This is my first hack on the original gameboy. I fill the background with a chessboard pattern, then change the vertical and horizontal scroll registers at every scanline during horizontal blanking, to achieve a distortion effect.
I cobbled together a "cartridge" for this test by sticking a 2kb EEPROM chip I had at hand, onto a gameboy cartridge breakout board.
Source code (mercurial repo): http://nuclear.mutantstargoat.com/hg/gb_test2
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https://www.youtube.com/watch?v=NxKRRTW5934