MiSTer FPGA Forum

Interesting.
There is a page in smspower with all the known bioses : https://www.smspower.org/Development/BIOSes

Some change could be made to Mister to add a bios load parameter, instead of loading the mboot.mif file by default, but that's some work because for the moment it's statically loaded.

Honestly I do not fully understand your question. As far as i know, there is no such thing as a 'custom bios', and I don't really see any reason to make or have one. This bios just initialize registers and memory to make games work. I am not even sure that the original machine has a bios. What could happen is that an exotic game does not work becau...

It's embedded. The compile process generates the file mboot.mif, which is loaded during the core generation by Quartus into a memory array.
It's not the original BIOS, it has been rewritten.

Hi,
There is a bios-replacement file in SMSBootLoader/smsboot.asm which initialize some ram and some registers if I remember correctly.
It does different things if SMS or GG is detected/selected.
Control is passed to the rom bios at reset.

If BIOS is disabled, this just doesn't happen. Some exotic games need this I guess.
hope this helps.