nanode it's a security measure in the h/w.

The logo is stored both on the cart and in the gameboy. At startup it is first copied from the cart to the screen,  scrolled down and then re read from the cart to compare byte for byte with the original nintendo logo.

The trick is to swap the custom logo and the original logo at the correct time.

So wait, is there no onboard system memory that you can modify to change it? And if it's purely on the cart's end, then how can it take up that much extra memory instead of just overwriting the memory of the boot-up logo?

5 wires, no additional ROM space required. Just modified the logo on an existing ROM in bank 3 on my cart.

Couldn't find my AtTiny's so used a 328p on a nano board. It'll work with any small micro with 3 I/O pins available.

code needs to be modified for GBC mode, ideally you would clock the micro from the PHI line which fixes that issue

https://youtu.be/1ywalk_TJTE

Sure, I guess an AtTiny would be the best choice for its size and cost. I'll try find a stash I have here put something together. it should only be 4 wires needed to do the ROM swap, and you'll loose 32k of ROM to the custom Startup Logo.

Do you plan on posting any info on how to do it in the future? I actually used your guide the other day to make my own design, but the actually application of it is whats getting me held up.

also, you cheeky bastard hahah.

Not without dedicated hardware in the cart to swap in the Nintendo logo during checksum time. Or else it wont get past the chime.

It can be done on my BennVenn cart with the help of any small microcontroller (avr/pic/tiny etc...)

I know it's possible to run in BGB, but has anyone successfully done it on the actual hardware?