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.