Branch instruction (B) in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to write a branch instruction that jumps to the label 'loop'.

ARM Architecture

B [1]

Drag options to blanks, or click blank then click option'

Astart

Bloop

Cend

Dmain

Attempts:

3 left

💡 Hint

Common Mistakes

Using a label that does not exist in the code.

Forgetting to write the label after the B instruction.

The B instruction is used to branch (jump) to a label. Here, the label is loop.

2fill in blank

medium

Complete the code to branch to the label 'exit' conditionally.

ARM Architecture

B[1] exit

Drag options to blanks, or click blank then click option'

AGT

BAL

CEQ

DNE

Attempts:

3 left

💡 Hint

Common Mistakes

Using unconditional branch (B) when a condition is needed.

Using wrong condition codes that do not match the desired condition.

The B instruction with condition EQ branches if the zero flag is set (equal condition).

3fill in blank

hard

Fix the error in the branch instruction to correctly jump to 'func'.

ARM Architecture

B [1]

Drag options to blanks, or click blank then click option'

Afunc

Bfunc()

C#func

D*func

Attempts:

3 left

💡 Hint

Common Mistakes

Adding parentheses after the label name.

Using immediate value syntax (#) incorrectly.

The branch instruction requires the label name without parentheses or symbols. Use func directly.

4fill in blank

hard

Fill both blanks to create a conditional branch that jumps to 'retry' if greater than zero.

ARM Architecture

B[1] [2]

Drag options to blanks, or click blank then click option'

AGT

BLT

Cretry

Dfail

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong condition code like LT instead of GT.

Jumping to the wrong label.

GT means branch if greater than (signed comparison). The label to jump to is retry.

5fill in blank

hard

Fill all three blanks to create a branch instruction with condition 'NE' to label 'loop_start' and add a comment.

ARM Architecture

B[1] [2] ; [3]

Drag options to blanks, or click blank then click option'

ANE

Bloop_start

Cbranch if not equal

DEQ

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong condition code like EQ instead of NE.

Forgetting to add a comment or using an unclear comment.

The instruction branches if not equal (NE) to the label loop_start. The comment explains the purpose.

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