Condition flags (N, Z, C, V) in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the Zero flag (Z) is set after an operation.

ARM Architecture

if (status_register & [1]) != 0:
    print("Zero flag is set")

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

A0x40000000

B0x20000000

C0x80000000

D0x10000000

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong bit mask for the Zero flag.

Confusing the Zero flag with the Negative flag.

The Zero flag (Z) is typically represented by bit 30 in the ARM status register, which corresponds to the mask 0x40000000.

2fill in blank

medium

Complete the code to set the Carry flag (C) in the status register.

ARM Architecture

status_register = status_register | [1]

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

A0x10000000

B0x40000000

C0x20000000

D0x80000000

Attempts:

3 left

💡 Hint

Common Mistakes

Setting the wrong bit for the Carry flag.

Using bit masks for other flags.

The Carry flag (C) is represented by bit 29 in the ARM status register, which corresponds to the mask 0x20000000.

3fill in blank

hard

Fix the error in the code that clears the Negative flag (N).

ARM Architecture

status_register = status_register & ~[1]

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

A0x40000000

B0x80000000

C0x20000000

D0x10000000

Attempts:

3 left

💡 Hint

Common Mistakes

Using the mask for the Zero or Carry flag instead of Negative.

Using OR instead of AND with negation.

The Negative flag (N) is the highest bit (bit 31) in the ARM status register, represented by 0x80000000. To clear it, we use bitwise AND with the negation of this mask.

4fill in blank

hard

Fill both blanks to check if the Overflow flag (V) is set and the Negative flag (N) is clear.

ARM Architecture

if (status_register & [1]) != 0 and (status_register & [2]) == 0:
    print("Overflow set and Negative clear")

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

A0x10000000

B0x80000000

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the masks for Overflow and Negative flags.

Checking for flags incorrectly with wrong operators.

The Overflow flag (V) is bit 28 (0x10000000) and the Negative flag (N) is bit 31 (0x80000000). The code checks if V is set and N is clear.

5fill in blank

hard

Fill all three blanks to create a dictionary mapping flag names to their bit masks.

ARM Architecture

flags = {
    'Negative': [1],
    'Zero': [2],
    'Carry': [3]
}

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

A0x80000000

B0x40000000

C0x20000000

D0x10000000

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping masks between flags.

Using the Overflow flag mask instead of Carry.

The Negative flag is bit 31 (0x80000000), Zero flag is bit 30 (0x40000000), and Carry flag is bit 29 (0x20000000).

Practice

1. Which condition flag in ARM architecture indicates that the result of an operation is zero?

easy

A. Z flag

B. N flag

C. C flag

D. V flag

Solution

1. Step 1: Understand the meaning of each flag

   The Z flag is set when the result of an operation is zero. The N flag indicates a negative result, C is for carry or borrow, and V is for signed overflow.

2. Step 2: Identify the zero result flag

   Since the question asks for the flag indicating zero result, the correct flag is Z.

3. Final Answer:

   Z flag -> Option A

4. Quick Check:

   Zero result = Z flag [OK]

Hint: Zero result always sets the Z flag [OK]

Common Mistakes:

• Confusing N (negative) with Z (zero)
• Thinking C means zero result
• Mixing V (overflow) with zero flag

2. Which of the following is the correct way to check the Negative flag (N) in ARM assembly condition codes?

easy

A. BMI (Branch if Minus)

B. BEQ (Branch if Equal)

C. BHS (Branch if Higher or Same)

D. BVS (Branch if Overflow Set)

Solution

1. Step 1: Recall ARM condition codes

   BEQ checks Z flag (zero), BMI checks N flag (negative), BHS checks C flag (carry set), and BVS checks V flag (overflow).

2. Step 2: Match the condition code to N flag

   The correct code for checking the Negative flag is BMI (Branch if Minus).

3. Final Answer:

   BMI (Branch if Minus) -> Option A

4. Quick Check:

   Negative flag = BMI [OK]

Hint: BMI always checks the Negative (N) flag [OK]

Common Mistakes:

• Using BEQ for negative instead of zero
• Confusing BHS with N flag
• Mixing BVS (overflow) with negative flag

3. Consider an ARM instruction that adds two 8-bit numbers: 0x7F + 0x01. Which condition flags will be set after this operation?

medium

A. N=0, Z=1, C=0, V=0

B. N=0, Z=0, C=1, V=0

C. N=1, Z=0, C=1, V=1

D. N=1, Z=0, C=0, V=1

Solution

1. Step 1: Calculate the sum and check overflow

   0x7F (127 decimal) + 0x01 (1 decimal) = 128 decimal (0x80). This fits in 8 bits but crosses the positive max for signed 8-bit (127), causing signed overflow.

2. Step 2: Determine flags

   N flag is 1 (MSB of 0x80 is 1, negative result), Z=0 since result is not zero, C=0 because no carry out beyond 8 bits, V=1 because signed overflow occurred.

3. Final Answer:

   N=1, Z=0, C=0, V=1 -> Option D

4. Quick Check:

   Signed overflow sets V=1, no zero or carry [OK]

Hint: Overflow sets V, zero result sets Z, carry sets C [OK]

Common Mistakes:

• Confusing carry with overflow
• Assuming zero flag set for non-zero result
• Misinterpreting negative flag for unsigned sum

4. An ARM instruction adds 0xFF and 0x02 and sets the condition flags. The flags are incorrectly showing C=0. What is the likely error?

medium

A. The addition result was zero, so C should be 0

B. The instruction did not update the condition flags

C. The V flag was set instead of C flag

D. The processor does not support carry flag

Solution

1. Step 1: Analyze the addition and expected flags

   0xFF (255) + 0x02 (2) = 0x101 (257), which exceeds 8 bits, so carry (C) should be set to 1.

2. Step 2: Identify why C flag is 0

   If C=0 incorrectly, likely the instruction did not update the condition flags because the 'S' suffix (e.g., ADDS) was missing, so flags were not changed.

3. Final Answer:

   The instruction did not update the condition flags -> Option B

4. Quick Check:

   Missing 'S' means flags unchanged [OK]

Hint: Use 'S' suffix to update flags after arithmetic [OK]

Common Mistakes:

• Assuming carry flag updates automatically
• Confusing overflow (V) with carry (C)
• Believing processor lacks carry flag

5. You want to write an ARM assembly routine that subtracts two numbers and branches if the result is negative or zero. Which condition flags and branch instructions should you use together?

hard

A. Check V flag with BVC and C flag with BCC, branch if both clear

B. Check C flag with BCS and V flag with BVS, branch if either set

C. Check N flag with BMI and Z flag with BEQ, branch if either set

D. Check Z flag with BNE and N flag with BPL, branch if both clear

Solution

1. Step 1: Identify flags for negative and zero results

   Negative result sets N flag, zero result sets Z flag.

2. Step 2: Choose branch instructions for these flags

   BMI branches if N=1 (negative), BEQ branches if Z=1 (zero). To branch if either negative or zero, use BMI and BEQ together.

3. Final Answer:

   Check N flag with BMI and Z flag with BEQ, branch if either set -> Option C

4. Quick Check:

   Negative = BMI, Zero = BEQ [OK]

Hint: Use BMI for negative and BEQ for zero branches [OK]

Common Mistakes:

• Using BNE instead of BEQ for zero
• Confusing carry or overflow flags for sign checks
• Trying to branch on both flags cleared instead of set