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SCHG

Sonic the Hedgehog (16-bit)/RAM Editing

From Sonic Retro

SCHG: Sonic the Hedgehog

Main System Memory Locations

This is a map of 68k memory as used by the main gameplay engine. Note that any numbers entered (for example, score or number of rings) will have to be converted to hex first.

These are the offsets in RAM. To convert them to Genecyst savestate offsets, add $2478. For use in disassemblies, simply add $FF0000 for addresses below $8000 and $FFFF0000 for addresses above or equal to $8000, for example, Sonic's top speed here is $F760, but in the disassembly it is referenced as $FFFFF760. This is useful for finding what bits of code do what when the purpose of a RAM variable is known, or conversely for figuring out the purpose of a RAM variable by looking at the referencing code.

RAM offset Description
$0000 - $A3FF Chunk mappings
$A400 - $A7FF Level layout (1 byte per chunk, $80 bytes per line, 8 lines for planes A and B interlaced)
$A800 - $A9FF 16 pixel scroll buffer primarily used for SLZ (4 bytes = one 16 pixel'd block scroll for FG and BG), this data is transfered/converter correctly to $CC00 - $CFFF (Scroll RAM cache).
$AA00 - $ABFF Nemesis decompression buffer
$AC00 - $AFFF Sprite table input. Consists of 8 different priority levels, each level taking up $80 bytes. The first word of each level is the number of objects currently in that priority level multiplied by 2, and the next $3F words are the addresses in RAM (inside the object RAM space) of the objects on that priority level. The information in this table is processed by BuildSprites to create the actual sprite table
$B000 - $C7FF Block mappings
$C800 - $CAFF Sonic's art buffer. Sonic's dynamic pattern reloading routine copies the relevant art over here, from where it is DMAed to VRAM every V-int.
$CB00 - $CBFF Sonic previous position array (used for invincibility stars and such)
$CC00 - $CF7F Scroll RAM cache (sent to VRAM every frame via DMA)
$CF80 - $CFFF Unused (not recommended because of scroll data overflowing)
$D000 - $D7FF Reserved object SST space
$D800 - $EFFF Main SST space
$F000 - $F5BF Flags used by the sound engine
$F5C0 - $F5FF Unused
$F600 Screen Mode
  • $00 - Sega Screen
  • $04 - Title Screen
  • $08 - Demo Mode
  • $0C - Normal Level
  • $10 - Special Stage
  • $14 - Continue Screen
  • $18 - Ending Sequence
  • $1C - Credits
$F601 Unused
$F602 Player's held buttons used in Sonic's moves, etc.
$F603 Player's pressed buttons used in Sonic's moves, etc.
$F604 Player 1's held buttons
$F605 Player 1's pressed buttons
$F606 Player 2's held buttons (unused)
$F607 Player 2's pressed buttons (unused)
$F608-$F60B Unused
$F60C-$F60D VDP command for mode register 2
$F60E-$F613 Unused
$F614-$F615 An universal counter, mainly used for time to display screens (like the title screen and the demo mode)
$F616-$F617 Plane A y-position for previous frame
$F618-$F619 Plane B y-position for previous frame
$F61A-$F61B Plane A x-position for previous frame
$F61C-$F61D Plane B x-position for previous frame
$F61E-$F61F Value of $F71C for previous frame
$F620-$F621 Value of $F718 for previous frame
$F622-$F623 Unused
$F624-$F625 Water's height onscreen. More specifically, VDP setting for the scanline H-int happens at. The value stored is $8Axx, where xx is the scanline number. This is moved to VDP each V-int.
$F626-$F627 Variables used in palette fade routines
$F628 Unknown
$F629 Unused
$F62A VBlank routine counter
$F62B Unused
$F62C Number of sprites on screen counter
$F62D-$F631 Unused
$F632-$F633 Palette cycle offset counter
$F634-$F635 Palette cycle wait timer
$F636-$F639 Random number generator seed & result
$F63A-$F63B Game paused state
$F63C-$F63F Unused
$F640-$F641 Used to hold the low word of a DMA transfer command, which is then sent to the VDP control port, to get around a hardware bug.
$F642-$F643 Unused
