You want the principles of operations manual (that's the ISA reference) if it's seriously troublesome stuff, but usually there is no need for anything like that

You want the principles of operations manual (that's the ISA reference) if it's seriously troublesome stuff, but usually there is no need for anything like that. Oftentimes issues are things like the system being Big Endian. If it were z/OS and not Linux you'd need to deal with a different object format and some weird calling convention stuff (even if sticking with z/OS Unix, it's not a SysV calling convention, and it's not ELF, but it is POSIX and SuS), and also EBCDIC interop possibly, so slightly more difficult, but still doesn't usually require doing any assembly. A lot of common nix software will compile and run just fine with no changes.