From Sonic Retro
<forumuser name="Tom41" />
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Tom41 arguably is the person who began the chain-reaction that led to the Sonic ROM hacking scene. In 1998, before the Sonic 2 Beta was made public, Tom released information on how to access Hidden Palace Zone using a hex editor. He also released several other bits of information, including a table of Blue Sphere stages for certain games when locked on to Sonic & Knuckles, on his web site.
Getting his first Mega Drive rather late in the decade, Tom41 quickly became a fan of the Sonic games, and enjoyed using the debug mode to cause glitches. Armed with an Action Replay cartridge, which allowed for cheat-searching and RAM hacks, Tom quickly found a lot of interesting things in the Sonic MD games. He originally discovered the "Let's Go" sign in Sonic 1 and other unused tiles, the infamous Hidden Palace Zone in Sonic 2, and the fact that Flying Battery's badniks were still programmed into the standalone Sonic 3 cart. Sadly, in those days the Internet wasn't really accessible to most people, so he had no way of sharing his discoveries with the rest of the world.
The turnaround started when the Genecyst emulator was released, allowing the Sonic games to be played on a PC for the first time. Genecyst already had out-of-the-box support for ROM patch codes (like those used by the Game Genie), but Tom was looking into some way of getting his RAM hack codes into the emulator. Intrigued by the idea of save states, Tom simply took a RAM dump and a save state at the exact same point in the game, then compared the two files with a hex editor. He found that Genecyst added a 2478H header to each save state, and the rest of the file was identical to the RAM dump. Therefore, in order to convert the Action Replay codes into savestate hacks, you would need to add 2478 to the original code. This had to be done with the calculator in hexadecimal mode, otherwise the result of the calculation would be incorrect. This resulted in Tom managing to convert the 'switch levels' code for Sonic 2, and thus being able to create a save state that would get you into Hidden Palace Zone (08) on an emulator. He posted this information on his website, as well as several other Sonic forums.
Tom remained active in the hacking community for a couple more years, finding other bits of lost levels and posting findings on forums. Later on though, many other people started producing whole ROM hacks and documenting the actual code used in the game. He still posts in the Sonic community from time to time though, discovering glitches and beta leftovers in the newer Sonic games - but can no longer compete with all the other expert Sonic hackers producing game disassemblies and complete ROM rewrites.