$F644-$F645 Set when water palette should be transferred next HBlank
$F646-$F647 Water height
$F648-$F649 Average water height
$F64A-$F64B Target water height
$F64C Water movement direction
$F64D Dynamic water routine counter
$F64E Flag that is set when water takes up the entire screen
$F64F Is set when sound driver should be ran on HBlank (not enough time before it)
$F650-$F661 SBZ palette cycling buffer (consists of 9 entries with 2 byte sized variables, first being the palette cycle timer, and the second being the palette cycle data index)
$F662-$F663 Unused, but is cleared upon loading the SEGA screen
$F664-$F67F Unused
$F680-$F6DF Pattern load queue. 16 entries with each being 6 bytes in size, consisting of a longword sized variable and a word sized variable. The first variable is a pointer to Nemesis compressed art. The second variable is the destination VRAM address. Once the queue is empty, the first entry's pointer variable is 0.
$F6E0-$F6FF Unknown pattern load queue variables
$F700-$F703 Plane A x-position (16.16 fixed point number)
$F704-$F707 Plane A y-position (16.16 fixed point number)
$F708-$F70B Primary plane B x-position (16.16 fixed point number)
$F70C-$F70F Primary plane B y-position (16.16 fixed point number)
$F710-$F713 Secondary plane B x-position (16.16 fixed point number)
$F714-$F717 Secondary plane B y-position (16.16 fixed point number)
$F718-$F71B Tertiary plane B x-position (16.16 fixed point number). In REV00, it's only used to scroll the background in special stages, but it ends up having use in normal levels in REV01.
$F71C-$F71F Tertiary plane B y-position (16.16 fixed point number). Unused in REV00, but it used in REV01.
$F720-$F721 Level's target left boundary (Unused)
$F722-$F723 Level's target right boundary (Unused)
$F724-$F725 Level's target upper boundary (Unused)
$F726-$F727 Level's target lower boundary
$F728-$F729 Level's left boundary
$F72A-$F72B Level's right Boundary
$F72C-$F72D Level's upper boundary
$F72E-$F72F Level's lower boundary
$F730-$F731 Written to by LevelSizeLoad, but seems to be unused
$F732-$F733 Upper level boundary +$240 (Seems to be unused)
$F734-$F739 Unused
$F73A-$F73B Number of pixels scrolled by the camera horizontally since the previous frame (multiplied by $100)
$F73C-$F73D Number of pixels scrolled by the camera vertically since the previous frame (multiplied by $100)
$F73E-$F73F Y-camera position, based on Sonic ($60 = default)
$F740-$F741 Unused (cleared by LevelSizeLoad)
$F742 Level's dynamic events routine ID
$F743 Unused
$F744 Scroll disable flag
$F745 Unused
$F746 Unused (cleared by LevelSizeLoad)
$F747 Unused
$F748 Unused (cleared by LevelSizeLoad)
$F749 Unused
$F74A Foreground horizontal plane redraw flag (used to indicate if the camera has passed a 16 pixel boundary)
$F74B Foreground vertical plane redraw flag (used to indicate if the camera has passed a 16 pixel boundary)
$F74C Primary background horizontal plane redraw flag (used to indicate if the camera has passed a 16 pixel boundary)
$F74D Primary background vertical plane redraw flag (used to indicate if the camera has passed a 16 pixel boundary)
$F74E Secondary background horizontal plane redraw flag (used to indicate if the camera has passed a 16 pixel boundary)
$F74F Possible the secondary background vertical plane redraw flag (used to indicate if the camera has passed a 16 pixel boundary), but is unused in both revisions.
$F750 Tertiary background horizontal plane redraw flag (used to indicate if the camera has passed a 16 pixel boundary) (unused in REV00, used in REV01)
$F751-$F753 Unused
$F754 Foreground plane redraw flags
Bit Hex Description
0 $01 Draw a new row on the top of the screen
1 $02 Draw a new row on the bottom of the screen
2 $04 Draw a new column on the left side of the screen
3 $08 Draw a new column on the right side of the screen
$F756 Primary background plane redraw flags (format is the same as above)
