Use the following codes at the game screen:
Invincibility: you are sod
Full energy: noble
Extra life: gimmie jimmie
Maximum continues: onandonandon
1000 bullets: popquizhotshot
Maximum plasma gun: justwhatyouseepal
Maximum life and energy: threemileisland
Homing missile: apollo13
Stick figure mode: hatman
Access first five levels: slaugtherhouse
Access first 11 levels: romario
Access all levels: getthecheesetosickbay
Display framerate: framerate
Display development team: iddqd
Displays testing team: idkfa