Enable the "Sound test" code. Press I, II then tap Select repeatedly until the debug screen appears. The following debug functions may now be enabled.

Level select
Enter 00 - 09 as a debug number.

Invincibility
Enter 77 as a debug number, then press Up, Down before exiting the debug screen.

Half game with many satellites
Enter 83 as a debug number.

Extra lives, bombs, and power
Enter the debug number for the desired level. Hold I + II and press Start to begin game play with 10 lives and 10 bombs. Additionally, you will continue from the previous location instead of the previous checkpoint after losing a life.