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EVM

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Definition

Turing-complete virtual machine that executes smart contracts on the Ethereum blockchain.

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State

  • Ethereum is a distributed state machine;

  • The machine state (or simply "state") is a data structure that keeps a record of all changes made to the blockchain like all accounts balances, all contracts, etc;

  • State is stored in a modified Merkle Patricia Trie (MPT);

  • State can change from block to block.

  • Rules for state changes are defined by the EVM.

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Storage

  • Key-value permanent store that is part of the state;

  • Key-value pairs = "slots";

  • Values = 256 bits words or items;

  • Keys = Keccak256 hashes of their contract's address & the word;

  • If a transaction tries to add an item that is too big, the transaction will be reverted;

  • Data types smaller than 256 bits can be packed into a single slot. For this, developers must order the same data types from the smallest to the biggest:

  • Mappings & dynamic arrays values are stored in different slots but contiguous;

  • Strings have 1 storage slot for the length, then 1 slot for each 32 bytes of the string;

  • A struct is stored in a single slot if it fits, otherwise it is stored in multiple contiguous slots;

  • Smart contracts have no storage size limit.

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Opcode

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STATICCALL

  • Similar to CALL but does not allow state modifications;

  • Introduced in the EIP-214arrow-up-right [3];

  • Used for view & pure functions.

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DELEGATECALL

  • Calls another contract's function using the current contract's context (storage, caller, value);

  • Introduced in the EIP-7arrow-up-right [4];

  • Used in proxy contracts.

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References

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