Inline Assembly

Solidity defines an assembly language that can be used without Solidity. This assembly language can also be used as "inline assembly" inside Solidity source code.

💡 NeoVM Difference

In Ethereum, inline assembly gives you direct access to the EVM stack and opcodes (e.g., sload, sstore, call). On NeoVM, the compiler lowers a limited Yul compatibility subset and warns on anything outside that subset because the opcodes and stack architecture fundamentally differ. Treat assembly blocks as portability scaffolding, not as EVM-opcode-level access.

How Neo DevPack for Solidity Handles Inline Assembly

When the Neo DevPack for Solidity compiler encounters an assembly { ... } block, it parses Yul syntax and lowers a documented compatibility subset to NeoVM IR. This is not EVM opcode parity: unsupported Yul constructs fall back to a compatibility warning and emit no assembly logic for that block.

The supported subset is meant for common library idioms, not arbitrary EVM assembly. It includes:

• Yul locals and assignments (let x := ..., x := ...)
• Reads from Solidity locals and parameters
• mstore, mload, and return over a bounded scratch memory buffer
• tstore and tload transient storage helpers
• returndatasize() and guarded returndatacopy(...)
• Arithmetic, bitwise, shift, and comparison helpers such as add, sub, mul, div, mod, and, or, xor, not, shl, shr, lt, gt, eq, and iszero
• Simple if, for, switch, and nested block forms when every nested expression is also in the supported subset

Unsupported EVM-only operations such as sload, sstore, raw call, and create do not map directly to NeoVM. Rewrite those paths with Solidity, devpack libraries, or explicit Neo native-contract/syscall wrappers.

function loadWord() public pure returns (uint256) {
    assembly {
        mstore(0, 42)
        return(0, 32)
    }
}

NeoVM Alternatives

If your contract requires Neo-specific low-level operations, prefer Neo's native intrinsics and Syscalls directly.

For example, to execute a cross-contract call with dynamic parameters or specific execution flags, use the Syscalls library rather than EVM's call opcode.

import {Syscalls, CallFlags} from "@neo/Syscalls.sol";

function callAnotherContract(address target, string memory method) public {
    // Replaces the need for low-level assembly calls
    Syscalls.contractCall(target, method, CallFlags.All, new any[](0));
}

For advanced mathematics, cryptography, or memory manipulation, the Neo Devpack exposes a comprehensive suite of native libraries designed explicitly for the NeoVM environment.