EVM -Storage

Storage Layout

1.

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contract X  {
    uint256 public a = 1;  // Slot 0
    uint128 public b = 2;  // Slot 1 (partially filled)
    uint128 public c = 3;  // Slot 1 (same as 'b', packed)
    bool public d = true;  // Slot 2 (partially filled)
    address public e = msg.sender; // Slot 3

    struct Data {
        uint256 x;
        uint256 y;
    }
    Data public data = Data(10, 20); // Slot 4 & 5 (separate storage)

    mapping(address => uint256) public balances; // Uses keccak256(address + slot)

    string public text = "Hello"; // Stored separately, reference stored in slot 6
}

Every Slot has: 256 bits (32 Bytes)

Slot	Data
Slot 0	a (32 bytes)
Slot 1	b and c (16+16 bytes)
Slot 2	d (1 byte) (31 waste)
Slot 3	e
Slot 4	x in struct
Slot 5	y in struct

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2.

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contract x {
    struct User {
        uint128 id;   // 16 bytes
        bool active;  // 1 byte
        uint8 level;  // 1 byte
        uint256 score; // 32 bytes (New slot)
    }
}

Slot	Data
Slot 0	id (uint128) + active (bool) + level (uint8)
Slot 1	score (uint256)

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3.

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contract {
    mapping(address => uint256) public balances;  // Slot 0

    function setBalance(address user, uint256 amount) public {
        balances[user] = amount;
    }
}

Storage slot = keccak256(abi.encode(0xABC, 0))

Slot	Data
Slot 0
Slot n

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Gas Optimization

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1.

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Unsafe:

uint128 a; // Slot 0
uint256 b; // Slot 1
uint128 c; // Slot 2

Safe:

uint128 a;
uint128 c;
uint256 b; // This ensures `a` and `c` are packed into one slot.

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Rule For Storage

1. Every storage slot can store up to 32 bytes of data.
2. If a variable is ≤ 32 bytes, it fits into a single slot.
3. If a variable is > 32 bytes, it gets its own slot and additional storage.