_Follow along with this video:_ --- ### Emitting Logs With Assembly - Log4 Let's start by taking a look at some of the Assembly being employed in `GasBadNFTMarketplace.sol`. ```js /* * @notice Method for listing NFT * @param nftAddress Address of NFT contract * @param tokenId Token ID of NFT * @param price sale price for each item */ function listItem(address nftAddress, uint256 tokenId, uint256 price) external { // Checks if (price <= 0) { revert NftMarketplace__PriceMustBeAboveZero(); } // Effects (Internal) s_listings[nftAddress][tokenId] = Listing(price, msg.sender); // emit ItemListed(msg.sender, nftAddress, tokenId, price); assembly { // This is the data mstore(0x00, price) log4( 0x00, 0x20, // keccak256("ItemListed(address,address,uint256,uint256)") 0xd547e933094f12a9159076970143ebe73234e64480317844b0dcb36117116de4, caller(), nftAddress, tokenId ) } // Interactions (External) IERC721(nftAddress).safeTransferFrom(msg.sender, address(this), tokenId); } ``` In the listItem function, our Assembly block is being used to emit an event, or emit logs. We can see how Log4 works through our handy reference in [**evm.codes**](https://www.evm.codes/). ::image{src='/formal-verification-3/3-emitting-logs-with-assembly/emitting-logs-with-assembly-log-41.png' style='width: 100%; height: auto;'} This op code will append our log record with 4 topics and takes 6 stack input elements. Let's consider how this is being used in our contract. ```js mstore(0x00, price); // This is storing our price parameter in memory log4( 0x00, // Start at position 0 in memory 0x20, // For as size of 32 bytes (this is the price we just stored in memory) // The first topic is the hash of our function selector: keccak256("ItemListed(address,address,uint256,uint256)") 0xd547e933094f12a9159076970143ebe73234e64480317844b0dcb36117116de4, caller(), // This Assembly acquires our msg.sender nftAddress, // nftAddress parameter passed to the function tokenId // tokenId parameter passed to the function ); ``` In an audit scenario we'd absolutely be double checking these parameters. We see this Assembly block repeated throughout GasBadNFTMarketplace.sol in the functions `listItem`, `cancelListing`, `buyItem`, and `updateListing`. In essence, with the exception of the bytes4 in `onERC721Received`, this is all that's being changed in this over our solidity implementation of this protocol. ### Wrap Up Ok, this was just a small walkthrough of the Assembly changes made in the Gas Bad implementation of this protocol. We know this _shouldn't_ have any effect on anything, but fortunately we don't have to hope, we can formally verify. In the next lesson we'll outline a game plan for how we'll apply Certora formal verification to this code base! See you there!