I read your comment to imply that the ISA is the only thing that matters since that's what defines the software interface and that software is the only thing th

I read your comment to imply that the ISA is the only thing that matters since that's what defines the software interface and that software is the only thing that matters. If that is what you're saying, I think you're wrong. As to the low level hand coded assembly. There's a fair amount of stuff in the Intel architecture that's not officially published. There are tools (from Intel) like VTune and the Intel C compilers that use non-published interfaces to optimize C or assembly, sometimes significantly. It is true you can write software without them, but many people purchase these Intel tools to make sure their systems run quickly and efficiently. I can't remember if the SiFive U-X4 core equivalents were defined in the open RISC-V specs or as part of SiFive specific interfaces. If the former, then your point is even stronger. If the latter, your point is still valid, though modulo the impact of having relied on a SiFive specific interface.