Data processing instructions (ADD, SUB, MOV, CMP) in ARM Architecture - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the effect of the ADD instruction

What will be the value in register R0 after executing the instruction ADD R0, R1, R2 if R1 contains 5 and R2 contains 10?

A 15

B 5

C 10

D 0

💡 Hint

ADD adds the values of the second and third registers and stores the result in the first register.

The ADD instruction adds the values in R1 and R2 (5 + 10) and stores the result (15) in R0.

📋 Factual

intermediate

Purpose of the CMP instruction

What is the primary purpose of the CMP instruction in ARM assembly?

A To add two register values and store the result

B To move data from one register to another

C To compare two registers and update condition flags without storing the result

D To subtract one register value from another and store the result

💡 Hint

CMP does not store a result but affects flags used for conditional instructions.

CMP subtracts the second operand from the first but does not store the result. Instead, it updates the condition flags (like zero, negative) to be used by conditional instructions.

🔍 Analysis

advanced

Result of SUB instruction with negative result

Given R3 = 8 and R4 = 12, what will be the value in R5 after executing SUB R5, R3, R4 in ARM assembly?

A 20

B -4 (represented as 0xFFFFFFFC in 32-bit two's complement)

C 4

D 0

💡 Hint

ARM registers hold unsigned 32-bit values but use two's complement for signed numbers.

SUB subtracts R4 from R3: 8 - 12 = -4. Since ARM registers are 32-bit, -4 is stored as 0xFFFFFFFC in two's complement form.

❓ Comparison

advanced

Difference between MOV and ADD instructions

Which statement correctly describes the difference between MOV and ADD instructions in ARM assembly?

A MOV copies a value from one register to another; ADD adds two values and stores the sum.

B Both MOV and ADD perform addition but store results differently.

C MOV adds two values; ADD copies a value from one register to another.

D MOV subtracts values; ADD copies values.

💡 Hint

Think about the basic function of each instruction.

MOV simply copies the value from the source register to the destination register. ADD performs arithmetic addition of two registers and stores the result.

❓ Reasoning

expert

Condition flags after CMP instruction

If R6 contains 20 and R7 contains 20, what will be the state of the Zero (Z) flag after executing CMP R6, R7?

A Z flag will be set to 0 (false) because the values are different

B Z flag will be set to 0 (false) because the values are equal

C Z flag will be set to 1 (true) because the values are different

D Z flag will be set to 1 (true) because the values are equal

💡 Hint

The Zero flag indicates if the subtraction result is zero.

CMP subtracts R7 from R6 (20 - 20 = 0). Since the result is zero, the Zero flag is set to 1, indicating equality.

Practice

1. What does the ADD instruction do in ARM architecture?

easy

A. Compares two register values without changing them

B. Subtracts one register value from another

C. Copies a value into a register

D. Adds values from two registers and stores the result in a register

Solution

1. Step 1: Understand the purpose of ADD

   The ADD instruction performs addition of two values stored in registers.

2. Step 2: Identify the result storage

   The result of the addition is stored back into a register.

3. Final Answer:

   Adds values from two registers and stores the result in a register -> Option D

4. Quick Check:

   ADD means addition operation [OK]

Hint: ADD always means adding two numbers and saving result [OK]

Common Mistakes:

• Confusing ADD with MOV which copies values
• Thinking ADD compares values like CMP
• Assuming ADD subtracts values

2. Which of the following is the correct syntax to move the value 5 into register R1 in ARM assembly?

easy

A. SUB R1, #5

B. MOV R1, #5

C. ADD R1, #5

D. CMP R1, #5

Solution

1. Step 1: Identify the instruction for copying values

   MOV is used to copy a value into a register.

2. Step 2: Check the syntax for immediate value

   The immediate value 5 is written as #5, so MOV R1, #5 copies 5 into R1.

3. Final Answer:

   MOV R1, #5 -> Option B

4. Quick Check:

   MOV copies value to register [OK]

Hint: MOV copies values; # means immediate number [OK]

Common Mistakes:

• Using ADD or SUB instead of MOV to copy values
• Forgetting # before immediate values
• Using CMP which only compares values

3. Given the instructions:
MOV R0, #10
SUB R0, R0, #3
What is the value in register R0 after execution?

medium

A. 7

B. 13

C. 3

D. 10

Solution

1. Step 1: Execute MOV instruction

   MOV R0, #10 sets register R0 to 10.

2. Step 2: Execute SUB instruction

   SUB R0, R0, #3 subtracts 3 from R0 (10 - 3 = 7) and stores result in R0.

3. Final Answer:

   7 -> Option A

4. Quick Check:

   10 minus 3 equals 7 [OK]

Hint: SUB subtracts immediate from register value [OK]

Common Mistakes:

• Adding instead of subtracting
• Not updating R0 after SUB
• Confusing immediate and register operands

4. Identify the error in the following ARM assembly code:
MOV R2, #8
CMP R2, #8
ADD R3, R2

medium

A. MOV instruction cannot use immediate values

B. CMP instruction syntax is incorrect

C. ADD instruction missing second operand

D. No error, code is correct

Solution

1. Step 1: Check MOV and CMP instructions

   MOV R2, #8 and CMP R2, #8 are correct syntax for moving and comparing immediate values.

2. Step 2: Analyze ADD instruction

   ADD requires three operands: destination, first source, second source. Here, ADD R3, R2 misses the third operand.

3. Final Answer:

   ADD instruction missing second operand -> Option C

4. Quick Check:

   ADD needs 3 operands [OK]

Hint: ADD needs destination and two sources [OK]

Common Mistakes:

• Assuming CMP syntax is wrong
• Thinking MOV can't use immediate values
• Ignoring missing operand in ADD

5. You want to check if the value in register R4 is equal to 15 and then move 1 into R5 if true, else move 0. Which sequence correctly uses ARM instructions?

hard

A. CMP R4, #15BEQ label_trueMOV R5, #0B label_endlabel_true: MOV R5, #1label_end:

B. MOV R5, #1CMP R4, #15BEQ label_trueMOV R5, #0label_true:

C. CMP R4, #15MOV R5, #1BEQ label_trueMOV R5, #0label_true:

D. MOV R5, #0CMP R4, #15MOV R5, #1BEQ label_truelabel_true:

Solution

1. Step 1: Compare R4 with 15

   CMP R4, #15 sets flags based on comparison.

2. Step 2: Use BEQ to branch if equal

   BEQ label_true jumps to label_true if R4 equals 15.

3. Step 3: Move 0 if not equal, else move 1

   If not equal, MOV R5, #0 executes; branch skips to label_end. At label_true, MOV R5, #1 sets R5 to 1.

4. Final Answer:

   Sequence in option A correctly implements the logic -> Option A

5. Quick Check:

   CMP + BEQ + conditional MOV sequence [OK]

Hint: Use CMP then BEQ to branch, MOV sets values [OK]

Common Mistakes:

• Moving values before comparison
• Not branching correctly on equality
• Incorrect order of instructions