Someone else already said it but it beats repeating: this is wrong > The competition is very much bound to a rather narrow ISA which is why CUDA is forward and

Someone else already said it but it beats repeating: this is wrong > The competition is very much bound to a rather narrow ISA which is why CUDA is forward and backwards compatible whilst ROCm isn’t. PTX isn't an ISA and it goes through another translation step before it hits the real ISA (SASS), which does change internally. Though you are right that PTX as intermediate is part of the reason for CUDA's success in that it makes the code portable across cards.