Gameboy Color Boot ROM Dumped After 10 Years

An anonymous reader writes "Costis was able to dump the elusive boot ROM from the Gameboy Color by using various voltage and clock glitching tricks. The boot ROM is what initializes the Gameboy hardware, displays the 'GAMEBOY' logo and animation, and makes the trademarked 'cling!' sound effect. Even decapping the CPU had failed previously, but now the boot image and specifics on how it was dumped (along with many photos) are available for download."

Comments

Cool

[dintech]

I really love reading about the lengths enthusiasts go to when trying to do this kind of thing. For some reason I had assumed that this had been done already since there is already emulation for gameboy color, right? Can someone explain the significance of this development?

Re:Cool

[Xeon3D]

I really love reading about the lengths enthusiasts go to when trying to do this kind of thing. For some reason I had assumed that this had been done already since there is already emulation for gameboy color, right? Can someone explain the significance of this development?

The gameboy bios was also "emulated" before, so this makes the emulation more "realistic". It happens the same with the GBA. While you can emulate games for the GBA without the need for a BIOS file, if you have one, they'll run better \ more accurately (or in some cases, they run instead of not running).

Re:Cool

[noidentity]

This allows Game Boy Color emulators to display an authentic intro before running the game, including the palette selection available when running a non-color game. There's otherwise no benefit that I can see. This includes initial register values, since those could already be determined via software. Some of the other initial state, like sound registers set by the boot ROM, is more difficult to determine, so this helped there.

When reverse-engineering hardware, it's nice to figure out every detail, and this was one of the much harder ones to figure out. Decapping usually reveals all, but even that failed here.

Re:Cool

[byuu]

It happens the same with the GBA. While you can emulate games for the GBA without the need for a BIOS file, if you have one, they'll run better \ more accurately (or in some cases, they run instead of not running).

It really just displays the logo and validates that the Nintendo copyrighted startup logo is present in the ROM. It was a trick to try and prevent third-party publishers from making their own releases. As soon as the Game Boy cartridge is started, the boot ROM is locked out completely and no longer needed. The GBA BIOS is quite different, it has tons of functions like various decompression routines, and without it you tend to have far less accurate timing and you may miss potential edge cases. Think of it as low-level emulation with the BIOS, and high-level emulation (in the vain of N64 graphics emulation) without it.

Re:What the ROM does

[Anonymous Coward]

Truly a powerful piece of technology if it's resenting CPUs.

Skynet's not far away now guys!

Re:What the ROM does

[BadAnalogyGuy]

Well, how do you think you would feel if you were dumped after 10 years?

Very short summary of how ROM was dumped

[noidentity]

Here's my summary of how he did it, since the linked blog posting is quite long:

When the Game Boy Color powers up, a small internal boot ROM is enabled inside the CPU. This displays the logo, verifies that the game ROM is "genuine", then starts executing it. Just before it starts executing user code, it disables the boot ROM by writing to an I/O register. Once disabled, there is no way to re-enable it, thus user code can't easily read the ROM.

Costis found that if he stopped the CPU clock for a few seconds, then restarted it, many of the CPU registers (including the program counter) would take on random values. So he placed NOP instructions in all external memory, along with a small dump routine, then stopped and restarted the clock just before the boot ROM wrote to the I/O location to disable itself. This caused the program counter to take on a value outside the boot ROM, and execute all the NOPs until it hit his small dump routine.

I smell double standards

[netpixie]

"Copyright 2009. Costis Sideris."

So copyright law is good enough for you, but not for Nintendo?

Re:I smell double standards

[Mr Z]

I assume you refer to the United States. The US was actually late to the party. The Berne Convention got the ridiculous-copyright-term ball rolling... Disney just gave it an extra push. In particular:

The Berne Convention states that all works except photographic and cinematographic shall be copyrighted for at least 50 years after the author's death

The Berne Convention is also what gives us the rule that daid303 stated, that you don't need to add a copyright notice to get copyright:

Under the Convention, copyrights for creative works are automatically in force upon their creation without being asserted or declared. An author need not "register" or "apply for" a copyright in countries adhering to the Convention. As soon as a work is "fixed", that is, written or recorded on some physical medium, its author is automatically entitled to all copyrights in the work and to any derivative works, unless and until the author explicitly disclaims them or until the copyright expires. Foreign authors are given the same rights and privileges to copyrighted material as domestic authors in any country that signed the Convention.

The US didn't sign on to Berne until 1988. The EU's been on board for awhile, as have many, many other countries. So, yes, you're technically correct that there are some people that are unaffected by the US's copyright protections (or in the case of Nintendo's IP, Japan's). But, a great many places have similar restrictions.

Re:Who even remembers the GBC? Who cares?

[LoonyMike]

This is a perfect example of useless, wasted effort.

I agree, you should have posted on Digg instead.

Re:Why can you not just read the rom??

[noidentity]

Why can't you just take the rom chip out of the gameboy, put it in a socket on a computer and just read the rom 1 byte at a time?

Because the boot ROM is built into the custom CPU. The data bus to this ROM isn't exposed on any of the pins; when enabled, it bypasses whatever is being sent to the external data bus pins on the CPU, so that its contents are never seen by the outside world.

A close comparison is the L1 cache inside a modern CPU. When the CPU is reading from it, you can't know what is in it, since the data isn't output to the bus.