Comment from SiFive:
SiFive is committed to supporting the open-source community. We are pleased to report that after discussions with our IP partners, we are now able to make available all the source code required to initialize the HiFive Unleashed board. The board’s boot sequence is described in the manual. The assembly code in the initial reset ROM is listed in the manual Chapter 6.1 “Reset Vector”. The firmware in the ZSBL mask ROM is directly readable by software on the chip, and we will be making the full source code available shortly. The source code for FSBL including the DDR initialization will also be available shortly. We can attest there is no other firmware run by the system during boot.