Data processing instructions (ADD, SUB, MOV, CMP) in ARM Architecture - Mini Project: Build & Apply

Understanding ARM Data Processing Instructions: ADD, SUB, MOV, CMP

📖 Scenario: You are learning how basic arithmetic and comparison operations work in ARM assembly language. These operations are essential for controlling the flow of programs and manipulating data in low-level programming.Imagine you are working with a simple calculator program that uses registers to hold numbers and perform operations like addition, subtraction, moving values, and comparing numbers.

🎯 Goal: Build a simple ARM assembly code snippet that uses the ADD, SUB, MOV, and CMP instructions to perform basic data processing tasks on registers.

📋 What You'll Learn

Create registers with initial values

Use MOV to set a register value

Use ADD to add values from two registers

Use SUB to subtract one register value from another

Use CMP to compare two register values

💡 Why This Matters

🌍 Real World

ARM assembly instructions like ADD, SUB, MOV, and CMP are used in embedded systems, mobile devices, and performance-critical applications to manipulate data efficiently.

💼 Career

Understanding these instructions is essential for roles in embedded programming, firmware development, and low-level system software engineering.

1

Set up initial register values

Write ARM assembly instructions to initialize registers R0 with value #10 and R1 with value #5 using the MOV instruction.

ARM Architecture

; Initialize registers R0 and R1
; Your code here

Hint

Use MOV to move an immediate value into a register, for example, MOV R0, #10.

2

Add values in registers

Add the values in registers R0 and R1 and store the result in register R2 using the ADD instruction.

ARM Architecture

MOV R0, #10
MOV R1, #5
; Add R0 and R1, store in R2
; Your code here

Hint

Use ADD destination, operand1, operand2 to add two registers and store the result.

3

Subtract values in registers

Subtract the value in register R1 from register R0 and store the result in register R3 using the SUB instruction.

ARM Architecture

MOV R0, #10
MOV R1, #5
ADD R2, R0, R1
; Subtract R1 from R0, store in R3
; Your code here

Hint

Use SUB destination, operand1, operand2 to subtract one register from another.

4

Compare register values

Use the CMP instruction to compare the values in registers R0 and R1. This sets condition flags based on the comparison.

ARM Architecture

MOV R0, #10
MOV R1, #5
ADD R2, R0, R1
SUB R3, R0, R1
; Compare R0 and R1
; Your code here

Hint

Use CMP operand1, operand2 to compare two registers and update flags for conditional instructions.

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