Ethereum Virtual Machine (EVM) in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

EVM Stack Behavior on PUSH and ADD

What is the final value on top of the EVM stack after executing these instructions?

PUSH1 0x03
PUSH1 0x05
ADD

Blockchain / Solidity

PUSH1 0x03
PUSH1 0x05
ADD

A 0x08

B 0x15

C 0x02

D 0x53

💡 Hint

Remember that ADD pops two values and pushes their sum.

PUSH1 0x03 pushes 3, PUSH1 0x05 pushes 5, ADD pops 5 and 3 and pushes 8 (0x08).

🧠 Conceptual

intermediate

Gas Cost for SSTORE Operation

Which of the following best describes the gas cost behavior of the SSTORE operation in the EVM?

A SSTORE always costs a fixed 20,000 gas regardless of the storage state.

B SSTORE costs more gas when changing a storage slot from zero to non-zero than from non-zero to zero.

C SSTORE costs less gas when changing a storage slot from zero to non-zero than from non-zero to zero.

D SSTORE gas cost is always refunded fully after execution.

💡 Hint

Think about how storage writes affect blockchain state and refunds.

Writing a zero storage slot to a non-zero value costs 20,000 gas, while clearing a non-zero slot to zero costs less and triggers a gas refund.

🔧 Debug

advanced

Identify the Error in EVM Bytecode Sequence

Given this EVM bytecode sequence, what error will occur during execution?

PUSH1 0x01
ADD

Blockchain / Solidity

PUSH1 0x01
ADD

A No error, outputs 0x01

B Out of gas error

C Invalid opcode error

D Stack underflow error

💡 Hint

ADD requires two values on the stack to operate.

ADD pops two values but only one is on the stack, causing stack underflow.

📝 Syntax

advanced

Correct EVM Assembly for Conditional Jump

Which EVM assembly code correctly performs a conditional jump to label 'loop' if the top stack value is non-zero?

A

PUSH1 loop
JUMPI

B

JUMPI
PUSH1 loop

C

PUSH1 loop
JUMP

D

PUSH1 0x00
JUMPI

💡 Hint

JUMPI expects the destination and condition on the stack.

PUSH1 loop pushes the destination, then JUMPI uses the top stack value as condition to jump.

🚀 Application

expert

Calculate Final Gas After Execution

A contract execution starts with 100,000 gas. It performs:
- 3 SSTORE operations changing zero to non-zero
- 2 SSTORE operations changing non-zero to zero
- 10 ADD operations

Given:
- SSTORE zero->non-zero costs 20,000 gas each
- SSTORE non-zero->zero costs 5,000 gas each and refunds 15,000 gas each
- ADD costs 3 gas each

What is the final gas left after execution (ignoring other costs and refunds)?

A 50,000 gas

B 40,000 gas

C 55,000 gas

D 35,000 gas

💡 Hint

Calculate total cost, then subtract refunds from gas used.

Total cost: (3*20,000) + (2*5,000) + (10*3) = 60,000 + 10,000 + 30 = 70,030 gas spent. Refund: 2*15,000 = 30,000 gas. Net gas used = 70,030 - 30,000 = 40,030. Gas left = 100,000 - 40,030 = 59,970 (rounded to closest option 55,000).

Practice

1. What is the main role of the Ethereum Virtual Machine (EVM) in the Ethereum network?

easy

A. It executes smart contract bytecode to run decentralized applications.

B. It stores user account balances and transaction history.

C. It mines new Ethereum blocks using proof of work.

D. It manages the Ethereum wallet user interface.

Solution

1. Step 1: Understand EVM's purpose

   The EVM is a virtual computer that runs smart contracts on Ethereum.

2. Step 2: Identify correct role

   It executes compiled bytecode from smart contracts, enabling decentralized apps to run safely and consistently.

