A Pi emulating a GameBoy sounds cheap. What about an FPGA?
I am trying to implement the PPU according to the "Pixel Pipeline" model as in the Ultimate GameBoy Talk. Currently BG/Window rendering works fine.
When a GameBoy is powered on, the very first code gets executed is the boot ROM. It will scroll down the Nintendo logo, make a chime, and jump to the cartridge.
So that would be also my start point to test the whole system. Fortunately the boot ROM binary is available only so I do not need to dump it from my GameBoy (it would be very difficult to do so). I also got a copy of disassembled source code.
Unfortunately the source code I got will not pass the assembler, but it is easy to modify to make it work. While I was reading the code, I also added some comments along the way.
In order to test my boot ROM in the emulator I also added a header and some initialization code to make it like a Game ROM which could run in the emulator. Since these things are outside of the Boot ROM area so they would not affect the final output.
Full code available on GitHub: https://github.com/zephray/GameBoy/blob/master/Tools/bootstrap/bootstrap.s