If-else implementation in assembly in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to compare two registers and branch if equal.

ARM Architecture

CMP R0, R1
BEQ [1]

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

AADD

BR2

CMOV

Dlabel_equal

Attempts:

3 left

💡 Hint

Common Mistakes

Using a register instead of a label after BEQ.

Confusing BEQ with MOV or ADD instructions.

The CMP instruction compares two registers. BEQ branches to the label if they are equal.

2fill in blank

medium

Complete the code to load immediate value 1 into R2 if R0 is greater than R1.

ARM Architecture

CMP R0, R1
BGT [1]
MOV R2, #0
B [2]
[1]:
MOV R2, #1
[2]:

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

Aset_one

Bend_if

Cstart

Dloop

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same label for both branches.

Forgetting to branch after setting R2 to 0.

BGT branches to set_one if R0 > R1. Otherwise, R2 is set to 0. Then it jumps to end_if to skip the next instruction.

3fill in blank

hard

Fix the error in the code to correctly implement if-else logic.

ARM Architecture

CMP R3, R4
BLT [1]
MOV R5, #10
B [2]
[1]:
MOV R5, #20
[2]:

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Aless_than

Belse_part

Cstart

Dend_if

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect or missing labels.

Not branching after the first MOV instruction.

BLT branches to less_than if R3 < R4. Otherwise, R5 is set to 10, then jumps to end_if to skip the else part.

4fill in blank

hard

Fill both blanks to complete the if-else structure that sets R7 to 5 if R6 equals R8, else sets R7 to 0.

ARM Architecture

CMP R6, R8
BEQ [1]
MOV R7, #0
B [2]
[1]:
MOV R7, #5
[2]:

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

Aequal_case

Bend_if

Cstart

Delse_case

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the labels for if and else blocks.

Forgetting the branch after MOV R7, #0.

BEQ branches to equal_case if R6 equals R8. Otherwise, R7 is set to 0, then jumps to end_if to skip the equal case.

5fill in blank

hard

Fill all three blanks to implement if-else that sets R9 to 100 if R10 is not zero, else sets R9 to 50.

ARM Architecture

CMP R10, #0
BNE [1]
MOV R9, #50
B [2]
[1]:
MOV R9, [3]
[2]:

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

Anot_zero

Bend_if

C#100

D#0

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong labels or missing branches.

Forgetting the # before immediate values.

BNE branches to not_zero if R10 is not zero. Otherwise, R9 is set to 50, then jumps to end_if. In the not_zero block, R9 is set to 100.

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