Game Boy boot ROM?

Hiya,

I've heard of the original Game Boy's built-in ROM chip being dumped, but despite many Google searches I've never found it. Anyone have it?

 

Yes, I have it. Can't share it though, that would be illegal.

 

sorry for asking, but what would you do with the GB's rom?

 

The ROM is responsible for initializing the console, verifying the legitimacy of the game and displaying the Nintendo logo; it's necessary for accurate emulation.

 

I know that it could be bypassed somehow though, and might not actually be needed by the game boy. I remember a unlincensed cart (I think some bomberman clone game) and several pirate games that did NOT display the nintendo logo when booting up the gameboy, instead they displayed a third party company logo with the same Nintendo "ding sound" though. I still had one of those carts some time ago.. I remember it refused to boot on my GBA back then.

 

The internal ROM simply checks to see if the game is licensed by comparing a string like "licensed by Nintendo", then displays the Nintendo logo. The ROM isn't needed for emulation because emulators can initialize values to a specific state. With real hardware though, it may have been easier to just let everything initialize to whatever state and not deal with a giant reset circuit.

I'm not sure how pirate games bypass the ROM. They probably use the "licensed by Nintendo" string. I'm not sure how they replace Nintendo with their logo though (I do know what you're talking about) since the Nintendo character data is actually in the internal ROM. It could be that the game includes code for a simulation of Nintendo's logo.

 

The character data for the logo is in the Boot ROM, but only as something to compare to the logo in the cartridge. As you could have guessed when booting a Gameboy with a dirty/uninserted cartridge, the logo that shows up on-screen is taken from the cart. Games that swap in their own logo probably have some sort of switching circuit that has the Nintendo logo available for the comparison, but their own logo available for when the logo patterns are loaded on-screen.

 

Yes, the logo is swapped out once the verification is complete. There was a document describing this awhile back. Here's a quote from GeeBee's GB FAQ (from http://www.devrs.com):

Is it possible to design a cart that displays something other than the Nintendo logo on power up ?

The GB Pocket & GB Classic at power up would display the graphic logo in the ROM and then test to make sure that the logo was accurate. For these units you could do some ROM hardware tricks that would swap the graphic logo ROM contents after the logo was displayed and then the game would boot properly.

The GBC has more sophisticated testing. In order to display your own logo on the GBC you need to alternate your logo and the Nintendo logo as follows: powerup with "Nintendo", "Nintendo" logo present for 19.2ms after reset, Custom logo present for 89.6ms, "Nintendo" logo present for 46.4ms, Custom logo present for 262.7ms, "Nintendo" logo present forever. (Info from Ken Kaarvik.)

 

http://www.geocities.com/kkaarvik/gameboy.html
There is a project using a PIC16F84 to change the boot logo.