Comment by drob518

Yes, but x86 had a goal of running every old piece of code on the new thing. That’s not true with RISC-V. Whereas x86 accreted more and more features, RISC-V solves this with profiles that are not guaranteed to be compatible with each other. RISC-V doesn’t even support running 32-bit code on 64-bit processors without a recompile. In a sense, 32-bit RISC-V is a vaguely similar but incompatible ISA to 64-bit RISC-V. That would be a train wreck if there weren’t profiles to specify a group of features that must be there (and some others that might be optional, with a register to flag whether they are or not). The expectation is that profiles act a lot more like the x86 ISA, accreting features while preserving backward compatibility. It took me a while to realize this, too, and I remember several WTF episodes while reading through the specs. And of course profiles go beyond the ISA and specify system level arch as well (like the “PC architecture” did for Windows and Linux). You can certainly argue that it should have been done differently, but it’s not completely crazy given that there is not legacy RISC-V code needing to be run on newer systems for the most part. That won’t be true forever, and so profiles help manage that.