Sonic the Hedgehog 2 (Sega Master System, Game Gear)

This page details one or more prototype versions of Sonic the Hedgehog 2 (Sega Master System, Game Gear).

The Sonic the Hedgehog 2 AutoDemo is a Game Gear EPROM cartridge demonstrating the then-upcoming game, sent to several magazines worldwide for preview purposes. On July 14, 2014, Orengefox from Sonic Retro obtained a copy, and the ROM was released to the public that August.

As the name may suggest, the AutoDemo is not playable and simply plays itself (a tactic Sega would later use to great effect for Sonic Adventure): it starts with a simplified introduction sequence and title screen (much like the final game) before continuing with Sonic running through an early version of Green Hills Zone (complete with loop), gliding through a largely empty Sky High Zone, and briefly riding one of Under Ground Zone's mine carts. Following this, a static "Coming Soon" screen appears and the demo loops from the beginning.

With a special Zone/Act editor and a modified ROM, though, there are more levels than the demo shows!

Much is different from the retail release of Sonic 2, most notably the introduction and title screens and the Green Hills Zone tileset. Tails is also completely missing.

Many early screenshots of the game shown during development are thought to originate from this demo, the primary giveaway being that as gameplay is always the same, the ring and time counts at specific points never change. Furthermore, as the HUD resets at the start of each of the three sections, Sonic is seen using a hang glider mere seconds into play - in the final game, there are no opportunities to use one this early.

Disabling the AutoDemo#

To disable the runtime of the AutoDemo, at offset 0x57E replace ​​0B 78 with 00 00. To play the AutoDemo, at offset 0x705 replace ​​AF 32 3C with C3 09 19.

To select a level, at offset 0x550 replace 00 with the desired value ​​below:

• 00 - Green Hills Zone Act 1
• 01 - Green Hills Zone Act 2
• 02 - Green Hills Zone Act 3
• 03 - Sky High Zone Act 1
• 04 - Sky High Zone Act 2
• 05 - Sky High Zone Act 3
• 06 - Removed Zone Act 1
• 07 - Removed Zone Act 2
• 08 - Removed Zone Act 3
• 09 - Under Ground Zone Act 1
• 0A - Under Ground Zone Act 2
• 0B - Under Ground Zone Act 3 (empty)
• 0C - Intro
• 0D and 0E - Title Screen

General Differences#

Sega Screen#

Proto	Final

• The Sega screen is the same as other Game Gear games, with no chant.

Intro#

Proto	Final (Master System)	Final (Game Gear)

• The opening cutscene merely shows Sonic running.
• The palette used in the sky is very reminiscent of the one seen in the Master System version.

Title Screen#

Proto	Final

• The title screen only features Sonic, who pops out of the ring, but doesn't wag his finger(s) like in the final.
• The logo and Sega copyright are in different positions than in the final version.
• The palette is quite different, having much more variety than the final version.
• It does not have TM, much less the "PRESS START BUTTON" for obvious reasons.

Others#

• Badniks, bosses, end-of-Act panels, Chaos Emeralds, and monitors are all entirely absent.
• Only Green Hills, Sky High, and Under Ground have artwork at this point. All other Zones are either not present at this point, or don't have any associated tiles yet.
• The Zone order is noticeably different from the final: Green Hills and Sky High are the first two, followed by three Acts with no data (trying to load any of these results in a blank screen). Under Ground is played next. Aqua Lake, Gimmick Mountain, Scrambled Egg, and Crystal Egg are not present in this build.
• Sonic has the idling animation from the Master System version of Sonic 1.
• A debug camera can be enabled: at offset 0x0064, replace 00 with 02. Press 1, after which you'll be able to move the camera with the D-Pad.

Audio#

• A single music track plays throughout the demo. There's no track like it in the final game.
• The introductory cutscene and the title screen are completely silent.
• The "rolling down a hill" sound is slightly different, with a cartoonish pitch shift in the end.

• The sound Sonic makes when falling does not play in the prototype. A similar sound however does exist, unused.
• The sound of jumping on a spring is the same as the "rolling down a hill" sound.

Level Differences#

Green Hills Zone#

• The art is completely different, being more similar to the credits sequence of the final version.
• Layouts for all three Acts are present, though they only vaguely resemble the final versions.

Sky High Zone#

• Seems to be an earlier version than the one featured in various magazines, and contains no trees or flowers at this point.
• The lightning flashes are different. Both the rain and the lightning flashes have priority, placing them in front of Sonic and the HUD.
• The raining Act has a deep pool of opaque water at the bottom that is not present in the final.

