via http://pdroms.de/

Chord is a monophonic synth program for the original Nintendo Game Boy. The idea was to make something that can be played like an instrument, but with the limitations of the few buttons available. No extra accessories. No midi keyboards.

You build “chords” of five notes and play them back using the pulsewave channel, with full sound register settings control and optional arpeggiator.

http://www.humbletune.com/chord/