Hi, Sifive did great job [1] [2] and everything is now opensource including mask rom loader. "Today we’re finally able to rectify this issue by releasing the FU540-C000’s ZSBL and FSBL as an open source project, which can be found on GitHub like all of SiFive’s other open source projects." And I guess someone could be interested in: "A Challenge Now that the bootloader code has been released it’s time for a little challenge. Since the ZSBL lives in a mask ROM on the FU540-C000 there is no way to replace it, but you can at least re-generate the contents of the ROM and then verify that it matches the contents of a real HiFive Unleashed. We’ve posted a copy of the contents of the mask ROM at https://github.com/sifive/freedom-u540-c000-bootloader, the first person to submit a pull request that can exactly reproduce that ROM will get a HiFive Unleashed board!" Thanks Rudolf [1] https://www.sifive.com/blog/2018/09/06/an-open-source-release-of-the-freedom... [2] https://github.com/sifive/freedom-u540-c000-bootloader