I looked at SiFive website but could not find it: is the HiFive1 Rev B bootrom source code available? I’m really curious to see how initialization is done.
Also, when I run example programs I see this on the console after reboot:
We’re sorry but the boot code is not public. The console messages you are seeing are expected and they are coming from the ESP32 wifi module. If you check Figure 3 of the HiFive Getting Started Guide you’ll see that the ESP32 has a serial interface that also comes out over USB.
The output you see is printed by bootloader (not bootrom) as a result of communication with the WiFi module. The bootloader is located at the beginning of the flash. If you’re really curious, you can use Ghidra to decompile it.