Branch instruction (B) in ARM Architecture - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

The Branch instruction (B) is used to change the flow of execution by jumping to a different part of the program, allowing the processor to execute instructions from a new address.

beginner

How does the Branch instruction (B) determine where to jump?

The Branch instruction uses a signed offset value that is added to the current program counter (PC) plus 4 to calculate the target address to jump to.

beginner

What happens to the program counter (PC) when a Branch instruction is executed?

When a Branch instruction executes, the program counter (PC) is updated to the target address calculated by adding the offset to the current PC plus 4, causing the CPU to continue execution from that new address.

intermediate

Can the Branch instruction (B) in ARM be conditional?

Yes, the Branch instruction can be conditional by adding condition codes (like BEQ for branch if equal), which makes the jump happen only if certain conditions are met.

intermediate

What is the difference between a Branch (B) and Branch with Link (BL) instruction in ARM?

Branch (B) jumps to a new address without saving the return address, while Branch with Link (BL) saves the return address in the link register (LR) so the program can return after a subroutine call.

What does the Branch instruction (B) do in ARM architecture?

A Loads data from memory

B Changes the flow of execution to a new address

C Performs arithmetic addition

D Stores data to memory

The Branch instruction changes the program flow by jumping to a new address.

How is the target address calculated in a Branch instruction?

A By subtracting the offset from the stack pointer

B By using a fixed absolute address

C By multiplying the offset with the program counter

D By adding a signed offset to the current program counter

The Branch instruction adds a signed offset to the current PC to find the jump target.

What does the 'B' instruction stand for in ARM?

A Buffer

B Break

C Branch

D Byte

'B' stands for Branch, which changes the execution flow.

Which instruction saves the return address when jumping to a subroutine?

A BL (Branch with Link)

B B (Branch)

C MOV

D STR

BL saves the return address in the link register for returning after subroutine.

Can the Branch instruction be conditional in ARM?

A Yes, by using condition codes

B No, it is always unconditional

C Only in ARMv7 and earlier

D Only when used with BL

Branch instructions can be conditional by adding condition codes like BEQ, BNE.

Explain how the Branch instruction (B) changes the flow of a program in ARM architecture.

Think about how the CPU knows where to go next.

Describe the difference between the Branch (B) and Branch with Link (BL) instructions.

Consider what happens when you want to come back after a jump.

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