CallData Fundamentals

[4 bytes: selector] + [ABI-encoded parameters]

const calldata = CallData.encode("transfer(address,uint256)", [
  Address("0x70997970C51812dc3A010C7d01b50e0d17dc79C8"),
  TokenBalance.fromUnits("1", 18)
]);

console.log(CallData.toHex(calldata));
// 0xa9059cbb                                                          // selector
//   00000000000000000000000070997970c51812dc3a010c7d01b50e0d17dc79c8  // address (32 bytes)
//   0000000000000000000000000000000000000000000000000de0b6b3a7640000  // uint256 (32 bytes)

const signature = "transfer(address,uint256)";
const hash = Keccak256.hashString(signature);
const selector = hash.slice(0, 4);
// 0xa9059cbb

Transaction {
  to: 0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48  // Contract address
  data: 0xa9059cbb...                             // CallData
}

Contract Bytecode (deployed at address)
  ↓
Execution starts at beginning of bytecode

CALLDATALOAD 0        // Load first 32 bytes
PUSH1 0xE0
SHR                   // Shift right to get first 4 bytes (selector)

DUP1
PUSH4 0xa9059cbb     // transfer(address,uint256)
EQ
PUSH2 0x0234         // Jump destination for transfer function
JUMPI

DUP1
PUSH4 0x70a08231     // balanceOf(address)
EQ
PUSH2 0x0456
JUMPI

REVERT               // Unknown function selector

PUSH1 0x04           // Offset 4 (after selector)
CALLDATALOAD         // Load bytes 4-36 → address
// Validate address...

PUSH1 0x24           // Offset 36 (0x24)
CALLDATALOAD         // Load bytes 36-68 → amount
// Execute transfer logic...

CALLDATALOAD offset → data

// Solidity: msg.data[4:36]
PUSH1 0x04
CALLDATALOAD  // Loads bytes 4-36

CALLDATASIZE → size

CALLDATASIZE  // Returns length of calldata

CALLDATACOPY destOffset, offset, length

PUSH1 0x20     // length: 32 bytes
PUSH1 0x04     // offset: start at byte 4
PUSH1 0x00     // destOffset: memory position 0
CALLDATACOPY   // Copy calldata[4:36] to memory[0:32]

// More expensive (non-zero padding)
0xa9059cbb0000000000000000000000001234567890123456789012345678901234567890

// Cheaper (many zero bytes)
0xa9059cbb0000000000000000000000000000000000000000000000000000000000000001

const tx = Transaction({
  to: Address("0x..."),
  value: Wei(1000000000000000000),
  data: CallData("0x"),  // Empty
});

const calldata = CallData.concat(
  contractBytecode,
  CallData.encode("(address,uint256)", [owner, initialSupply])
);