Conditional branch with flags in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to branch if the zero flag is set.

ARM Architecture

B[1] label_zero

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

AGT

BNE

CEQ

DLT

Attempts:

3 left

💡 Hint

Common Mistakes

Using NE (not equal) instead of EQ.

Using GT or LT which check other flags.

The EQ condition code means "equal" and is used to branch if the zero flag (Z) is set, indicating the previous operation resulted in zero.

2fill in blank

medium

Complete the code to branch if the negative flag is clear.

ARM Architecture

B[1] label_positive

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AMI

BPL

CVS

DVC

Attempts:

3 left

💡 Hint

Common Mistakes

Using MI which branches if negative flag is set.

Confusing VS and VC which relate to overflow flag.

The PL condition code means "plus" and branches if the negative flag (N) is clear, indicating a positive or zero result.

3fill in blank

hard

Fix the error in the branch instruction to branch if carry flag is set.

ARM Architecture

B[1] label_carry

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ACS

BCC

CNE

DEQ

Attempts:

3 left

💡 Hint

Common Mistakes

Using CC which branches if carry flag is clear.

Using NE or EQ which check zero flag instead.

The CS condition code means "carry set" and branches if the carry flag (C) is set. Using CC means carry clear, which is incorrect here.

4fill in blank

hard

Fill both blanks to branch if the overflow flag is clear and zero flag is set.

ARM Architecture

B[1][2] label_special

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AVC

BEQ

CNE

DVS

Attempts:

3 left

💡 Hint

Common Mistakes

Using VS instead of VC for overflow flag.

Using NE instead of EQ for zero flag.

VC means overflow clear, and EQ means zero flag set. Combining them checks both conditions.

5fill in blank

hard

Fill all three blanks to branch if negative flag is set, zero flag clear, and carry flag set.

ARM Architecture

B[1][2][3] label_complex

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

AMI

BNE

CCS

DPL

Attempts:

3 left

💡 Hint

Common Mistakes

Using PL instead of MI for negative flag.

Using EQ instead of NE for zero flag.

Using CC instead of CS for carry flag.

MI means negative flag set, NE means zero flag clear, and CS means carry flag set. Combining these condition codes checks all three flags.

Practice

1. What does the zero flag (Z) indicate in ARM conditional branching?

easy

A. The processor is in privileged mode

B. The previous operation caused an overflow

C. The previous operation resulted in zero

D. The previous operation resulted in a negative value

Solution

1. Step 1: Understand the zero flag meaning

   The zero flag (Z) is set when the result of the last operation is exactly zero.

2. Step 2: Relate zero flag to conditional branching

   Conditional branches use the zero flag to decide if the program should jump when a zero result occurs.

3. Final Answer:

   The previous operation resulted in zero -> Option C

4. Quick Check:

   Zero flag means result is zero [OK]

Hint: Zero flag means result is zero, check it first [OK]

Common Mistakes:

• Confusing zero flag with negative flag
• Thinking zero flag means overflow
• Assuming zero flag indicates processor mode

2. Which of the following is the correct syntax for a conditional branch in ARM assembly that jumps if zero flag is set?

easy

A. BEQ label

B. BNE label

C. BGT label

D. BLT label

Solution

1. Step 1: Identify the condition codes for branch instructions

   BEQ means 'Branch if Equal', which checks if the zero flag (Z) is set.

2. Step 2: Match the zero flag condition to the instruction

   Since zero flag set means equality, BEQ is the correct instruction to branch on zero flag.

3. Final Answer:

   BEQ label -> Option A

4. Quick Check:

   BEQ branches if zero flag set [OK]

Hint: BEQ means branch if zero flag set [OK]

Common Mistakes:

• Using BNE which branches if zero flag is clear
• Confusing BGT and BLT which check signed comparisons
• Mixing branch instructions with function calls

3. Given the ARM assembly snippet:

SUBS R0, R1, R2
BEQ equal_label
BNE notequal_label

If R1 equals R2, which label will the program jump to?

medium

A. equal_label

B. notequal_label

C. No jump, continue sequentially

D. Undefined behavior

Solution

1. Step 1: Understand the SUBS instruction effect on flags

   SUBS subtracts R2 from R1, updates flags including zero flag if result is zero.

2. Step 2: Check condition when R1 equals R2

   If R1 equals R2, subtraction result is zero, so zero flag is set.

3. Step 3: Determine which branch executes

   BEQ branches if zero flag set, so program jumps to equal_label.

4. Final Answer:

   equal_label -> Option A

5. Quick Check:

   Equal inputs set zero flag, BEQ jumps [OK]

Hint: If subtraction zero, BEQ jumps to equal_label [OK]

Common Mistakes:

• Choosing BNE which jumps if zero flag clear
• Assuming no jump happens on equality
• Confusing SUBS with non-flag updating SUB

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

CMP R3, R4
BGT greater_label
BLT less_label
BEQ equal_label

Assuming R3 and R4 are signed integers, what is wrong?

medium

A. CMP does not update flags

B. BGT and BLT cannot be used after CMP

C. BGT and BLT check unsigned, should use BHI and BLS

D. No error, code is correct

Solution

1. Step 1: Understand CMP and flag updates

   CMP subtracts R4 from R3 and updates flags for signed comparisons.

2. Step 2: Check branch instructions for signed comparisons

   BGT (branch if greater than) and BLT (branch if less than) are correct for signed comparisons.

3. Step 3: Confirm code correctness

   All branches correctly use signed condition codes after CMP, so no error.

4. Final Answer:

   No error, code is correct -> Option D

5. Quick Check:

   CMP + BGT/BLT correct for signed [OK]

Hint: BGT/BLT are for signed, BHI/BLS for unsigned [OK]

Common Mistakes:

• Confusing signed and unsigned branch instructions
• Thinking CMP does not update flags
• Assuming BGT/BLT are invalid after CMP

5. You want to write ARM assembly code that jumps to handle_negative if a subtraction results in a negative value, and to handle_zero if the result is zero. Which conditional branch instructions should you use after SUBS R5, R6, R7?

hard

A. BGT handle_negative and BNE handle_zero

B. BLT handle_negative and BEQ handle_zero

C. BLE handle_negative and BNE handle_zero

D. BHS handle_negative and BLO handle_zero

Solution

1. Step 1: Understand SUBS and flag updates

   SUBS subtracts and updates flags including negative (N) and zero (Z).

2. Step 2: Match flags to branch instructions

   BLT branches if negative flag set (result negative), BEQ branches if zero flag set (result zero).

3. Step 3: Confirm correct instructions for conditions

   BLT for negative result and BEQ for zero result are the correct choices.

4. Final Answer:

   BLT handle_negative and BEQ handle_zero -> Option B

5. Quick Check:

   Negative uses BLT, zero uses BEQ [OK]

Hint: Use BLT for negative, BEQ for zero after SUBS [OK]

Common Mistakes:

• Using BGT or BHS for negative values
• Mixing zero and negative flag conditions
• Using unsigned branch instructions incorrectly