Flute is one of a family of free and open-source RISC-V CPUs from Bluespec, Inc.

 

Bluespec offers RISC-V cores for Xilinx specific FPGA’s, portable cores for other FPGA brands (Altera, Lattice etc) as well as highly domain specific cores.

 

Flute’s microarchitecture is a 5-stage, in-order pipeline. The source code is highly parameterized to produce hundreds of variants, ranging from a small, bare-metal RV32 CPU to a medium-size, Linux-capable RV64 CPUs with multiple privilege levels.

 

You can download the Flute core here