15 Ethereum Virtual Machine Concepts
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ACCOUNT TYPES
There are 2 types of accounts:
- External accounts: controlled by public-private key pairs, address determined from the public key.
- Contract accounts: controlled by the code stored with the account, address determined at the creation time.
TRANSACTIONS
They are messages sent from one account to another.
They can include:
- Binary data (payload)
- Ether
If the target account contains code, that code is executed.
Payload is provided as input data.
If the target account is not set, a new contract is created.
GAS
A transaction is charged with GAS.
First, a gas price is set by the creator, at gas_price * gas cost.
If some gas is left after the execution, it's refunded.
If the gas is used up at any point, an exception is triggered, and all the modifications are reverted.
STORAGE
It's a key-value store that maps 256-bit words to 256-bit words.
Each account has persistent storage between function calls & transactions.
It's not possible to enumerate storage from within a contract, it's costly to read, and even more to initialize and modify it.
MEMORY
Contract's space instance for each msg call.
Memory can be addressed at byte level:
- Reads are limited to 256 bits
- Writes can be 8 bits/256 bits
Memory is expanded on access by a 256-bit, and the gas cost must be paid at expansion time (it scales quadratically).
STACK
The EVM is a stack machine: all computations are performed on a data area (the stack).
Max size of 1024 elements with 256-bit words.
Possible actions:
- Copy one of the topmost 16 elements to the top of the stack.
- Swap the topmost element with one of the 16 below.
STACK COMPLEX OPERATIONS
Other ops take the topmost 2 elements from the stack and push the result onto it.
Stack elements can be moved to storage/memory to get deeper access.
It's not possible to just access arbitrary elements in the stack without first removing the top.
INSTRUCTION SET
All instructions operate on 256-bit words, or on memory slices.
Arithmetic, bit, logical, comparison operations are present.
Conditional/unconditional jumps are possible.
Contracts can access relevant properties of the current block (number, timestamp,...).
MESSAGE CALLS
Similarly to transactions, they have:
- Source
- Target
- Data
- Payload
- Ether
- Gas
- Return data
Transaction: top-level msg call that can create further message calls.
Contracts: can call other contracts or send Ether to non-contract accounts with msg calls.
DELEGATECALL
A msg call variant, BUT:
- Target address code executed on calling the contract context.
- msg.sender/msg.value don't change values.
A contract can dynamically load code from a different address at runtime.
Storage, address, balance refer to the calling contract.
LOGS
Data can be stored in a specially indexed data structure that maps all the way up to the block level.
Contracts cannot access log data after creation, but they can be efficiently accessed from outside the blockchain.
This feature is used by Solidity to implement events.
CREATE CALLS
Contracts can create other contracts using a special opcode.
Differences between create calls and normal message calls:
- The payload data is executed.
- The result is stored as code.
- The caller/creator receives the address of the new contract on the stack.
SELF DESTRUCT
Operation on the contract's address to remove code from the blockchain.
ETH is sent to a designated target.
Storage + code are removed from the state.
Removing the contract is dangerous: if someone sends Ether to the removed contracts, the ETH is lost.
DEACTIVATE
If you want to deactivate your contracts, you should instead disable them by changing some internal state which causes all functions to revert.
This makes it impossible to use the contract, as it returns Ether immediately.
PRECOMPILED CONTRACTS
Contract addresses with ranges 1-8 are special.
They can be called like any other contract, but their behavior and gas cost is not defined by EVM code stored at that address but implemented in the EVM execution environment itself.
Sources: Francesco Ciulla's tweet
