Twin Cobra

Level select:
Press Up, Down, Right, Left, Start after the helicopter lands at the title screen.
The phrase "Stage Select" will appear to confirm correct code entry.

Full weapon power:
Pause game play and press Up, Down, Right, Left, A + Start.
This will also bring any weapons collected to full power.
Note: This code must be re-enabled with each new life.

Eight bombs:
Pause game play and press Up, Down, Right, Left, B + Start.
Note: This code must be re-enabled with each new life.

View ending sequence:
Press Start to display the green opening screen and press Up, Down, Right, Left, A, B, C, Start.

Fourteen continues:
Press Start to display the green opening screen and tap A to increase the continues.
The maximum amount allowed is 14.