Under Ground Zone#

• Contains many tiles that didn't make the cut, including gray bricks and curvy stone pathways.
• Most of the Zone appears to be made of layered minecart rails. It's likely that the tile definitions were changed to accommodate the minecart rails seen in the demo, and the existing Act layouts were not updated.
• Act 3 uses alternate and shorter 16×16 block data in which neither the background nor the minecart rails are defined. The way tile definitions are laid out matches perfectly with the layout of the two other Acts (minus the lack of a background). However, since Act 3 is empty, this difference can't be normally seen.
• A large stone building and what appear to be non-functional horizontal crushers can be seen at the top-right of Act 1. Neither of these elements made it into the final version.
• The minecart wheel sits above the rail, rather than inside.

Removed Zones#

Although Zones with IDs 06, 07, and 08 do not work properly, it is possible to access them in another way by changing the values ​​of 0x54B from 00 to 02 and changing the value of 0x550 for the Acts.

All of these levels use Green Hills' palette and Sky High's layouts, without the tiles loaded.

Unused Title Card Screen#

Setting ROM address 0x30D to 02 will cause an early version of the Title Card to appear! However, it still works and will take you to the level you were programmed to be in. Although this will no longer make the game play like a demo, for some reason Sonic will still be uncontrollable, resulting in him getting stuck in the level, and the AutoDemo music will not play.

It is also possible to access other levels on the Title Card by changing the RAM values: D287​​ for Zones and D288​​ for Acts.

Zone:

• 00 - Green Hills Zone
• 01 - Sky High Zone
• 02 - Removed Zone
• 03 - Under Ground Zone

Act:

• 00 - Act 1
• 01 - Act 2
• 02 - Act 3

Unused Graphics#

Fonts#

Artwork	Description
	An 8×8 font with an unknown purpose, although it is used in the final Master System version for the Level Select. It's also used in a Sonic Chaos prototype for a placeholder title screen.
	An 8×8 font for the unused title card screen.
	The 16×16 font for the unused title card screen. Only 8 of the 41 characters get used.

Sonic#

Artwork	Description
	Sonic running with a hang glider. While not used at any point in the demo, it is functional and can be accessed at address D502 and changing the value from 01 to 11 in RAM.
	Sonic falling from a hang glider, also not used at any point in the demo.
	Sonic laughing in a rather creepy manner, purpose unknown.
	A sprite of Sonic's waiting animation from the Master System Sonic 1, altered to make him look happy.
	Likely early versions of the skidding sprites, which are actually closer to those of the Genesis version.
	Sonic can't die in the prototype, so this isn't used. It's the same as the early death sprite in the final, taken from the Master System Sonic 1.
	Sonic can't drown in the prototype, so this isn't used. Taken from the Master System Sonic 1.
	Sonic cannot catch an air bubble in this build due to there being no aquatic stages and much less bubbles. Also taken from the Master System Sonic 1.

Others#

Artwork	Description
	No monitors appear in any level, but graphics for them are loaded at the Sega and title screens.
	Switch sprites are located within the Under Ground Zone tileset, but aren't used anywhere.
	Under Ground Zone only has downward-facing spikes at this point, rendering these upward-facing ones unused.
	There's nothing to explode in the prototype, so the explosion tiles that are located within the number/icon sprites are never seen in action.
	Blocks can't be destroyed in the prototype, leaving this fragment unused.
	The trees seen in the final Sky High Zone and in some early screenshots are present in the ROM, but not actually used.

Unused Sounds#

ASCII Text#

Build Date#

While the build date is present at 0x000C0, it is likely incorrect and marks the start of development, rather than this build's date.

MS SONIC.THE.HEDGEHOG.2 Ver0.00 1991/12/05 Aspect Co.,Ltd

Early Title Screen#

At 0x00869, there's a string of text reading out the title of the game. This may have been used for an early title screen similar to that of a prototype of Sonic Chaos. Interestingly, the text is wider than the Game Gear's resolution, so it may have been intended for Master System builds of the game. Note that the copyright symbol in the ROM is Line Feed as there is no symbol, so it has been corrected in the image.

It is possible to reactivate this by modifying the addresses: 0x52E from 22 to 02, however due to the graphics of the new title screen it will make your font graphics look all strange, and after a few seconds the game will restart and crash, curiously the palette of this is also loaded.

Error Handler#

At 0x70, there is a routine for error handling.

It still works in the game and when activating it a message will appear saying "ERROR" lasting 3 seconds and then the game will restart.

Maps#

Green Hills#

Act 1#

Proto
Final

Act 2#

Proto
Final

Act 3#

Proto
Final

Sky High#

Act 1#

Proto
Final

Act 2#

Proto
Final

Act 3#

Proto
Final

Under Ground#

Act 1#

Proto
Final

Act 2#

Proto
Final