Enter the following codes at the 'Press Start' screen.

Invincibility:

A, Z, Z, Up, Left, C-Left, C-Left, C-Down.

Unlock All Missions and Battle Mode:

A, R, Z, Up, Up, C-Up, C-Down, C-Down.

Infinite Primary Weapon:

A, R, Z, Right, C-Up, C-Right, C-Down, C-Down.

Infinite Secondary Weapon:

B, B, Z, Left, Left, C-Up, C-Left, C-Right.

Infinite Solaris Missiles:

B, L, L, Z, Up, Down, C-Up, C-Up.

Infinite Titan Missiles:

A, B, L, Up, Up, C-Up, C-Up, C-Right.

Infinite Weapon Energy:

L, Z, Left, Right, Down, Down, C-Down, C-Down.

One Shot Kills:

B, B, B, L, R, Left, Down, Down.

Stealth Mode:

Up, Up, Up, Up, Right, Down, C-Left, C-Left

Gore Mode:

Z, Down, C-Up, C-Left, C-Left, C-Left, C-Left, C-Down.

Psychedelic Mode:

A, R-Button, Left, Right, Down, C-Up, C-Left, C-Down.

Turbo Crazy mode:

B, B, R-Button, Up, Left, Down, C-Up, C-Left.

Wire Frame Mode:

L-Button, L-Button, R-Button, Z, Left, Right, C-Up, C-Right.

Freeze Enemies:

R, Z, Right, Right, C-Up, C-Left, C-Right, C-Down

Skip Opening Ads:

To skip the long string of advertisements, simply press the reset button on the N64 on the first screen that shows up (the one with the white text and black background ). You should see it "fly" towards the bottom-right hand side of the screen, then it will reapear. Now press Start at all the screens to skip them.