Compare and branch patterns in ARM Architecture - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding ARM Compare and Branch Instructions

In ARM assembly, compare and branch instructions are used to control program flow based on conditions. Which of the following best describes the purpose of the CBZ instruction?

A It compares a register to zero and branches if the register is zero.

B It compares two registers and branches if they are equal.

C It compares a register to zero and branches if the register is not zero.

D It unconditionally branches to a label.

💡 Hint

Think about what the abbreviation CBZ stands for.

CBZ stands for Compare and Branch on Zero. It checks if a register's value is zero and branches if true.

📋 Factual

intermediate

ARM Branch Instruction Condition Codes

Which ARM condition code suffix causes a branch if the previous comparison found the values to be not equal?

A NE

B EQ

C GT

D LT

💡 Hint

Recall that NE stands for 'Not Equal'.

The NE condition code means 'Not Equal' and causes a branch if the compared values differ.

🚀 Application

advanced

Predicting Branch Outcome from ARM Code

Given the following ARM assembly snippet, what will be the value of register R2 after execution if R1 contains 0?

    CMP R1, #0
    BNE skip
    MOV R2, #1
skip:
    MOV R2, #2

A 0

B 1

C Undefined

D 2

💡 Hint

Consider what the BNE instruction does when R1 is zero.

CMP R1, #0 sets flags comparing R1 to zero. Since R1 is zero, the zero flag is set. BNE branches if not equal (zero flag clear), so it does not branch. The instruction MOV R2, #1 executes, then MOV R2, #2 executes unconditionally, so final R2 is 2.

🔍 Analysis

advanced

Identifying Error in ARM Compare and Branch Sequence

Examine the following ARM code snippet. What error will occur when this code runs?

    CMP R0, R1
    BEQ label
    MOV R2, #5
label:
    MOV R2, #10

A Logical error: MOV R2, #5 is always skipped.

B No error; R2 will be 10 regardless of comparison.

C Runtime error because label is undefined.

D Syntax error due to missing colon after label.

💡 Hint

Check the label syntax and branching logic carefully.

The label syntax is correct with a colon. The branch BEQ jumps to label if R0 equals R1, skipping MOV R2, #5. Otherwise, MOV R2, #5 runs followed by MOV R2, #10. No syntax or runtime error occurs.

❓ Reasoning

expert

Choosing the Correct Branch Instruction for a Condition

You want to branch to done only if the value in R3 is less than the value in R4. Which ARM instruction should you use immediately after CMP R3, R4?

A BLE done

B BGT done

C BLT done

D BNE done

💡 Hint

Remember that BLT means 'Branch if Less Than'.

After CMP R3, R4, BLT branches if R3 is less than R4. Other options check different conditions.