Conditional branch with flags in ARM Architecture - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a conditional branch in ARM architecture?

A conditional branch is an instruction that changes the flow of execution only if certain conditions, based on processor flags, are met.

beginner

Which processor flags are commonly used for conditional branching in ARM?

The common flags are N (Negative), Z (Zero), C (Carry), and V (Overflow). These flags reflect the result of previous operations.

beginner

How does the Zero (Z) flag affect conditional branching?

If the Z flag is set (meaning the last result was zero), a branch instruction with a condition checking Z will execute the branch.

beginner

What does the 'BEQ' instruction do in ARM assembly?

BEQ stands for 'Branch if Equal'. It causes a branch if the Zero (Z) flag is set, indicating the previous comparison found equality.

beginner

Why are flags important for conditional branches?

Flags store results of operations like comparisons. Conditional branches use these flags to decide if the program should jump to a different part of code.

Which flag indicates if the last operation result was zero?

A Z (Zero)

B N (Negative)

C C (Carry)

D V (Overflow)

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

What does the ARM instruction 'BNE' do?

A Branch if Negative

B Branch if Not Equal

C Branch if Equal

D Branch if Overflow

BNE branches if the Zero flag is clear, meaning the compared values were not equal.

Which flag is set when an arithmetic operation results in a negative number?

A N (Negative)

B C (Carry)

C V (Overflow)

D Z (Zero)

The N flag is set when the result of an operation is negative.

What does the 'BGT' instruction check before branching?

A Branch if Greater Than (checks Z flag only)

B Branch if Greater Than (checks C flag only)

C Branch if Greater Than (checks N and V flags)

D Branch if Greater Than (ignores flags)

BGT branches if the result is greater than, which involves checking the Negative and Overflow flags along with Zero.

If the Carry (C) flag is set, what does it usually indicate?

A An overflow occurred

B The last result was zero

C The result was negative

D A carry out from the last addition or borrow in subtraction

The Carry flag is set when there is a carry out from addition or borrow in subtraction.

Explain how conditional branches use flags in ARM architecture to control program flow.

Think about how the processor decides to jump or continue based on previous results.

Describe the role of the Zero (Z) and Negative (N) flags in conditional branching.

Consider what these flags tell about the last operation's outcome.

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