Using tiles other than those from ROM

In the ROM, offset $1B664 is your friend. First two bytes are the X coords for Sonic's starting point, the second two are the Y coords.

Although if you only go by that, moving Sonic to a precise spot may be a bit of a problem, but that's why we have savestates. Turn on debug and go to the special stage. Move Sonic to wherever you want his starting point to be, then make a savestate. Now go to $F480 in the savestate to get his X coords, and $F484 to get his Y coords, and punch those into the ROM ($1B664) to change his starting point.

 

The file extension wasn't added obviously.

 

Well, it is the same savestate, just without .gsx as the file extension. Gens doesn't automatically add a file extension to savestates, which is a very stupid idea. Just right click the file and add .gsx, and you'll be fine :P

 

Go to 00F480, and retrieve the X position of Sonic; go to 00F484, and retrieve his Y position in the savestate. Now in the ROM, go to 01B664, and punch in the values you read from the savestate.

 

Hex editor.

By the way, $F480.w is the position in RAM. Check Nemesis' guides for the start location of main RAM, then add that to $F480 using a hex calculator. Then go to THAT location in the savestate.

 

Wrong.

EDIT: Image link now, because of some unexplained transparency.

 

You can do that with SonED and ESE, you know that?

 

I don't think you can change it with ESE. I know you can with SonED though. But I might be wrong. I haven't used ESE in quite a while.