Atari 2600 version:
Press Down, A(26), Down at the title screen.

All weapons:
Press A, B, Up, C, A, C, A at the title screen.

Nine lives:
Press Right, A, Down, B, Right, A, B, Up, Down at the title screen.

Extra lives and weapons:
Press Left, A, Down, B, Right, A, B, Up, Down, A, B, Up, C, A, C, A at the title screen.

Unlimited continues:
Press C(4), Left, A, Down, Up, Down at the title screen.

Level select:
Press B, Right, A, Down, Right, Up, B, Left, A, Up, Right, A at the title screen, then press Up or Down to select a starting level.

Speed mode:
Press B, A, Right, C, Right, Up, Down at the title screen.

Lever sequence practice:
Press B, A, Down, C, Right, A, B at the title screen.

View credits:
Press C, Right, Down, C, Right, Down, C, Right, Down at the title screen.