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Sonic the Hedgehog 2 (Genesis)/Revision Differences

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This is a sub-page of Sonic the Hedgehog 2 (Genesis).

Sonic 2 has various regional and revisional differences, sometimes with them being based off earlier revisions. Of the many, the three main ones are Revision 00, Revision 01 and Revision 02, which will just be called REV00, REV01 and REV02 from now on.

Region Switch Code

On the title screen, press Up, Down, Down, Down, Up, and a ring sound will be heard. This switches the game between Japanese and International mode, like if the game was put in a Japanese console. In Japanese mode, references to "Tails" are changed to Miles (his official name in Japan), and the trademark symbols are removed from the SEGA screen and title screen. It can be re-entered to revert it back to International mode. It works the same way on the Japanese hardware (but it's the other way around obviously).

REV00 and REV01

The first two retail releases, being compiled on September 26th and September 29th respectively and with REV01 being the most mass-produced version.

Sonic2CasinoNightSignpostRev00.png Sonic2CasinoNightSignpostRev01.png
  • The signpost in Casino Night Zone is a little too deep into the ground in REV00, resulting in the bottom portion of the goalpost being visible through the floor.


  • The developers forgot to add a ring cap to monitors in REV00, meaning 999+ can be collected, bugging out the ring counter.
Sonic2KalinskeRev00.png Sonic2KalinskeRev01.png
  • Thomas Kalinske (Sega of America's president at the time) was misspelled as "Tohmas Kalinske" in REV00. REV01 fixed his typo, however they forgot to fix Phoenix Rie (Rieko Kodama) being misspelled Phenix Rie.
  • REV00 forgot to set the 'remember state' flag for checkpoints in Wing Fortress, meaning dying will make them all look activated.


  • Sonic and Tails briefly display garbled art after activating a Teleport Monitor in 2-player mode. This seems to be the result of both players using the other players' tilemap but applied to their own graphics. REV01 fixes this by making the players invisible until they start moving again.
  • The yellow spring walls in Metropolis Zone can be bounced on while hurt in REV00, which freezes the character's animation and locks the controls.
  • If the big Casino Night Zone springs are landed on while hurt in REV00, the character enters a glitched state: they don't enter their rolling state, and remain able to walk, but are still treated as if they're attached to the spring.
Sonic2CasinoNightFlippers2PRev00.png Sonic2CasinoNightFlippers2PRev01.png
  • Two flippers in the 2-player version of Casino Night Zone Act 1 changed positions in REV01.
  • The Crawl placement in the 2-player version of Casino Night Zone in REV01 matches Beta 8's placement.
  • "Invisible wall" and yellow spring collision objects are visible in Debug Mode REV00, but not REV01.
  • A slight code change was made to the routine to draw sprites (BuildSprites in the Github disassembly) in REV01 in an attempt to prevent the game from crashing if an object is placed after dying in Debug Mode. Specifically, an ID check instead points to something else instead of code that makes the game read off an odd address (which causes it to crash as the Genesis can't), however a mappings check was also removed meaning the crash still happens.
  • REV01 is overall more tight, removing unnecessary stuff such as Hidden Palace's background positioning routine.

Sonic Mega Collection

The version included in Sonic Mega Collection is the REV01 version (oddly instead of the REV02 version), however it now fixes a bug where the 14 continues cheat (1, 1, 2, 4 on the options menu) causes the Oil Ocean theme to play continually. This is because, despite what the sound test says, the sound IDs actually start at $80 internally, so the 2 player results would be $81 and so on. It's possible that the person programming the cheat mistook the continue jingle's sound test ID ($3F) with its actual ID ($BF), which causes the sound driver to malfunction and be unable to play any new sounds.

This special revision would later go on to be used in 3D Sonic the Hedgehog 2 unaltered, with all the new content in the game being handled by the emulator instead.


A third revision that never saw a standalone release, being used as the base for future versions such as Sonic Classics, Knuckles in Sonic 2, the Mega Play arcade version, Sonic Jam and Sonic the Hedgehog 3 instead. Unlike the previous two revisions, this wasn't found in the Sega QA archives.

This version fixed several major bugs including:

  • A crash that would sometimes happen if Sonic defeated a Rexon in Hill Top Zone.

