Understanding EVM: an introduction to Ethereum OPCODES

Table of Content

The Ethereum Virtual Machine or EVM

The Ethereum Virtual Machine is the software platform that developers can use to create decentralized applications (DApps) on Ethereum. At any given block in the chain, Ethereum has one and only one ‘canonical’ state, and the EVM is what defines the rules for computing a new valid state from block to block.

The EVM design simplified

The EVM behaves as a mathematical function would: Given an input, it produces a deterministic output. It therefore is quite helpful to more formally describe Ethereum as having a state transition function:

$$ Y(S, T)= S’ $$

Given an old valid state $(S)$ and a new set of valid transactions $(T)$, the Ethereum state transition function $Y(S, T)$ produces a new valid output state $S'$

State

In the context of Ethereum, the state is an enormous data structure called a modified Merkle Patricia Trie, which keeps all accounts linked by hashes and reducible to a single root hash stored on the blockchain. Transactions

Transactions

Transactions are cryptographically signed instructions from accounts. There are two types of transactions: those which result in message calls and those which result in contract creation.

Contract creation results in the creation of a new contract account containing compiled smart contract bytecode. Whenever another account makes a message call to that contract, it executes its bytecode.

EVM instructions

The EVM executes as a stack machine with a depth of 1024 items. Each item is a 256-bit word, which was chosen for the ease of use with 256-bit cryptography (such as Keccak-256 hashes or secp256k1 signatures).

During execution, the EVM maintains a transient memory (as a word-addressed byte array), which does not persist between transactions.

Contracts, however, do contain a Merkle Patricia storage trie (as a word-addressable word array), associated with the account in question and part of the global state.

Compiled smart contract bytecode executes as a number of EVM opcodes, which perform standard stack operations like XOR, AND, ADD, SUB, etc. The EVM also implements a number of blockchain-specific stack operations, such as ADDRESS, BALANCE, BLOCKHASH, etc.

So, we could define the EVM as a set of rules that allow us to write content in the blockchain according to conditions of the smart contracts.

It is important to note that, All implementations of the EVM must adhere to the specification described in the Ethereum Yellowpaper.

Ethereum OPCODE list

Remember that: The size of a “word” in EVM is 256 bits.

Some additional notes

• Both suicide() and selfdestruct() Solidity calls are represented internally by opcode 0xff as SELFDESTRUCT.
• Push opcodes are the only way to add new content to the stack.

References

• https://ethereum.github.io/yellowpaper/paper.pdf
• https://github.com/crytic/evm-opcodes
• https://www.ethervm.io

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