Bit manipulation in ARM in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to perform a bitwise AND operation between registers R0 and R1, storing the result in R2.

ARM Architecture

AND [1], R0, R1

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

AR0

BR2

CR1

DR3

Attempts:

3 left

💡 Hint

Common Mistakes

Using one of the operand registers as the destination, which overwrites original data.

The AND instruction syntax is AND destination, operand1, operand2. Here, R2 is the destination register to store the result.

2fill in blank

medium

Complete the code to shift the bits in register R4 left by 3 positions, storing the result in R5.

ARM Architecture

LSL [1], R4, #3

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

AR5

BR4

CR3

DR6

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same register as source and destination, which overwrites the original value.

The LSL (Logical Shift Left) instruction syntax is LSL destination, source, #shift_amount. Here, R5 stores the shifted result.

3fill in blank

hard

Fix the error in the code to clear bit 5 of register R7 using bitwise operations.

ARM Architecture

BIC [1], R7, #[2]

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

A0x10

BR8

C0x20

DR7

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong mask value that does not correspond to bit 5.

Using a different destination register that does not update R7.

The BIC instruction clears bits set in the mask. To clear bit 5, use mask 0x20 (which is 1 shifted left 5 times). The destination should be R7 to update the register.

4fill in blank

hard

Fill both blanks to set bits 0 and 3 in register R9 without affecting other bits.

ARM Architecture

ORR [1], R9, #[2]

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

AR9

B0x09

C0x0F

DR10

Attempts:

3 left

💡 Hint

Common Mistakes

Using a mask that sets extra bits.

Using a different destination register that does not update R9.

The ORR instruction sets bits specified by the mask. Bits 0 and 3 correspond to 0x09 (binary 00001001). The destination register is R9 to update it.

5fill in blank

hard

Fill all three blanks to create a mask that extracts bits 4 to 7 from register R11 and stores the result shifted right by 4 in R12.

ARM Architecture

AND [1], R11, #[2]
LSR [3], [1], #4

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

AR12

B0xF0

DR11

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong mask that includes bits outside 4 to 7.

Using different registers for intermediate and final results causing errors.

First, AND R12 with mask 0xF0 to keep bits 4-7. Then, logical shift right (LSR) R12 by 4 to move those bits to the lower positions. The destination for both instructions is R12.

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