Data processing instructions (ADD, SUB, MOV, CMP) in ARM Architecture

Introduction

Data processing instructions help the computer do math and move information inside it. They are simple commands that tell the processor to add, subtract, copy, or compare numbers.

When you want to add two numbers together, like adding scores in a game.

When you need to subtract one number from another, such as calculating change.

When you want to copy a value from one place to another, like saving a number for later.

When you want to check if two numbers are equal or which one is bigger.

Core Concept

ARM Architecture

ADD Rd, Rn, Operand2
SUB Rd, Rn, Operand2
MOV Rd, Operand2
CMP Rn, Operand2

Rd is the destination register where the result is stored.

Rn is the first source register.

Operand2 can be a register or an immediate value (a number).

Key Points

Adds the value in R2 and R3, then stores the result in R1.

ARM Architecture

ADD R1, R2, R3

Subtracts 5 from the value in R4 and stores the result back in R4.

ARM Architecture

SUB R4, R4, #5

Moves the number 10 into register R0.

ARM Architecture

MOV R0, #10

Compares the values in R5 and R6 by subtracting R6 from R5, but does not store the result. It sets flags for later decisions.

ARM Architecture

CMP R5, R6

Detailed Explanation

This program moves 20 into R0 and 10 into R1. Then it adds R0 and R1, storing 30 in R2. It subtracts R1 from R0, storing 10 in R3. Finally, it compares R2 and R3 to see which is bigger.

ARM Architecture

MOV R0, #20
MOV R1, #10
ADD R2, R0, R1
SUB R3, R0, R1
CMP R2, R3

Important Notes

The CMP instruction does not save a result but sets special flags used to make decisions later.

Immediate values start with a # sign, like #5 or #10.

Registers are like small boxes inside the processor that hold numbers temporarily.

Summary

ADD and SUB do math by adding or subtracting numbers in registers.

MOV copies a number or value into a register.

CMP compares two numbers and sets flags for decision-making.

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