$F758 Secondary background plane redraw flags (format is the same as above)
$F75A Tertiary background plane redraw flags (unused in REV00, but is used in REV01) (format is the same as above)
$F755
$F757
$F759
$F75B
Unused
$F75C Flag that is set if the level's lower boundary is moving.
$F75D-$F75F Unused
$F760-$F761 Sonic's top speed
$F762-$F763 Sonic's acceleration
$F764-$F765 Sonic's deceleration
$F766 Sonic's previous DPLC frame ID
$F767 Flag that is set when Sonic's art needs to be reloaded in VRAM.
$F768 Angle value at Sonic's left sensor
$F769 Possibly unused
$F76A Angle value at Sonic's right sensor
$F76B Possibly unused
$F76C Object manager's routine ID
$F76D Unused
$F76E-$F76F Copy of $F700 used by object position loader
$F770-$F77F Flags used by object position loader
$F780-$F781 Special stage's angle
$F782-$F783 Special stage's rotation speed
$F784-$F78F Unused
$F790-$F791 Demo's button press counter (Addition to proper address)
$F792 Seems to be duration to increment button press counter
$F793 Unused
$F794-$F795 Palette fade out delay counter
$F796-$F799 Direct address of collision pointer of the level
$F79A-$F79B Palette cycle counter for Special Stage
$F79C-$F79D Palette cycle delay for Special Stage
$F79E-$F79F Unknown purpose (Something to do with special stage palette cycle)
$F7A0-$F7A1 Something to do with Special Stage BG deformation
$F7A2-$F7A3 Unused
$F7A4-$F7A5 Stomping chained metal block Y position (used to align a pushing block on top of it)
$F7A6 Unused
$F7A7 Boss fight flags (0 = before/during boss fight, 1 = boss defeated, 2 = prison capsule destroyed)
$F7A8-$F7A9 Sonic's recorded position array's counter
$F7AA Flag to prevent Sonic from going past the right level boundary
$F7AB Unused
$F7AC-$F7AF Chunk IDs for loops and S tubes (first 2 are for loops, with the first ID being the loop with half of its collision enabled, and the second being the loop with the other half enabled, the last 2 are for S tube chunks) (Only used in GHZ and SLZ)
$F7B0 Animated art tile offset counter
$F7B1 Animated art timer
$F7B2 Animated art tile offset counter
$F7B3 Animated art timer
$F7B4 Animated art tile offset counter
$F7B5 Animated art timer
$F7B6 Animated art tile offset counter
$F7B7-$F7BD Unused, but could possibly have been used for animated art tile variables like above.
$F7BE-$F7BF Big ring art offset counter
$F7C0 Conveyor belt direction (0 = normal, 1 = reversed)
$F7C1-$F7C6 (?) Array of flags used by LZ and SBZ conveyor belt platforms
$F7C7 Flag to prevent Sonic from using the normal walking animation in wind tunnels and to make air bubbles move horizontally in the wind tunnels
$F7C8 Ignore mode flag (Ignoring all player control)
$F7C9 Flag to make Sonic move in the wind tunnels (0 = enabled)
$F7CA Lock controls (Except jumping)
$F7CB Flag to prevent multiple large moving platforms from SBZ3 from spawning at once
$F7CC Joypad control lock flag
$F7CD Flag that is set when Sonic enters a big ring at the end of a level
$F7CE Used by Obj56 (SYZ/SLZ floating block) in REV01, Unused in REV00
$F7CF Unused
$F7D0-$F7D1 Number of enemies destroyed without touching the ground
$F7D2-$F7D3 End of level time bonus
$F7D4-$F7D5 End of level ring bonus
$F7D6 Flag to update the bonus counters in the end level card
$F7D7 Ending sequence routine ID
$F7D8-$F7D9 Used for water deformation effects in LZ in REV01, Unused in REV00
$F7DA-$F7DF Unused
$F7E0-$F7EF Switch statuses
$F7F0-$F7F1
$F7F2-$F7F3
$F7F4-$F7F5
$F7F6-$F7F7
Scroll section Y sizes
$F7F8-$F7FF Unused
$F800-$FA7F Sprite table buffer (sent to VRAM every frame via DMA)
$FA00-$FA7F Target underwater palette (is overwritten by the "BuildSprites" routine after being processed)
$FA80-$FAFF Normal underwater palette
$FB00-$FB7F Normal palette
$FB80-$FBFF Target palette (calling Pal_FadeTo will change the normal palette until it is the same as the target palette)
