"The RISC-V architecture also provides for customization in the form of user defined instructions (UDIs), and MIPS said this would be useful in many high-end applications, while also keeping full compatibility with standard RISC-V development tools and software libraries."
And how long until we have a dozen processors, from different companies, all offering their own custom things (look, this one helps with mining!), all of which are totally incompatible with each other?
Biting the hand that feeds IT