3. Final Answer:

   It executes smart contract bytecode to run decentralized applications. -> Option A

4. Quick Check:

   EVM runs smart contracts = B [OK]

Hint: EVM runs code, not stores or mines [OK]

Common Mistakes:

• Confusing EVM with blockchain storage
• Thinking EVM mines blocks
• Mixing EVM with wallet software

2. Which of the following is the correct way to represent a simple EVM instruction to add two numbers?

easy

A. ADD 5 3

B. PUSH1 5 PUSH1 3 ADD

C. LOAD 5 3 ADD

D. SUM 5 3

Solution

1. Step 1: Recall EVM instruction format

   EVM uses stack operations like PUSH1 to push values, then ADD to add top stack items.

2. Step 2: Identify correct instruction sequence

   "PUSH1 5 PUSH1 3 ADD" pushes 5 and 3, then adds them on the stack.

3. Final Answer:

   PUSH1 5 PUSH1 3 ADD -> Option B

4. Quick Check:

   Push values then add = C [OK]

Hint: EVM uses PUSH and then operation instructions [OK]

Common Mistakes:

• Using invalid instructions like ADD 5 3 directly
• Confusing LOAD with PUSH
• Using non-EVM commands like SUM

3. Given this EVM bytecode snippet:

PUSH1 0x02
PUSH1 0x03
ADD

What is the result on the stack after execution?

medium

A. 0x05

B. 0x06

C. 0x23

D. 0x00

Solution

1. Step 1: Understand PUSH1 and ADD operations

   PUSH1 0x02 pushes 2, PUSH1 0x03 pushes 3, ADD adds top two stack values.

2. Step 2: Calculate addition result

   2 + 3 = 5, so 0x05 remains on the stack.

3. Final Answer:

   0x05 -> Option A

4. Quick Check:

   2 + 3 = 5 = 0x05 [OK]

Hint: Add pushed hex values on stack [OK]

Common Mistakes:

• Adding hex as strings instead of numbers
• Confusing 0x02 + 0x03 as 0x06
• Misreading PUSH1 as PUSH2

4. Identify the error in this EVM bytecode sequence:

PUSH1 0x01
ADD

medium

A. No error; code is correct.

B. PUSH1 opcode is invalid.

C. ADD cannot be used after PUSH1.

D. Missing second operand on stack for ADD instruction.

Solution

1. Step 1: Check stack before ADD

   PUSH1 0x01 pushes one value, but ADD needs two values on stack.

2. Step 2: Identify missing operand

   Since only one value is pushed, ADD causes stack underflow error.

3. Final Answer:

   Missing second operand on stack for ADD instruction. -> Option D

4. Quick Check:

   ADD needs two stack values [OK]

Hint: ADD requires two values on stack [OK]

Common Mistakes:

• Assuming ADD works with one operand
• Thinking PUSH1 is invalid
• Believing code is correct as is

5. You want to write an EVM bytecode sequence that calculates (4 + 5) * 3. Which sequence correctly achieves this?

hard

A. PUSH1 0x03 PUSH1 0x04 ADD PUSH1 0x05 MUL

B. PUSH1 0x04 PUSH1 0x05 MUL PUSH1 0x03 ADD

C. PUSH1 0x04 PUSH1 0x05 ADD PUSH1 0x03 MUL

D. PUSH1 0x04 PUSH1 0x03 MUL PUSH1 0x05 ADD

Solution

1. Step 1: Understand operation order

   (4 + 5) * 3 means add 4 and 5 first, then multiply by 3.

2. Step 2: Match EVM instructions to operations

   PUSH1 0x04 PUSH1 0x05 ADD pushes 4 and 5, adds them; then PUSH1 0x03 MUL multiplies result by 3.

3. Step 3: Check other options

   Options B, C, and D change operation order or operands, producing incorrect results (B: 23, C: 35, D: 17).

4. Final Answer:

   PUSH1 0x04 PUSH1 0x05 ADD PUSH1 0x03 MUL -> Option C

5. Quick Check:

   Add then multiply = A [OK]

Hint: Add first, then multiply in EVM code [OK]

Common Mistakes:

• Changing operation order incorrectly
• Confusing MUL and ADD positions
• Ignoring operator precedence