Why branching controls program execution in ARM Architecture - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to show the instruction that causes a branch in ARM assembly.

ARM Architecture

B [1]

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

AMOV

BSUB

CADD

Dlabel

Attempts:

3 left

💡 Hint

Common Mistakes

Using MOV, ADD, or SUB instead of a label after B.

Confusing branch instructions with data movement instructions.

In ARM assembly, the B instruction is used to branch to a label, which changes the flow of execution.

2fill in blank

medium

Complete the code to branch if zero flag is set in ARM assembly.

ARM Architecture

BEQ [1]

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

AADD

BMOV

Czero_label

DSUB

Attempts:

3 left

💡 Hint

Common Mistakes

Using instructions like MOV or ADD instead of a label after BEQ.

Not understanding that BEQ requires a label to branch to.

The BEQ instruction branches to the given label if the zero flag is set, indicating equality.

3fill in blank

hard

Fix the error in the branch instruction to correctly jump to the label.

ARM Architecture

B [1]

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

Aloop_start

BMOV

C1234

DADD

Attempts:

3 left

💡 Hint

Common Mistakes

Using numeric values instead of labels after B.

Confusing instructions with labels.

The branch instruction must be followed by a valid label name, not a number or instruction.

4fill in blank

hard

Fill both blanks to create a conditional branch that jumps if not equal.

ARM Architecture

B[1] [2]

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

ANE

Bequal_label

Cnot_equal_label

DEQ

Attempts:

3 left

💡 Hint

Common Mistakes

Using EQ condition with a not equal label.

Mixing condition codes and labels incorrectly.

The BNE instruction branches if the zero flag is clear, meaning not equal. It must be followed by the target label.

5fill in blank

hard

Fill all three blanks to create a branch with link instruction to a function label.

ARM Architecture

BL [1] [2] [3]

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

ASUB

Bfunction_label

CADD

DMOV

Attempts:

3 left

💡 Hint

Common Mistakes

Adding instructions after BL that are not labels.

Misunderstanding BL usage as multiple arguments.

The BL instruction branches to a function label and saves the return address. The following tokens are invalid here, so only the label is correct; the other blanks are distractors to fill.

Practice

1. What is the main purpose of branching instructions in ARM architecture?

easy

A. To change the order in which instructions are executed

B. To perform arithmetic calculations

C. To store data in memory

D. To load data from registers

Solution

1. Step 1: Understand the role of branching

   Branching instructions allow the program to jump to different parts of the code instead of running instructions one after another.

2. Step 2: Identify the effect on execution order

   This jump changes the normal sequential flow, enabling decisions and loops.

3. Final Answer:

   To change the order in which instructions are executed -> Option A

4. Quick Check:

   Branching controls execution flow [OK]

Hint: Branching changes instruction flow order [OK]

Common Mistakes:

• Confusing branching with arithmetic operations
• Thinking branching stores or loads data
• Assuming branching only repeats instructions

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

easy

A. BRANCH label

B. B label

C. JMP label

D. GOTO label

Solution

1. Step 1: Recall ARM branch instruction syntax

   In ARM assembly, the unconditional branch uses the mnemonic B followed by the label to jump to.

2. Step 2: Compare options

   Only B label uses the correct ARM mnemonic; options A, C, and D are invalid in ARM assembly.

3. Final Answer:

   B label -> Option B

4. Quick Check:

   Unconditional branch = B label [OK]

Hint: ARM unconditional branch uses 'B' mnemonic [OK]

Common Mistakes:

• Using non-ARM mnemonics like JMP or GOTO
• Writing full words like BRANCH instead of B
• Confusing conditional and unconditional branch syntax

3. Consider this ARM assembly snippet:

BNE skip
MOV R0, #1
skip:
MOV R1, #2

What happens if the zero flag is set before executing this code?

medium

A. Both R0 and R1 remain unchanged

B. R0 is not changed, R1 is set to 2

C. Only R0 is set to 1

D. R0 is set to 1, then R1 is set to 2

Solution

1. Step 1: Understand BNE behavior with zero flag set

   BNE means 'Branch if Not Equal', which branches if zero flag is clear. Since zero flag is set, branch is NOT taken.

2. Step 2: Trace instruction execution

   Because branch is not taken, MOV R0, #1 executes, then MOV R1, #2 executes.

3. Final Answer:

   R0 is set to 1, then R1 is set to 2 -> Option D

4. Quick Check:

   Zero flag set means no branch, both MOV run [OK]

Hint: BNE skips only if zero flag is clear [OK]

Common Mistakes:

• Assuming branch always happens regardless of flags
• Confusing zero flag set with clear
• Thinking R0 is unchanged when branch not taken

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

BNE label
MOV R0, #5
label:
MOV R1, #10

Assuming the zero flag is set, what is wrong?

medium

A. BNE should be BEQ to branch when zero flag is set

B. MOV instructions cannot follow a branch

C. Label is missing a colon

D. Branch instruction must be unconditional

Solution

1. Step 1: Analyze branch condition and zero flag

   BNE branches if zero flag is clear. Since zero flag is set, branch will not happen.

2. Step 2: Identify correct branch for zero flag set

   To branch when zero flag is set, BEQ (Branch if Equal) should be used instead of BNE.

3. Final Answer:

   BNE should be BEQ to branch when zero flag is set -> Option A

4. Quick Check:

   Branch condition must match zero flag [OK]

Hint: Use BEQ to branch when zero flag is set [OK]

Common Mistakes:

• Thinking BNE branches when zero flag is set
• Believing MOV cannot follow branch
• Confusing label syntax with error

5. You want to repeat a block of instructions 5 times using ARM assembly. Which branching method best achieves this?

hard

A. Use multiple unconditional branches to skip instructions

B. Use an unconditional branch to jump forward once

C. Use a conditional branch that loops until a counter reaches zero

D. Use a branch that only executes once

Solution

1. Step 1: Understand looping with branching

   To repeat instructions multiple times, a loop is needed that checks a counter and branches back if not done.

2. Step 2: Identify correct branch type for looping

   A conditional branch that tests the counter and loops until it reaches zero is the correct method.

3. Final Answer:

   Use a conditional branch that loops until a counter reaches zero -> Option C

4. Quick Check:

   Looping requires conditional branch with counter check [OK]

Hint: Loop with conditional branch and counter check [OK]

Common Mistakes:

• Using unconditional branch without loop control
• Skipping instructions instead of looping
• Assuming branch executes multiple times automatically