Sonic the Hedgehog

Sonic the Hedgehog is the first Sonic game and the first Genesis game with Sonic. It was released in 1991 and introduced the new Sega mascot.

International version
There is 2 codes, the first one is Up, C, Down, C, Left, C, Right, C and the other one is '''C, C, Up, Down, Left, Right. '''The main requirement for the code is pressing C twice before the final input of Right. After entering the code, press Start while holding A and continue holding A until Sonic is in the level. From here, you have access to a variety of things:
 * Invincibility
 * Infinite Time
 * Debug Coordinates (in place of score)
 * Sprite Counter (in place of time)
 * Placement Mode: Press B during gameplay. From there, press A to move forward one object, hold A and press C to move backward one object and press C to place the object.
 * Pause Options: Press A to go back to the title screen, press B to enable slow motion and press C to enable frame advance.

Japanese version
When running on a Japanese console, there are separate codes for the pause options and the rest of the debug features. As usual, they are entered on the title screen and can be combined by interpolating a D-Pad direction between them.

Pause Options: Debug Mode:
 * C, C, Up, Down, Down, Down, Left, Right
 * C, C, C, C, Up, Down, Down, Down, Left, Right, hold A and press Start.

Level Select
Press Up, Down, Left, Right on the title screen (three Downs if playing on a Japanese console), then hold A and press Start. From REV00, Labyrinth Zone was supposed to be the 2nd zone, Marble Zone was supposed to be the 3rd zone, Star Light Zone was supposed to be the 4th zone and Spring Yard Zone was supposed to be the 5th zone.

Green Hill Zone
TBA

Marble Zone
TBA

Spring Yard Zone
TBA

Labyrinth Zone
TBA

Star Light Zone
TBA

Final Zone
TBA

Others
TBA

Special Stage
TBA

Misc. unused stuff
TBA

Warping effect
This warp effect activates when Sonic runs into the area in which the object is placed. Sonic disappears as he runs into it, and a warping noise plays. On the ROM hack Sonic 1 Beta Remake, this teleports Sonic to an early special stage. This is similar to the warping effect on Sonic CD.

Hidden credits
Hidden credits on the Sonic Team Presents screen, but unseen on every region outside of Japan. If the game is running on a Japanese console or switch the region to Japanese, the code C, C, C, C, C, C, Up, Down, Down, Down, Left, Right, which make a ring noise, then when the demo starts, hold Down plus A plus B plus C. The screen will turn white showing the credits. This does not work on Sonic Classics.

Misplaced badniks and objects
In Green Hill Zone Act 1, there is a misplaced Motobug near the first group of spikes. An misplaced Crabmeat on Green Hill Zone Act 2. Near the boss fight on Act 3 of Green Hill Zone there is a misplaced rock, but if Sonic stands on the rock for a few seconds he dies. There is also a Buzz Bomber that is impossible to reach. In Labyrinth Zone Act 1, there is a misplaced group of spikes, but it is impossible to see them without Debug Mode.

Marble Zone
TBA

Labyrinth Zone
TBA

Star Light Zone
TBA

Scrap Brain Zone
TBA

Unused 2X2 tiles
TBA

Exception Handler
The game contains a very basic error handler. In the event a 68000 exception occurs (e.g. a bug causes the game to crash), the game will stop and print some basic information to the screen. This can be triggered in the first two acts of Labyrinth Zone. If the camera is panned down at the moment the results screen is about to appear at the end of the first two Acts, the game may crash with an illegal instruction exception.

Revisional Differences
TBA

Sonic Jam
This version of the game is also based on REV01, and is not simply an emulated rom - it uses rebuilt code, which allows the toggle-able features.
 * A toggle-able Spin Dash move present in the other Sonic games. Turning on the Spin Dash makes spike damage happen like later Sonic games, rather than ignoring the invulnerability after taking damage.
 * Easy and Normal modes, which reduce the difficulty of the game by different measures.
 * Skid marks appear when changing direction while running, much like in the other Sonic games.
 * Unlike most of the other re-releases, the version of the game included uses CD audio (like Sonic CD), allowing for enhanced sound.

Sonic Mega Collection
Sonic Mega Collection (and Sonic Mega Collection Plus) include both the REV00 and REV01 versions, as well as a new revision, REVXB, which changes the spike damage behavior.

3D Sonic the Hedgehog
Like Sonic Jam, this version of Sonic 1 uses rebuilt code for extra options. These options include:
 * Stereoscopic 3D graphics that take advantage of the 3DS's hardware.
 * REV00 (labeled as "International") and REV01 (labeled as "Japanese") are both selectable.
 * Customizable controls.
 * A CRT-style filter.
 * The Spin Dash.
 * The Stage Select being unlocked by default.

Green Hill Zone
TBA

Labyrinth Zone
TBA

Star Light Zone
TBA

Scrap Brain Zone/Clock Work Zone
TBA

Special Stage
TBA

Sonic 1 CD
Sonic 1 was supposed to be for the Sega CD, but it didn't, and so does Sonic 2 CD.

Currently missing/under construction

 * Prerelease pictures of Green Hill Zone, Labyrinth Zone, Star Light Zone, Scrap Brain Zone and Special Stages
 * Revisional differences
 * Unused 2X2 tiles
 * Unused level chunks
 * Other unused graphics
 * Zone-specific unused graphics
 * Unused sound