Believe it or not, I'm walking on air.

  • The ability to transform into Super Sonic at the end of a level, which would get the player stuck in the air (and prevent the level from being completed)


  • The background speed and offset variables not being reset if the player continues from a checkpoint in Wing Fortress Zone. This is only a problem if the player gets a Time Over during the WFZ ending sequence.

Additionally, some under the hood changes were made:

  • Various aspects of the generated machine code differ, such as data being padded (automatic 'dc.b' directive padding), some instructions being optimized ('addi'/'subi' instructions becoming 'addq'/'subq'), and others being unoptimized ('lea ADDRESS(pc)' instructions becoming 'lea (ADDRESS).l'). These differences suggest that the assembler used to build the ROM from the game's source code was changed.
  • Even more unused code was removed including Wood Zone's level layout drawing routine, resulting in it taking awhile to draw segments of the screen.

Sonic Classics

Though it's based on REV02, the Sonic Classics release introduces several bugs:
Sonic2Rev02FlipBug.png Sonic2Rev02FlipBugGrabber.png

  • Grabber's legs and Mecha Sonic's flame and sparks aren't properly flipped if they're facing right.
  • The player cannot Spin Dash off the Tornado in Sky Chase Zone.
  • The vertical conveyor belt platforms in Wing Fortress Zone do not make the player fall when they retract into the wall, causing them to walk on air.

Knuckles in Sonic 2

Aside from the obvious addition of Knuckles instead of Sonic, there were some minor changes.

  • The game now saves the player's rings after exiting a Special Stage, a la Sonic 3. However, this has the unintended side effect of allowing the player to keep their rings after death if they enter a Special Stage beforehand.
  • The bottom two rows of pixels of the screen in Emerald Hill were fixed.
Sonic the Hedgehog 2 Knuckles in Sonic 2
Sonic 2 Chemical Plant No Monitors.png If you're wondering, these are life and ring monitors.
  • Object layouts were altered, mostly adding monitors and rings in places Knuckles can reach but Sonic and Tails can't.
To do:
Cover everywhere that there was a change in object layouts.
  • The shield and invincibility stars were changed to gray; this is because Sonic uses 4 shades of blue, while Knuckles uses 3 shades of red and 1 shade of green. However, water surfaces in Aquatic Ruin Zone weren't changed for some reason, which makes them look a bit weird.
  • All of the cheat codes were changed:
    • Level Select: On the title-screen, press Up, Up, Up, Down, Down, Down, Left, Right, Left, Right then hold A and press Start.
    • Debug Mode: On the level select, play sounds 01, 09, 09, 04, 01, 00, 01, 08 (October 18th 1994, which is Sonic & Knuckles release date).
    • All Emeralds: On the level select, play sounds 01, 06, 07, 07, 07, 02, 01, 06.
  • For unknown reasons, all objects except for Rings and Monitors were removed from Debug Mode, with the Monitor now being a Ring instead of the 2-Player Teleport.
  • Path swappers (which are used by the engine to switch Sonic onto another plane) are now visible in Debug Mode, like they were in some early prototypes.
  • Two-player mode and the options screen were removed, alongside a playable Tails.
    • As a result, all tracks linked to the aforementioned two-player mode (tracks 01, 03, 08, and 0C) are now unused outside of the sound test.
  • The Tails monitor at Hidden Palace Zone now grants an extra life.

Sonic Jam

  • Unlike previous versions, Sonic Jam's version of Sonic 2 directly ported the code to the Sega Saturn's hardware. As a result, the game can now use CD audio which prevents channels from going silent, and load times are reduced.
  • This version also fixes the bottom two pixels of Emerald Hill Zone.
  • There are three difficulty settings, Easy, Normal and Original; Easy removes a few levels and changes the object layout, Normal has all the levels but still alters the object layouts, and Original uses the original object layouts.
  • There's a small bug with Chemical Plant's water palette, as the darkest purple one is even darker than it should be (appearing black).
  • Instead of playing the Sonic 2 invincibility theme, Sonic Jam instead plays the Sonic 1 invincibility theme; the Sonic 2 version is on the disc (and is even used in two-player mode), meaning it was likely a mistake by the porting team.