Common Mistakes:

• Trying to combine conditions in one CMP
• Skipping branches for nested logic
• Using STR instead of branch instructions