simple example of how solidity calculates storage refs for structs

assemblyStorageRef.sol

pragma solidity 0.8.12;

// contains refs to hardhat console.sol and DSTest.sol contract
import '../utils/forge.sol';

contract test is t {

    struct Test {
        uint256 a;
        uint256 b;
        uint256 c;
    }
    
    mapping(uint256 => Test) tmp;

    function test__structStoragePointer() public {
        assembly {
            mstore(0x00, 0x4444)
            mstore(0x20, tmp.slot)

            let ptr := keccak256(0x00, 0x40)

            sstore(add(ptr, 0x00), 0xfff0)

            sstore(add(ptr, 0x01), 0xfff1)

            sstore(add(ptr, 0x02), 0xfff2)
        }

        console.log(tmp[0x4444].a);

        console.log(tmp[0x4444].b);

        console.log(tmp[0x4444].c);
    }
}