_Follow along with this video:_ --- ### updateHorseNumber Recap <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 ✅ 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> Take a moment above to appreciate how much low level knowledge you've gleaned from just this one small Solidity smart contract! You should be very proud of yourself! We just went through the entire execution of the `updateHorseNumber` function, So let's recap what we've learnt. ### Function Dispatch The first thing our contract does is perform a `function dispatch` to determine where our `call data` should be appropriately sent. ```js DUP1 ✅ PUSH4 0xcdfead2e ✅ EQ ✅ PUSH1 0x34 ✅ JUMPI ✅ DUP1 ✅ PUSH4 0xe026c017 ✅ EQ ✅ PUSH1 0x45 ✅ JUMPI ✅ JUMPDEST ✅ PUSH0 ✅ DUP1 ✅ REVERT ✅ ``` If call data doesn't include a function selector which matches the transaction will revert. ### Program Counters From here, Solidity is clever enough to prepare our stack with a number of program counters we'll need to jump through as we execute our function call. It also prepares us for our CALLDATASIZE check! ```js JUMPDEST ✅ PUSH1 0x43 ✅ PUSH1 0x3f ✅ CALLDATASIZE ✅ PUSH1 0x04 ✅ PUSH1 0x59 ✅ JUMP ✅ ``` ### CALLDATASIZE Check Next our execution is going to check our `CALLDATASIZE`. We want to be sure that the data received, when the `function selector` is removed, is still large enough to satisfy the requirements of our function parameter - in this case we need a 32 byte integer or our `CALLDATASIZE - func_selector` to be > `0x20` ```js JUMPDEST ✅ PUSH0 ✅ PUSH1 0x20 ✅ DUP3 ✅ DUP5 ✅ SUB ✅ SLT ✅ ISZERO ✅ PUSH1 0x68 ✅ JUMPI ✅ ``` ### Isolating Passed Data If the `call data` passes this size check, we next jump and isolate the parameter from the `call data` by loading 32 bytes of `call data` starting at `0x04`: ```js JUMPDEST ✅ POP ✅ CALLDATALOAD ✅ SWAP2 ✅ SWAP1 ✅ POP ✅ JUMP ✅ ``` ### Add Data to Storage Lastly, we take this value and finally store it in storage slot 0 and then stop execution: ```js JUMPDEST ✅ PUSH0 ✅ SSTORE ✅ JUMP ✅ JUMPDEST ✅ STOP ✅ ``` That's it! The transaction is done! It's easy to see at first glance (if you remember back to our Huff implementation), that we wrote _much less_ Huff code. It's clear why this is the case, now, isn't it? We performed many fewer checks and we didn't have to jump around! If we were focused on hyper gas optimized code, we may want to remove some of these checks that Solidity performs - but this is a double edged sword. These checks are often in place for a reason and to protect against potential vulnerabilities and bugs! There's more for us to do still! One function down, let's keep going.