Branch instruction (B) in ARM Architecture - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the basic function of the B instruction

What is the primary purpose of the ARM B (Branch) instruction?

A To perform an arithmetic operation

B To load data from memory into a register

C To store data from a register into memory

D To jump to a different address unconditionally

💡 Hint

Think about what 'branch' means in programming.

The B instruction in ARM is used to jump to a different part of the program without any condition. It changes the flow of execution by updating the program counter to a new address.

📋 Factual

intermediate

Range of the Branch instruction offset

What is the maximum range (in bytes) that the ARM B instruction can branch to using its 24-bit signed offset?

A ±32 MB

B ±16 MB

C ±8 MB

D ±4 MB

💡 Hint

Consider that the offset is 24 bits and is shifted left by 2 bits.

The 24-bit signed offset in the B instruction is shifted left by 2 bits (because instructions are 4 bytes), giving a range of ±32 MB (2^23 * 4 bytes).

🔍 Analysis

advanced

Effect of B instruction on the program counter

Given the ARM instruction B label, what happens to the program counter (PC) after this instruction executes?

A PC is set to the address of <code>label</code>, and execution continues from there

B PC increments by 4 and continues sequentially

C PC is set to the address of <code>label</code> plus 4 bytes

D PC remains unchanged

💡 Hint

The branch instruction changes where the CPU fetches the next instruction.

The B instruction updates the PC to the target label's address, causing the CPU to continue execution from that new location.

❓ Comparison

advanced

Difference between B and BL instructions

Which statement correctly describes the difference between the ARM B and BL instructions?

A B branches unconditionally; BL branches conditionally

B B branches and saves return address; BL only branches

C B branches unconditionally; BL branches and saves return address in LR

D B branches conditionally; BL branches unconditionally

💡 Hint

Consider what 'BL' stands for and what it does with the link register.

The BL (Branch with Link) instruction branches to a target address and saves the return address in the link register (LR), enabling function calls. The B instruction just branches without saving return info.

❓ Reasoning

expert

Predicting the outcome of a branch with an invalid offset

If an ARM B instruction is encoded with an offset that exceeds its 24-bit signed range, what will happen when the CPU executes it?

A The CPU will raise a hardware exception immediately

B The CPU will branch to an incorrect address causing unpredictable behavior

C The instruction will be ignored and execution continues sequentially

D The CPU will branch to the nearest valid address within range

💡 Hint

Think about how the CPU interprets the offset bits in the instruction.

The offset field is limited to 24 bits. If the encoded offset is invalid or out of range, the CPU interprets the bits as a signed value, causing it to branch to an unintended address, which can lead to unpredictable program behavior.

Practice

1. What does the ARM B instruction do in a program?

easy

A. It adds two numbers and stores the result.

B. It jumps to a specified label to change the flow of execution.

C. It loads data from memory into a register.

D. It compares two values and sets flags.

Solution

1. Step 1: Understand the purpose of the B instruction

   The B instruction is used to jump to a different part of the program, identified by a label.

2. Step 2: Compare with other instructions

   Other instructions like add, load, or compare do not change program flow by jumping.

3. Final Answer:

   It jumps to a specified label to change the flow of execution. -> Option B

4. Quick Check:

   Branch (B) = Jump to label [OK]

Hint: B means Branch: jump to a label in code [OK]

Common Mistakes:

• Confusing B with arithmetic instructions
• Thinking B loads or stores data
• Assuming B compares values

2. Which of the following is the correct syntax for a branch instruction in ARM assembly?

easy

A. B label

B. branch to label

C. jmp label

D. goto label

Solution

1. Step 1: Recall ARM branch syntax

   ARM uses the instruction B followed by the label name to branch.

2. Step 2: Eliminate incorrect syntax

   Instructions like 'branch to', 'jmp', or 'goto' are not valid ARM syntax.

3. Final Answer:

   B label -> Option A

4. Quick Check:

   Correct syntax = B label [OK]

Hint: Branch uses 'B' followed by label name [OK]

Common Mistakes:

• Using non-ARM keywords like 'jmp' or 'goto'
• Writing full words instead of 'B'
• Omitting the label after B

3. Given the following ARM assembly code snippet, what will be the value in register r0 after execution?

B skip
MOV r0, #1
skip:
MOV r0, #5

medium

A. 5

B. 1

C. 0

D. Undefined

Solution

1. Step 1: Analyze the branch instruction

   The instruction B skip jumps over the MOV r0, #1 line directly to the label skip.

2. Step 2: Determine which MOV executes

   Since the jump skips the first MOV, only MOV r0, #5 runs, setting r0 to 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Branch skips first MOV, r0 = 5 [OK]

Hint: Branch skips code, so last MOV sets r0 [OK]

Common Mistakes:

• Ignoring the branch and thinking both MOV run
• Assuming r0 stays 0 by default
• Confusing label position

4. Identify the error in this ARM assembly code snippet:

B 100
MOV r0, #10
100:
MOV r1, #20

medium

A. MOV instructions cannot use immediate values.

B. The branch instruction must be 'BR' instead of 'B'.

C. There is no error; the code is correct.

D. Label '100' is invalid because it starts with a number.

Solution

1. Step 1: Check label naming rules

   Labels in ARM assembly cannot start with a digit; '100' is invalid.

2. Step 2: Verify branch and MOV syntax

   The branch instruction 'B' is correct, and MOV can use immediate values like #10.

3. Final Answer:

   Label '100' is invalid because it starts with a number. -> Option D

4. Quick Check:

   Labels cannot start with digits [OK]

Hint: Labels must start with a letter, not a number [OK]

Common Mistakes:

• Thinking 'B' is wrong branch syntax
• Believing MOV can't use immediate values
• Assuming numeric labels are allowed

5. You want to create a loop in ARM assembly that repeats 3 times using the branch instruction B. Which approach correctly uses B to implement this loop?

hard

A. Use B with labels named as numbers only.

B. Use multiple unconditional branches without a counter.

C. Use a counter register, decrement it, and branch back if not zero.

D. Use B to jump forward only, no looping possible.

Solution

1. Step 1: Understand loop control with branch

   Loops require a way to repeat code, usually by checking a counter and branching back if needed.

2. Step 2: Evaluate options for looping

   Using a counter register and branching back when not zero is the standard method. Branching forward only or using invalid labels won't create a loop.

3. Final Answer:

   Use a counter register, decrement it, and branch back if not zero. -> Option C

4. Quick Check:

   Loop = counter + branch back [OK]

Hint: Loop needs counter and branch back to label [OK]

Common Mistakes:

• Trying to loop without a counter
• Using only forward branches
• Using invalid label names for loops