This was a feature of the GameShark for Nintendo 64 as well for finding new codes beyond what they published. For example, if you had 100 rounds of ammo left,

This was a feature of the GameShark for Nintendo 64 as well for finding new codes beyond what they published. For example, if you had 100 rounds of ammo left, you would press a button on the cartridge, and have it search the ram for the value 100. Drop back into the game, fire off a shot, and hit the button again to search for which of those values changed to 99. Repeat until you found target area in memory and make a code for it. The device also had a parallel port interface to talk to it via a pc, but I remember less about using that when I was a kid. Pretty basic technique, but as a kid interested in technology, it's stuck with me to this day!