nesdev.org

Does NES have a BIOS? I tried searching and looks like it does not.

SO it means when we power on NES, nothing happens inside CPU or PPU?
It is only after we insert cartridge, CPU starts executing instructions, setting some memory?

Well, my question is, Is ROM the only source of instructions for CPU?

Pranav

Yes. No BIOS.

Newbie Help Center might be a better area for these questions.

Original NES and Famicom consoles do not have a BIOS (some clones might have, if they have built-in games and the like). ROM in the cartridge is mapped to $8000-$FFFF from the time the console is turned on, and the CPU will immediately look at $FFFC-$FFFD (the reset vector) to find out where the program starts.

The only BIOS in any NES or Super NES game is a pretend BIOS like the one shown at the beginning of Mega Man X. If it were done on NES, it might look like this.

tokumaru wrote:some clones might have [a BIOS]

I think that's part of the point of doctorlai's thread.

The Famicom Disk System (FDS) does have a BIOS though.
It is a Japan-only extension to the Famicom that loads games from 3.5 inch diskettes.

Actually they are 2.8" disks that are basically QuickDisk except with a physical extension of the plastic enclosure making the disk taller to include the NINTENDO logo on the disk. They are not 3.5" disks like those used on IBM PCs.