Boot to FDS BIOS - MiSTer FPGA How-To
This is how you make the NES core boot to the Famicom Disk System bios when loaded on your MiSTer FPGA.
You will need a valid Famicom Disk System bios file, and you will have to rename it to boot0.rom and place it in the "/games/NES/" folder.
You'll also need a blank FDS disk file with only the first 16 bytes used for the FDS header, everything else is 0x0 (any regular FDS ROM can be copied and then edited in a hex editor to achieve this). Next, rename that to boot2.rom and put it in the same folder as boot0.rom was placed.