Bit manipulation in ARM in ARM Architecture - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the purpose of the ARM BIC instruction

What does the ARM BIC (Bit Clear) instruction do?

A It clears bits to 0 where the mask has 1s, leaving other bits unchanged.

B It sets bits to 1 where the mask has 1s, leaving other bits unchanged.

C It toggles bits where the mask has 1s, leaving other bits unchanged.

D It copies bits from the mask to the destination register.

💡 Hint

Think about how clearing bits works with a mask.

The BIC instruction clears bits in the destination register where the mask bits are 1, effectively performing a bitwise AND with the inverse of the mask.

📋 Factual

intermediate

ARM instruction for setting specific bits

Which ARM instruction is used to set specific bits to 1 without affecting other bits?

A EOR

B AND

C ORR

D BIC

💡 Hint

Setting bits to 1 usually involves a bitwise OR operation.

The ORR instruction performs a bitwise OR, which sets bits to 1 where the mask has 1s, leaving other bits unchanged.

🔍 Analysis

advanced

Result of combined ARM bitwise instructions

Given the initial register value R0 = 0xF0F0, what is the value of R0 after executing these instructions?

ORR R0, R0, #0x0F0F
BIC R0, R0, #0x00FF

A 0xF00F

B 0xF0FF

C 0xFF00

D 0xF000

💡 Hint

Apply each instruction step-by-step, converting hex to binary if needed.

First, ORR sets bits: 0xF0F0 OR 0x0F0F = 0xFFFF. Then BIC clears bits: 0xFFFF AND NOT 0x00FF = 0xF000.

❓ Comparison

advanced

Difference between EOR and BIC in ARM

Which statement correctly describes the difference between the ARM EOR and BIC instructions?

A EOR clears bits where the mask is 1; BIC toggles bits where the mask is 1.

B EOR toggles bits where the mask is 1; BIC clears bits where the mask is 1.

C EOR copies bits from the mask; BIC sets bits to 1 where the mask is 1.

D EOR sets bits where the mask is 1; BIC clears bits where the mask is 0.

💡 Hint

Think about how toggling and clearing bits differ.

EOR performs bitwise exclusive OR, toggling bits where the mask has 1s. BIC clears bits where the mask has 1s.

❓ Reasoning

expert

Determining the effect of a sequence of ARM bitwise instructions

Starting with R1 = 0x12345678, what is the value of R1 after executing the following instructions?

BIC R1, R1, #0x00FF00FF
EOR R1, R1, #0xFF0000FF

A 0xEDCB5600

B 0xEDCB56FF

C 0x12340000

D 0xEDCB0000

💡 Hint

Apply BIC first, then EOR. Convert hex to binary or decimal to track bits clearly.

BIC clears bits in R1 where mask 0x00FF00FF has 1s, resulting in 0x12340000. Then EOR toggles bits where mask 0xFF0000FF has 1s, resulting in 0xEDCB5600.

Practice

1. Which ARM instruction is used to set specific bits to 1 without changing other bits?

easy

A. BIC

B. ORR

C. EOR

D. LSR

Solution

1. Step 1: Understand ORR instruction

   ORR performs a bitwise OR, setting bits to 1 where either operand has 1.

2. Step 2: Compare with other instructions

   BIC clears bits, EOR toggles bits, LSR shifts bits right. Only ORR sets bits without clearing others.

3. Final Answer:

   ORR -> Option B

4. Quick Check:

   Set bits = ORR [OK]

Hint: ORR sets bits; BIC clears bits; remember ORR for setting [OK]

Common Mistakes:

• Confusing BIC with ORR
• Thinking EOR sets bits
• Mixing shift instructions with bit setting

2. Which of the following is the correct syntax to clear bits using BIC in ARM assembly?

easy

A. BIC R0, R1, R2

B. BIC R0, R1, #R2

C. BIC R0, #R1, R2

D. BIC R0, R1, LSR #1

Solution

1. Step 1: Review BIC syntax

   BIC syntax is BIC destination, source, operand2 (register or immediate). BIC R0, R1, R2 matches this format.

