_Follow along with this video:_ --- ### Function Selector Size Check Ok! We've reach a section that may finally be new to us. Not sure what's going to be done next, but we'll find out as we go through the operations! <details> <Summary> Op Codes </summary> bytecode - 0x6080604052348015600e575f80fd5b5060a58061001b5f395ff3fe6080604052348015600e575f80fd5b50600436106030575f3560e01c8063cdfead2e146034578063e026c017146045575b5f80fd5b6043603f3660046059565b5f55565b005b5f5460405190815260200160405180910390f35b5f602082840312156068575f80fd5b503591905056fea2646970667358fe1220fe01fe6c40d0ed98f16c7769ffde7109d5fe9f9dfefe31769a77032ceb92497a64736f6c63430008140033 ```js PUSH1 0x80 ✅ PUSH1 0x40 ✅ MSTORE ✅ CALLVALUE ✅ DUP1 ✅ ISZERO ✅ PUSH1 0x0e ✅ JUMPI ✅ PUSH0 ✅ DUP1 ✅ REVERT ✅ JUMPDEST ✅ POP ✅ PUSH1 0xa5 ✅ DUP1 ✅ PUSH2 0x001b ✅ PUSH0 ✅ CODECOPY ✅ PUSH0 ✅ RETURN ✅ INVALID ✅ PUSH1 0x80 ✅ PUSH1 0x40 ✅ MSTORE ✅ CALLVALUE ✅ DUP1 ✅ ISZERO ✅ PUSH1 0x0e ✅ JUMPI ✅ PUSH0 ✅ DUP1 ✅ REVERT ✅ JUMPDEST //<---- We are Here! POP PUSH1 0x04 CALLDATASIZE LT PUSH1 0x30 JUMPI PUSH0 CALLDATALOAD PUSH1 0xe0 SHR DUP1 PUSH4 0xcdfead2e EQ PUSH1 0x34 JUMPI DUP1 PUSH4 0xe026c017 EQ PUSH1 0x45 JUMPI JUMPDEST PUSH0 DUP1 REVERT JUMPDEST PUSH1 0x43 PUSH1 0x3f CALLDATASIZE PUSH1 0x04 PUSH1 0x59 JUMP JUMPDEST PUSH0 SSTORE JUMP JUMPDEST STOP JUMPDEST PUSH0 SLOAD PUSH1 0x40 MLOAD SWAP1 DUP2 MSTORE PUSH1 0x20 ADD PUSH1 0x40 MLOAD DUP1 SWAP2 SUB SWAP1 RETURN JUMPDEST PUSH0 PUSH1 0x20 DUP3 DUP5 SUB SLT ISZERO PUSH1 0x68 JUMPI PUSH0 DUP1 REVERT JUMPDEST POP CALLDATALOAD SWAP2 SWAP1 POP JUMP INVALID LOG2 PUSH5 0x6970667358 INVALID SLT KECCAK256 INVALID ADD INVALID PUSH13 0x40d0ed98f16c7769ffde7109d5 INVALID SWAP16 SWAP14 INVALID INVALID BALANCE PUSH23 0x9a77032ceb92497a64736f6c63430008140033 ``` </details> ```js JUMPDEST // [msg.value] POP // [] PUSH1 0x04 // [0x04] CALLDATASIZE // [calldata_size, 0x04] LT // ``` Continuing from our `JUMPDEST` (this is if `msg.value == 0`), we are then clearing our stack with `POP`. Next we push `0x04` to our stack with `PUSH1`, we'll see why soon. Then we execute an opcode we haven't seen before. `CALLDATASIZE`.  We can see that this opcode takes no stack input, but the stack output is the `byte size of the calldata`. A couple examples: If calldata = 0x04 - CALLDATASIZE = 0x01 if calldata = 0x05284a06 - CALLDATASIZE = 0x04 We then hit another new opcode `LT` this stands for `less than`.  The LT opcode will return 1 if the top item of the stack is less than the second from top item in the stack. Functionally this is checking the the calldata being received is long enough to satisfy the required length of a contracts function selector (4 bytes). ```js JUMPDEST // [msg.value] POP // [] PUSH1 0x04 // [0x04] CALLDATASIZE // [calldata_size, 0x04] LT // [calldata_size < 0x04] PUSH1 0x30 // [0x30, calldata_size <0x04] JUMPI // [] PUSH0 CALLDATALOAD ``` After our stack has the `LT` conditional, we're then pushing the program counter (effectively our `JUMPDEST`) and executing the `JUMPI` code. This is going to jump our execution to after our `function dispatcher` (more on that coming!): ```js PUSH1 0x30 // [0x30, calldata_size < 0x04] JUMPI // [] // if calldata_size < 0x04, Jump to 0x30 PUSH0 CALLDATALOAD PUSH1 0xe0 SHR DUP1 PUSH4 0xcdfead2e EQ PUSH1 0x34 JUMPI DUP1 PUSH4 0xe026c017 EQ PUSH1 0x45 JUMPI // this JUMPDEST is 0x30 JUMPDEST // [] PUSH0 // [0x00] DUP1 // [0x00, 0x00] REVERT // [] ``` The summarize all this, we're checking our `CALLDATASIZE`, if it is less than the size of a `function selector` we are jumping our execution to the `0x30` offset and reverting the transaction. The Solidity compiler again is smart enough to know if a contract possesses a `fallback` function and how to handle `calldata` that can't be processed by a contract's functions by reverting the call when the `calldata` passed is less then 4 bytes long. In the next lesson we'll see how `calldata` is handled when it passes this `CALLDATASIZE` check!

bytecode - 0x6080604052348015600e575f80fd5b5060a58061001b5f395ff3fe6080604052348015600e575f80fd5b50600436106030575f3560e01c8063cdfead2e146034578063e026c017146045575b5f80fd5b6043603f3660046059565b5f55565b005b5f5460405190815260200160405180910390f35b5f602082840312156068575f80fd5b503591905056fea2646970667358fe1220fe01fe6c40d0ed98f16c7769ffde7109d5fe9f9dfefe31769a77032ceb92497a64736f6c63430008140033