The first video game I ever owned was Pokemon Yellow for the Gameboy Colour. Mine was a special gameboy that had pokemon on the cover. Although I do not still own it outside of a VBA/VBC emulator, I have managed to exploit a lot of the glitches that was in the original game. Some examples include the Mew glitch, and Missingno.
Most- if not all - cheat codes can be easily found online. Websites such as gamefaqs provide a page that gives a player access to the cheat codes. I have found that cheat codes come in numerous different categories:
Key Combination
This type of cheat is common to a lot of console games. This type of cheat requires you to punch in a certain order of keys on the game controller in order for the cheat to be activated. This cheat, I find, is becoming more common. It is also used on handheld consoles too such as the Nintendo DS. An example of it would be Lego Star Wars where on the beginning menu you can punch in cheat codes in order to increase your money or unlock stuff.
Glitches
I think this is probably one of the first RPG to be for a handheld console. Therefore those games were tempermental and was full of glitches and crashed a lot. I should know- I played all three in the Pokémon Blue/Red series. Numerous websites are devoted to these glitches, it is not even funny! I think people still play those games for the sole purpose of exploiting glitches!! Can't say I blame them because they do save people the time of getting 99 of a certain item or going to a Nintendo thing to catch Mew.
Missingno is the Glitch Pokémon that appears after viewing the Old Man's tutorial in Viridian City on how to catch Pokémon, then immediately Flying to Cinnabar Island and Surfing along the right hand side of the island, near the Gym, until one is encountered.
When you talk to the old man in Viridian City, the game needs to change the player's name to "OLD MAN" so that it will display the "OLD MAN" sprite instead of the original character. Because of this, the programmers needed a place to store the player's information and name while the "OLD MAN" data replaced it, so they decided to store it in the area where wild Pokémon information is. This was done because of the lack of memory on an original Game Boy. This normally wouldn't cause any glitches because the correct data for wild Pokémon available is written to this area in memory whenever the player travels to an area where it is possible to catch wild Pokémon.
Cinnabar Island's coast has no data for indicating which wild Pokémon are available because no pokemon can be caught in the general area. The same is true for both Viridian City and Cinnabar Island itself. The game uses whatever data was already in the nearby area, or whatever area the player was in last, when determining which wild Pokémon are encountered and what their levels are. This is seen when exiting the Safari Zone and flying directly to a place with no available pokemon memory such as Cinnibar Island. When this is done, Safari Zone Pokemon can be encountered along the right side of the island because Cinnibar Island is using the Safari Zone's available Pokémon data.
Normally this memory space ,found on the side of Cinnibar Island, would hold the data of the last area visited where wild Pokémon were catchable and their level data. But since the player's last visited area had no Pokémon and instead the player's temporarily stored name (Viridian City), this leaves the game placing in the temporarily stored data in the available Pokémon data's place. This is what causes Missingno's. availability.
The name of the player has much to do with the level and appearance of MissingNo.. As stated earlier, MissingNo. appears due to a lack of Pokemon data in Cinnibar and the use of the player's info (which has been stored in the pokemon info data in Viridian City) inserted into this empty space. The type of Pokémon or MissingNo. the player encounters is determined by the characters used in the 3rd, 5th, and 7th characters of the player's name. The level of the Pokémon or MissingNo. is based on the 2nd, 4th, and 6th characters.
Game Modifications
A positive thing about Game Modifications (Mods) are that they areone of the most sneakiest and evil ways of cheating. This is because you can do whatever you want with the XML files. For example, in Empire at War I prefer playing the Empire as my faction. So, if I want to modify the game I would make it in such a way that the Empire would win such as making my troops immortal, make the intervals between their special abilitites shorter and stuff. Like, I was able t give myself two Darth Vaders and two special AT-ATs in the game from modifiying the XML files. I am posting a picture as evidence.
Gamesharks can allow you to do anything on a game console. They are available for all kinds of games and can either make a video game harder or easier with the entering of a code. Other than Pokémon, I am not familiar with the codes for other games. Although, since I am starting to broaden my hand-held horizon beyond Pokémon, I am sure I'll find a few!

