Barrel shifter operations in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to specify the barrel shifter operation for a logical left shift.

ARM Architecture

MOV R0, R1, LSL #[1]

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

A5

B0

C1

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 means no shift, which does not change the value.

Confusing LSL with LSR (Logical Shift Right).

The logical shift left (LSL) shifts bits to the left by the specified number. Here, #3 means shifting left by 3 bits.

2fill in blank

medium

Complete the code to perform a logical right shift using the barrel shifter.

ARM Architecture

MOV R2, R3, LSR #[1]

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

A6

B2

C4

D8

Attempts:

3 left

💡 Hint

Common Mistakes

Using a shift amount larger than the register size.

Confusing LSR with ASR (Arithmetic Shift Right).

LSR means Logical Shift Right. Here, #4 shifts the bits right by 4 positions.

3fill in blank

hard

Fix the error in the barrel shifter operation to perform an arithmetic right shift.

ARM Architecture

MOV R4, R5, [1] #3

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

ALSR

BASR

CLSL

DROR

Attempts:

3 left

💡 Hint

Common Mistakes

Using LSR instead of ASR for signed values.

Using ROR which rotates bits instead of shifting.

ASR stands for Arithmetic Shift Right, which preserves the sign bit during the shift.

4fill in blank

hard

Fill both blanks to perform a rotate right operation by 8 bits.

ARM Architecture

MOV R6, R7, [1] #[2]

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

AROR

BLSL

C8

DASR

Attempts:

3 left

💡 Hint

Common Mistakes

Using LSL or ASR instead of ROR for rotation.

Using incorrect shift amount.

ROR rotates bits to the right. The #8 means rotate by 8 bits.

5fill in blank

hard

Fill all three blanks to perform a logical left shift by 2 bits and store the result in R9.

ARM Architecture

MOV [1], [2], [3] #2

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

AR9

BR8

CLSL

DR7

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up source and destination registers.

Using the wrong shift operation.

The instruction moves the value from R8 shifted left logically by 2 bits into R9.

Practice

1. What is the primary function of a barrel shifter in ARM architecture?

easy

A. To store data temporarily during program execution

B. To perform arithmetic calculations like addition and subtraction

C. To quickly shift or rotate bits left or right in a single operation

D. To manage memory addresses for data storage

Solution

1. Step 1: Understand the role of barrel shifters

   Barrel shifters are hardware components designed to shift or rotate bits efficiently in one operation.

2. Step 2: Differentiate from other components

   Unlike arithmetic units or memory managers, barrel shifters specifically handle bit-level shifts and rotations.

3. Final Answer:

   To quickly shift or rotate bits left or right in a single operation -> Option C

4. Quick Check:

   Barrel shifter = fast bit shifts/rotations [OK]

Hint: Barrel shifters move bits fast left/right or rotate [OK]

Common Mistakes:

• Confusing barrel shifter with arithmetic logic unit
• Thinking barrel shifter manages memory
• Assuming barrel shifter stores data temporarily

2. Which of the following is the correct syntax to apply a logical left shift by 3 bits using a barrel shifter in ARM assembly?

easy

A. ROR R1, R2, #3

B. LSR R1, R2, #3

C. ASR R1, R2, #3

D. LSL R1, R2, #3

Solution

1. Step 1: Identify the shift types

   LSL means Logical Shift Left, LSR is Logical Shift Right, ROR is Rotate Right, ASR is Arithmetic Shift Right.

2. Step 2: Match the operation to the syntax

   To shift left by 3 bits, the correct syntax is LSL R1, R2, #3.

3. Final Answer:

   <code>LSL R1, R2, #3</code> -> Option D

4. Quick Check:

   LSL = Logical Shift Left [OK]

Hint: LSL means shift bits left logically [OK]

Common Mistakes:

• Using LSR instead of LSL for left shift
• Confusing rotate (ROR) with shift
• Using ASR which is arithmetic right shift

3. Given the ARM instruction MOV R0, R1, LSR #2 and R1 contains the binary value 0b11010000, what will be the value in R0 after execution?

medium

A. 0b01101000

B. 0b00110100

C. 0b00001101

D. 0b11010000

Solution

1. Step 1: Understand the instruction and initial value

   The instruction moves R1 shifted right logically by 2 bits into R0. R1 = 0b11010000.

2. Step 2: Perform logical shift right by 2 bits

   Shifting 0b11010000 right by 2 bits gives 0b00110100 (the two rightmost bits are dropped, zeros fill left).

3. Final Answer:

   0b00110100 -> Option B

4. Quick Check:

   Logical shift right 2 bits = 0b00110100 [OK]

Hint: Logical shift right drops bits, fills zeros left [OK]

Common Mistakes:

• Confusing logical shift right with rotate right
• Not filling zeros on left after shift
• Using arithmetic shift instead of logical

4. Identify the error in this ARM assembly snippet using barrel shifter:
MOV R0, R1, LSL #33
Assuming a 32-bit register size.

medium

A. Shift amount exceeds register size; valid range is 0-31

B. LSL cannot be used with MOV instruction

C. R1 cannot be source register in MOV

D. Missing comma between operands

Solution

1. Step 1: Check shift amount validity

   In ARM, shift amounts must be between 0 and 31 for 32-bit registers. Here, #33 is invalid.

2. Step 2: Verify instruction syntax and operands

   MOV supports barrel shifter operations and R1 is a valid source register; commas are correctly placed.

3. Final Answer:

   Shift amount exceeds register size; valid range is 0-31 -> Option A

4. Quick Check:

   Shift amount must be ≤ 31 [OK]

Hint: Shift count must be 0-31 for 32-bit registers [OK]

Common Mistakes:

• Ignoring shift amount limits
• Thinking MOV disallows barrel shifter
• Assuming syntax error due to commas

5. You want to rotate the bits of a 32-bit register R2 right by 8 positions using a barrel shifter in ARM. Which instruction correctly achieves this without losing any bits?

hard

A. ROR R2, R2, #8

B. LSR R2, R2, #8

C. LSL R2, R2, #8

D. ASR R2, R2, #8

Solution

1. Step 1: Understand rotation vs shift

   Rotation moves bits around the ends without loss; shifting moves bits out and fills with zeros or sign bits.

2. Step 2: Identify correct instruction for rotate right

   ROR (Rotate Right) rotates bits right by specified amount without losing bits.

3. Final Answer:

   <code>ROR R2, R2, #8</code> -> Option A

4. Quick Check:

   Rotate right = ROR instruction [OK]

Hint: Use ROR to rotate bits without losing any [OK]

Common Mistakes:

• Using LSR which shifts and loses bits
• Using LSL which shifts left instead of rotate
• Confusing ASR with rotate