I know nothing about RISC-V, but my usual way of finding out how to do something standard with a given instruction set is to ask a compiler. There are online compilers, such as this one: https://cx.rv8.io/ Just type in "__int128 add(__int128 a, __int128 b) { return a + b; }" on the left, stick "-O2" into the "Compiler options", and see what you get! I don't know whether that particular compiler is doing the right thing, of course, but it gives you an idea and it's definitely less work than reading the spec.