SCAL: An Open-Source Scalable Core Adaptation Layer for Interfacing RISC-V ISA Extensions

Instruction Set Architecture eXtensions (ISAX) are one of the key methods to improve the performance of software-programmable processors. However, adding them into processor cores is cumbersome and becomes even more challenging if the ISAX needs to be portable among multiple different cores. Existing extension interfaces aim to alleviate these difficulties. However, they lack an abstraction layer that encapsulates commonly used adaptation mechanisms for mediating between the base core and the ISAX. Typical adaptation issues include latency matching, shared resource management, or hazard handling. For this purpose, we propose the Scalable Core Adaptation Layer (SCAL), which builds on an existing open-source interface, SCAIE-V. By using SCAL, we show that for ISAXes reading and returning a result, their interface width and semantics remain unchanged independent of their latency, number, or base core. We demonstrate this for four cores with different microarchitectures. Moreover, the ISAX is agnostic of its execution mode (in-pipeline or in parallel to the main pipeline), and its datapath does not change even if the result is written to the memory, program counter, or a new internal state element. This brings us closer to fast ISAX development and integration into multiple cores.