2. Step 2: Analyze other options

   Options using # before register names are invalid; the option attempting a shift lacks the required register operand, making it invalid syntax.

3. Final Answer:

   BIC R0, R1, R2 -> Option A

4. Quick Check:

   BIC syntax = BIC Rd, Rn, Operand2 [OK]

Hint: BIC clears bits: BIC Rd, Rn, Operand2 [OK]

Common Mistakes:

• Using # before register names
• Confusing immediate and register operands
• Adding unnecessary shifts in basic syntax

3. Given the ARM instruction EOR R0, R1, #0xFF and R1 contains 0x0F0F, what will be the value in R0 after execution?

medium

A. 0xF0F0

B. 0x0F00

C. 0xF0F0FF

D. 0x0FF0

Solution

1. Step 1: Understand EOR operation

   EOR performs bitwise exclusive OR between R1 and immediate 0xFF (which is 255 decimal).

2. Step 2: Calculate bitwise EOR

   R1 = 0x0F0F (binary 0000111100001111), immediate 0xFF affects only lower 8 bits (0000000011111111). XOR lower 8 bits: 00001111 XOR 11111111 = 11110000 (0xF0). Upper bits remain 0x0F. So result is 0x0F (upper) and 0xF0 (lower) combined = 0xF0F0.

3. Final Answer:

   0xF0F0 -> Option A

4. Quick Check:

   EOR 0x0F0F ^ 0x00FF = 0xF0F0 [OK]

Hint: EOR toggles bits; XOR lower byte 0x0F with 0xFF = 0xF0 [OK]

Common Mistakes:

• Applying immediate to full 16 bits
• Confusing OR with XOR
• Ignoring bitwise operation effects

4. Identify the error in this ARM assembly snippet intended to clear bit 3 of R2:
BIC R2, R2, #3

medium

A. BIC cannot use immediate values

B. Syntax error: missing comma after R2

C. Immediate value #3 clears bits 0 and 1, not bit 3

D. Should use ORR instead of BIC

Solution

1. Step 1: Understand BIC with immediate

   BIC clears bits set in the immediate value. #3 in binary is 00000011, which clears bits 0 and 1.

2. Step 2: Check which bit to clear

   Bit 3 corresponds to binary 00001000 (decimal 8). Using #3 clears wrong bits.

3. Final Answer:

   Immediate value #3 clears bits 0 and 1, not bit 3 -> Option C

4. Quick Check:

   Clear bit 3 needs #8, not #3 [OK]

Hint: Immediate #3 clears bits 0 and 1, use #8 for bit 3 [OK]

Common Mistakes:

• Using wrong immediate value for bit position
• Thinking BIC can't use immediate
• Confusing ORR and BIC roles

5. You want to toggle bits 0 and 2 of register R3 without affecting other bits. Which ARM instruction and operand should you use?

hard

A. AND R3, R3, #0x05

B. ORR R3, R3, #0x05

C. BIC R3, R3, #0x05

D. EOR R3, R3, #0x05

Solution

1. Step 1: Identify bits to toggle

   Bits 0 and 2 correspond to binary 00000101 (hex 0x05).

2. Step 2: Choose instruction for toggling bits

   EOR toggles bits where mask bits are 1. ORR sets bits, BIC clears bits, AND masks bits.

3. Step 3: Apply EOR with mask 0x05

   EOR R3, R3, #0x05 flips bits 0 and 2, leaving others unchanged.

4. Final Answer:

   EOR R3, R3, #0x05 -> Option D

5. Quick Check:

   Toggle bits = EOR with mask [OK]

Hint: Toggle bits with EOR and mask of bits to flip [OK]

Common Mistakes:

• Using ORR or AND instead of EOR for toggling
• Wrong mask value for bits 0 and 2
• Confusing clearing with toggling