Understanding the Ethereum Virtual Machine (EVM)

The Ethereum Virtual Machine (EVM) is the heart of Ethereum. It's a decentralized computing engine that powers smart contracts and manages blockchain state transitions. This article breaks down what the EVM is and how it works.

What is the EVM?

• Computation Engine: The EVM is Ethereum's computation engine, similar to interpreters in other bytecode-compiled languages like Java.
• Smart Contract Execution: It handles the deployment and execution of smart contracts, which are self-executing programs on the blockchain.
• State Updates: Nearly everything that happens on Ethereum involves a state update computed by the EVM.
• Global Decentralized Computer: Think of the EVM as a global decentralized computer containing millions of executable objects (smart contracts), each with its own permanent data store.

Ethereum as a State Machine

• Transaction-Based State Machine: Ethereum can be described as a transaction-based state machine.
• EVM's Job: The EVM's job is to update Ethereum's state by computing valid state transitions resulting from smart contract execution.

World State

The Ethereum state consists of a mapping of Ethereum addresses (160-bit values) to Account objects. There are two types of accounts:

• Externally Owned Accounts (EOAs)
  • Balance: Holds an amount of Ether (in Wei).
  • Nonce: Counts the number of transactions sent from this address.
  • Storage and Code: EOAs have no storage or associated code.
• Smart Contracts
  • Balance: Holds Ether (in Wei).
  • Nonce: Counts the number of contract creations.
  • Storage: Contains the contract's permanent data store.
  • Program Code: Contains the smart contract's code, which the EVM executes.

EVM Instruction Set - Bytecode Operations

The EVM uses a low-level, stack-based language consisting of various opcodes. Below is a breakdown of the main categories of EVM instructions:

Arithmetic Operations

• ADD - Add the top two stack items.
• MUL - Multiply the top two stack items.
• SUB - Subtract the top two stack items.
• DIV - Integer division.
• SDIV - Signed integer division.
• MOD - Modulo (remainder) operation.
• SMOD - Signed modulo operation.
• ADDMOD - Addition modulo any number.
• MULMOD - Multiplication modulo any number.
• EXP - Exponential operation.
• SIGNEXTEND - Extend the length of a two's complement signed integer.
• SHA3 - Compute the Keccak-256 hash of a block of memory.
• Note: All arithmetic is performed modulo 2^256.

Stack Operations

• POP - Remove the top item from the stack.
• MLOAD - Load a word from memory.
• MSTORE - Save a word to memory.
• MSTORE8 - Save a byte to memory.
• SLOAD - Load a word from storage.
• SSTORE - Save a word to storage.
• MSIZE - Get the size of active memory in bytes.
• PUSHx - Place x byte item on the stack (x = 1 to 32).
• DUPx - Duplicate the x-th stack item (x = 1 to 16).
• SWAPx - Exchange the 1st and (x+1)-th stack items (x = 1 to 16).

Process Flow Operations

• STOP - Halt execution.
• JUMP - Set the program counter to a specific value.
• JUMPI - Conditionally alter the program counter.
• PC - Get the value of the program counter.
• JUMPDEST - Mark a valid destination for jumps.

System Operations

• LOGx - Append a log record with x topics (x = 0 to 4).
• CREATE - Create a new account with associated code.
• CALL - Message-call into another account.
• CALLCODE - Message-call into this account with another account's code.
• RETURN - Halt execution and return output data.
• DELEGATECALL - Message-call into this account with alternative account's code.
• STATICCALL - Static message-call into an account.
• REVERT - Halt execution, revert state changes, return data and remaining gas.
• INVALID - Designated invalid instruction.
• SELFDESTRUCT - Halt execution and register account for deletion.

Logic Operations

• LT - Less-than comparison.
• GT - Greater-than comparison.
• SLT - Signed less-than comparison.
• SGT - Signed greater-than comparison.
• EQ - Equality comparison.
• ISZERO - Simple NOT operator.
• AND - Bitwise AND operation.
• OR - Bitwise OR operation.
• XOR - Bitwise XOR operation.
• NOT - Bitwise NOT operation.
• BYTE - Retrieve a single byte from a full-width word.

Environmental Operations

• GAS - Get the amount of available gas.
• ADDRESS - Get the address of the currently executing account.
• BALANCE - Get the account balance of any given account.
• ORIGIN - Get the address of the EOA that initiated this EVM execution.
• CALLER - Get the address of the caller immediately responsible for this execution.
• CALLVALUE - Get the Ether amount deposited by the caller.
• CALLDATALOAD - Get the input data sent by the caller.
• CALLDATASIZE - Get the size of the input data.
• CALLDATACOPY - Copy the input data to memory.
• CODESIZE - Get the size of code running in the current environment.
• CODECOPY - Copy the code running in the current environment to memory.
• GASPRICE - Get the gas price specified by the originating transaction.
• EXTCODESIZE - Get the size of any account's code.
• EXTCODECOPY - Copy any account's code to memory.
• RETURNDATASIZE - Get the size of the output data from the previous call.
• RETURNDATACOPY - Copy data output from the previous call to memory.

Block Operations

• BLOCKHASH - Get the hash of one of the 256 most recently completed blocks.
• COINBASE - Get the block's beneficiary address.
• TIMESTAMP - Get the block's timestamp.
• NUMBER - Get the block's number.
• DIFFICULTY - Get the block's difficulty.
• GASLIMIT - Get the block's gas limit.