$FC00-$FCFF Object respawn status table
$FC44 Error message ID (If crash happens)
$FD00-$FDFF System stack
$FE00 Stack pointer
$FE00-$FE01 Unused
$FE02-$FE03 Level reload flag
$FE04-$FE05 Internal level counter (number of frames passed since the level fades from black)
$FE06 Debug mode item ID
$FE07 Unused
$FE08-$FE09 Object placement flag (Set if debug mode is being used)
$FE0A Debug mode speed timer
$FE0B Debug mode moving speed
$FE0C-$FE0F Vertical interrupt counter (increments after each V-INT)
$FE10 Zone ID
$FE11 Act ID
$FE12 Number of lives Sonic has
$FE13 Unused
$FE14-$FE15 Air remaining (When underwater)
$FE16 Last special stage entered ID
$FE17 Unused
$FE18 Number of continues
$FE19 Unused
$FE1A Time over flag
$FE1B Extra life flags (bits 1 and 2 are set if you have received the extra life at 100 and 200 rings)
$FE1C Flag to update the life counter in the HUD
$FE1D Flag to update the ring counter in the HUD (if negative, reset the counter to 0, otherwise, just update it with the new ring amount)
$FE1E Flag to update the timer in the HUD
$FE1F Flag to update the score in the HUD
$FE20-$FE21 Number of rings collected
$FE22-$FE23 Minutes counter for the level timer
$FE24 Seconds counter for the level timer
$FE25 Frame counter for the level timer
$FE26-$FE29 Score
$FE2A-$FE2B Unused
$FE2C Shield flag (Set to take a single hit without losing rings)
$FE2D Invincibility flag (Set to make Sonic invincible)
$FE2E Speed shoes flag (Set to activate speed shoe physics)
$FE2F Unused (cleared when level starts, possibly another power up flag, like the goggles)
$FE30-$FE54 Lamppost Variables
  • $FE30 - Lamppost Counter
  • $FE31 - Copy of Lamppost Counter
  • $FE32-$FE33 - Copy of Sonic's X Position
  • $FE34-$FE35 - Copy of Sonic's Y Position
  • $FE36-$FE37 - Copy of Ring Counter
  • $FE38-$FE3B - Copy of Time Counter
  • $FE3C-$FE3D - Copy of Resize Level Routine Counter
  • $FE3E-$FE3F - Copy of Levels lower boundary
  • $FE40-$FE41 - Copy of Plane A X-position
  • $FE42-$FE43 - Copy of Plane A Y-position
  • $FE44-$FE4F - Copy of every background sections' X and Y positions
  • $FE50-$FE51 - Copy of average water height
  • $FE52 - Copy of water routine counter
  • $FE53 - Copy of water fullscreen flag
  • $FE54 - Copy of extra lives flags
$FE55-$FE56 Unused
$FE57 Number of emeralds collected
$FE58-$FE5D Array of collected emeralds
$FE5E-$FE9F(?) Oscillation values
$FEA0-$FEBF Unused
$FEC0 Rotating spikes in GHZ animation timer
$FEC1 Rotating spikes in GHZ animation frame
$FEC2 Ring animation timer
$FEC3 Ring animation frame
$FEC4 Animation timer (Unused)
$FEC5 Animation frame (Unused)
$FEC6 Ring loss timer
$FEC7 Lost ring animation frame
$FEC8 Lost ring animation accumulator
$FEC9-$FEEF Unused
$FEF0-$FEF3 Debug mode variables
  • $FEF0-$FEF1 - Copy of level's upper boundary
  • $FEF2-$FEF3 - Copy of level's bottom boundary
$FEF4-$FF0F Unused
$FF10-$FF37 Copy of $F700-$F72F and $F754-$F75B, used in "LoadTilesAsYouMove" routine
$FF38-$FF7F Unused
$FF80-$FF81 Timer for level select selection movement
$FF82-$FF83 Level select selection number
$FF84-$FF85 Level select sound ID
$FF86-$FFBF Unused
$FFC0-$FFC3 Copy of current score (Unused)
$FFC4-$FFDF Unused
$FFE0 Level select enabled flag
$FFE1 Slow motion enabled flag
$FFE2 Debug mode enabled flag
$FFE3 Japanese credits enabled flag
$FFE4-$FFE5 Correct button press counter for level select cheat
$FFE6-$FFE7 C button press counter (Title Screen only)
$FFE8-$FFE9 Unused
$FFEA-$FFEB Unused (However is cleared on the Title Screen)
$FFEC-$FFEF Unused (Possibly used for debugging the collision engine while in development)
$FFF0-$FFF1 Demo mode flag (level demo if positive, credits demo if negative)
$FFF2-$FFF3 Demo ID
$FFF4-$FFF5 Credits index ID
$FFF6-$FFF7 Unused
$FFF8 Hardware version register (Bits 6 and 7 of lower byte)
$FFF9 Unused
$FFFA-$FFFB Debug mode flag
$FFFC-$FFFF Checksum passed flag

