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Written by Leon "Hivebrain" White.

ROM Contents:

0		Header, Sonic 2 ROM
100000		Dr. Robotnik's Mean Bean Machine ROM
200000		Sonic 1 ROM
280000		Sonic Compilation ROM
2805C8		ASCII text - "Dr Robotnik's"
2805DE		ASCII text - "Sonic the Hedgehog"
2805F4		ASCII text - "Sonic the Hedgehog 2"
28060A		Code
280616		ASCII text - "Select cursor/game"
280682		Code
280794		ASCII text - region error message
280830		Code
2814CA		Pallete - red, green and blue (only those 3 colours)
2814D0		Palletes (4) - entirely black
281550		Palletes (4) - entirely white
2815D0		Code
282760		Pallete - region error message
282780		Code
282D5C		Pallete - Sega logo
282D7C		Pallete - Sega logo
282D9C		Code
282DBA		Nemesis Compressed Art - Sega logo
283124		Code
2837E0		Nemesis Compressed Art - small yellow text
283CBE		Nemesis Compressed Art - yellow text with shaded background (unused)
284252		Uncompressed Art - letters from Dr Robotnik's MBM high score entry
286652		Pallete - yellow text and blue background
286672		Pallete - Dr Robotnik's image #1
286692		Pallete - Sonic 1 image #1
2866B2		Pallete - Sonic 2 image #1
2866D2		Pallete - unknown
2866F2		Pallete - Dr Robotnik's image #2
286712		Pallete - Sonic 1 image # 2
286732		Pallete - Sonic 2 image # 2
286752		Pallete - Dr Robotnik's image # 3
286772		Pallete - Sonic 1 image # 3
286792		Pallete - Sonic 2 image # 3
2867B2		Mappings - blue background
2869E2		Mappings - "Press Start Button"
286A2A		Mappings - "Game Select"
286A56		Mappings - Dr Robotnik's image
286B46		Mappings - Sonic 1 image
286C36		Mappings - Sonic 2 image
286D26		Nemesis Compressed Art - images and large text from game select screen
287E84		Empty (filled with FFs) to end of ROM

Palletes:

Megadrive colour palletes are usually 16 colours long, each colour is represented by two bytes in the format 0B GR, where B = blue; G = green; and R = red. The B, G and R can be any of the following values:

0 = 0
2 = 32
4 = 64
6 = 96
8 = 128
A = 160
C = 192
E = 224
(and in rare cases, F = 224)

There are a few palletes in the ROM whose purpose are unclear. Such as the group of palletes starting at address 2814CA which don't seem to do anything, and another at address 2866D2 pictured here:

Mappings:

Each image on the game select screen is made up of 8x8 pixel tiles. The way these tiles are arranged is called mappings. Here are all the available tiles from the VRAM:

Each 8x8 tile is given a two-byte value (see column of numbers on the left), so that the word "Press" would be:

0101	0102	0103	0104	0104
010B	010C	010D	010E	010E

The blue background is made up entirely of tile 0100. The tile is also flipped horizontally, vertically or both. To do this with any given tile, add 0800 (flip horizontally), 1000 (flip vertically) or 1800 (flip both ways) to the tile's value. In the case of the blue background, this gives an equal mix of 0100, 0900, 1100 and 1900.

Game Select Pointers:

Pointer address	Address it points to	Game it points to
2801BC	100200	Dr Robotnik's
2801C2	1007B4
2801C8	10054E
2801D4	200200	Sonic 1
2801DA	2010DA
2801E0	200AC4
2801EC	206	Sonic 2
2801F2	F1C
2801F8	3CE

These triplets of pointers determine which game you start playing when you select a game on the game select menu. The pointer triplets are all interchangeable, so you could make it so when you select Sonic 1 on the game select menu, you start playing Sonic 2 instead. To clarify, these are the changes you'd need to make:

At address 2801D4, change 0020 0200 to 0000 0206
At address 2801DA, change 0020 10DA to 0000 0F1C
At address 2801E0, change 0020 0AC4 to 0000 03CE

Also, you can use a similar method to completely skip the game select menu, and go directly to any of the three games as soon as you start playing the ROM. To do this, you need to copy the header from another ROM (i.e. the one you want to play in Sonic Compilation) and paste it over the normal Sonic Compilation header. The new header should contain three pointers, which you need to alter to match the pointers listed in the above table. You only need to copy and paste the first 0x100 bytes of the header.

Unused Shaded Text:

Located at address 283CBE is a piece of compressed art which appears not to be used anywhere in the game. The art is identical to the yellow alphabet used on the region error screen and game select menu, except it has a shaded background:

Normal text	Unused shaded text

By changing a pointer, I made the unused text appear instead of the normal text.