The Wikipedia on Tyrian says:
It is possible to switch between the Single Player and (1 person) Arcade modes using a secret routine (not cheat codes). Since it is possible to find exotic weapons early in Arcade Mode (by contrast in Single Player, such weapons can only be bought at the end), using this switch will allow such weapons to be brought over to single player. Likewise, generator and shield upgrades from single player will result in a better ship for Arcade Mode.
What is this secret routine?