Cheat Mode #1:
During loading, hold down the fire button (or switch to autofire if available) until the game has loaded. "CHEAT MODE #1 ACTIVATED" appears on screen and you have no time limit.

Cheat Mode #2:
During loading, hold down the F1 key until the game has loaded. Go to "Introduction" in the main menu. You should see "Cheat Mode 2" along with the first part of the instructions on how to use it. Part two can be found under "Credits".

Note: Both cheats (Mode #1 and #2) cannot be activated at the same time!