SCAIE-V: an open-source SCAlable interface for ISA extensions for RISC-V processors

Custom instructions extending a base ISA are often used to increase performance. However, only few cores provide open interfaces for integrating such ISA Extensions (ISAX). In addition, the degree to which a core's capabilities are exposed for extension varies wildly between interfaces. Thus, even when using open-source cores, the lack of standardized ISAX interfaces typically causes high engineering effort when implementing or porting ISAXes. We present SCAIE-V, a highly portable and feature-rich ISAX interface that supports custom control flow, decoupled execution, multi-cycle-instructions, and memory transactions. The cost of the interface itself scales with the complexity of the ISAXes actually used.