Storage Instructions

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Overview

• Contract state maintained across transactions
• Account-specific storage per contract instance
• Complex gas metering with refunds (EIP-2200, EIP-2929, EIP-3529)
• Cold/warm access tracking for gas optimization

• Transaction-scoped temporary storage (Cancun, EIP-1153)
• Cleared at end of transaction, not persisted
• Fixed 100 gas cost (no refunds, no access tracking)
• Common for reentrancy guards and local state

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Instructions

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Storage Model

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Persistent Storage

contract MyContract {
  uint256 public count;        // Storage slot 0
  mapping(address => uint) balances;  // Slot 1+ (hash-based)
}

• Committed to blockchain state
• Persisted across transactions and blocks
• Accessible to all transactions and external callers
• Refundable when clearing slots (EIP-3529)

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Transient Storage

// During transaction execution
TSTORE(key, value)   // Write to transient storage
TLOAD(key)           // Read from transient storage
// Transaction ends
// Transient storage cleared (not persisted)

• Reentrancy protection (guard flags)
• Call context passing (inter-contract communication)
• Temporary work variables
• Avoiding expensive storage refunds

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Gas Costs

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SLOAD (Persistent Read)

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SSTORE (Persistent Write)

• Sentry: SSTORE requires >= 2300 gas remaining
• Gas varies by current vs original value
• Refunds only 4800 per cleared slot (EIP-3529 reduced from 15000)

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TLOAD/TSTORE (Transient)

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Common Patterns

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Persistent Storage Patterns

// Simple counter
uint256 public count;

function increment() external {
  count++;  // SLOAD, ADD, SSTORE
}

// Multiple reads from same slot - use local var
function expensive() external {
  uint256 value = state[key];  // Cold SLOAD (2100)
  for (let i = 0; i < 10; i++) {
    // ... use value ...
    // Next reads are warm (100) if cached
  }
}

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Transient Storage Patterns

contract ReentrancyGuard {
  // Check pattern using transient storage
  function _nonReentrant() internal {
    uint256 locked = 1;
    assembly {
      // tstore(key, locked)
      // Before call: check tload(key) == 0
      // After call: tstore(key, 0)
    }
  }
}

// Pass data between contract calls without storage
TSTORE(contextKey, contextValue)  // Store in caller
CALL(...)                          // Callee can TLOAD(contextKey)
// Context available only during transaction

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See Also

• SLOAD (0x54) - Persistent storage read
• SSTORE (0x55) - Persistent storage write
• TLOAD (0x5c) - Transient storage read
• TSTORE (0x5d) - Transient storage write
• EIP-2929: Gas-cost increases for state access opcodes
• EIP-2200: Structured Definitions for Net Gas Metering
• EIP-3529: Reduction in refunds
• EIP-1153: Transient storage opcodes