From Sonic Retro
Sonic Crackers (referred to as Sonic Studium in the ROM header and often identified incorrectly as Sonic the Hedgehog 4) is an early prototype build of the game Knuckles' Chaotix, incorporating many of the elements that would appear in the final game. Its name comes from a peculiar, and clearly quickly made, ASCII-art title screen. Sonic Crackers is thought to be an engine test, composed of various different game engines, which are swapped out using RAM coding.
The game features Sonic and Tails, who, in the main demonstration of its link to Knuckles' Chaotix, are held together by rings that they carry and that are joined by an elastic force. Also, Sonic can pick up Tails and throw him upwards, to make it easier to reach higher platforms. The stages and music are also similar to those in Chaotix. There are no sound effects at all during gameplay. There are no badniks to be found, however there are spikes, and you lose rings when hitting them (even though you have none).
Interestingly, the date on the lower right of the title screen reads "19940401". The 0401, meaning April 1st, lead to some debate on if the title screen, or the ROM was a April Fools joke. With the discovery of the Chaotix protos (especially of prototype 1207), this is clearly not the case.
There are four playable levels in Sonic Crackers, which are accessible either by pressing start on "1P START" on the title screen, or by using the "SELECT" menu. They are listed below:
These are standard sidescrolling levels similar to those seen in previous Sonic games. By pressing the pause button and pressing a direction key, you can move Sonic in the corresponding direction to anywhere in the level. While moving, Sonic and Tails' arms and Tails' tails become detached and stay at the original location until the game is resumed. When dying by Time Over (or, in the case of the first level, reaching the top of the screen) you enter a field stage.
Starting level and World 2 levels
This is an industrial themed level which can be completed by reaching the top, whereupon the Game Over sound occurs and a field level is entered. This is the only level with a obstacle which causes harm to the player (spikes) and makes this the most complete level in the game. The palette and music changes each time the level is entered, which indicates that time of day was already planned by the time of this prototype. There are four themes for this level, which are all prototype versions of Chaotix songs (Walkin', Hyper-Hyper, Evening Star and New Moon). Game over activates when the timer reaches 3 minutes, or when the top of the level is reached.
World 1 level
This stage is a circus/carnival themed level which can't be beaten; rather than having a level ending it just loops. The collision detection is glitchy and makes the floor difficult to walk on when entering the normal way. The best way to play it is by entering it using the level select as this will make the ground solid. Tails seems to be able to walk on the ground without any trouble, though. The characters' arms and Tails' tails will glitch sometimes when they are on slopes. The music that plays in this level is a prototype version of Electoria. Game over activates after 1 minute. You cannot die from falling into a pit, you are simply stuck there unless you debug your way out.
Worlds 3-7 and Premium 1-2
No work has been done on these levels yet. If you try to enter them, the title card will appear and music (carried over from World 2) will play for a few seconds, but immediately afterwards you will get a game over and get sent to a field level.
The incomplete overhead field stages, commonly referred to as Special Stages, even though they are not listed as such: the "Special Stage" selection on the level select menu causes the ROM to crash. These field levels were never properly completed and don't have any collision detection. Holding a button will cause Tails to remain in place, as in the Attraction Levels; by doing this, Sonic can move any distance away from Tails. Music that plays in these levels is a prototype version of "Electoria" from Knuckles' Chaotix. To exit these stages, the player can simply pause and hit any button. The game will then proceed to an attraction stage.
The prototype ROM will crash the system on a real Mega Drive/Genesis machine, but only if the player does not skip the SEGA screen quick enough or delays at the title screen for too long, the cause of this crash is due to the sound driver (which is held on and controlled by the Z80 sub-processor) when it isn't given a music track ID to process quick enough after it is setup.
A detailed explanation of the crash and how to fix it, can be found here
When bypassing the crash stated above and obtaining access to the other selections in the menu, the game can crash in the level select option Named "SELECT". When selecting the "PREMIUM1 - ATTRACTION" selection, the game crashes with a plain blue screen, the same happens when selecting "PREMIUM1 - FIELD", which will lead to the world 2 field stage first, and will happen upon pausing and pressing A.