SNES Version ONLY
To get all upgrades and armor of such sort, but no bosses defeated:
Enter this for all bosses defeated, and all upgrades:
Both SNES and PC
For this code to work, you'll need to have acquired all of the Heart Tanks, Sub-Tanks, enemy weapons, and power-ups. Then, go to Armored Armadillo's level. Journey through the level until you reach the final mine cart, which launches you through the air at the end of the level. Just before you reach the wall, jump up and climb the wall. At the top, there will be a large Energy Unit. Get it, and then jump into the pit and die. Repeat this step five times. On the fifth time through, a capsule will appear on that same ledge. Dr. Light will then give you a Ryu-style fireball for Mega Man to use. It can only be used at full energy, and is done by pressing down, down-forward, forward, and Y.
PC Version ONLY
For all upgrades and bosses gone, enter "xstuff" (no "") at anytime in gameplay. It also fills up your sub tanks and can be done as many times as wanted. Note: some computers only understand the code if you type in "xstuf" so make sure not to type another "f" on accedent.