Object Status Table Format

The Object Status Table is located at RAM address $D000. Each entry is $40 bytes long. Here is a list of all the values for one entry in the OST:

Offset Size Description
$00 byte Object type
$01 byte Render flags. The bitfield looks like this:
Bit Hex Description
0 $01 This is the horizontal mirror flag. If set, the object will be flipped on its horizontal axis.
1 $02 This is the vertical mirror flag.
2/3 $04/$08 These are the coordinate system. If 0, the object will be positioned by absolute screen coordinates. This is used for things like the HUD and menu options. If 1, the object will be positioned by the playfield coordinates, i.e. where it is in a level. If 2 or 3, the object will be aligned to the background somehow (perhaps this was used for those MZ UFOs).
4 $10 This is the assume height flag. The object will be drawn if it is vertically within x pixels of the screen where x is #$20 if this flag is clear or $16(a0) if it is set.
5 $20 This is the raw mappings flag. If set, just 5 bytes will be read from the object's mappings offset when the BuildSprites routine draws the object, and these will be interpreted in the normal manner to display a single Genesis sprite. This format is used for objects such as breakable wall fragments.
6 $40 This is Sonic's "Ride B" flag
7 $80 This is the on-screen flag. It will be set if the object is on-screen, and clear otherwise.
$02 word Art tile
Bit Hex Description
0-10 $01-$400 Tile number in VRAM. To get the actual address in VRAM, multiply this number by $20 (32 decimal).
11 $800 If clear, sprites will be drawn normally. If set, sprites will be flipped horizontally.
12 $1000 If clear, sprites will be drawn normally. If set, sprites will be flipped vertically.
13/14 $2000/$4000 Default palette line the sprite uses.
15 $8000 If clear, sprite will be drawn on low plane. If set, sprite will be drawn on high plane.
$04 long Mappings offset
$08 word
  • If in playfield positioning mode, it is the X playfield coordinate.
  • If in screen positioning mode, it is the X screen coordinate.
$0A word
  • If the object is in playfield positioning mode, this is the X subpixel coordinate.
  • If in screen positioning mode, it's the Y screen coordinate.
$0C word
  • If in playfield positioning mode, it is the Y playfield coordinate.
  • If in screen positioning mode, it is unused.
$0E word
  • If the object is in playfield positioning mode, this is the Y subpixel coordinate.
  • If in screen positioning mode, it is unused.
$10 word Horizontal (X) velocity
$12 word Vertical (Y) velocity
$14 word Ground velocity (inertia).
$16 byte Vertical radius of the object's hitbox.
$17 byte Horizontal radius of the object's hitbox.
$18 byte Sprite priority ($00 to $07, lower value has bigger priority).
$19 byte Horizontal radius of the object's visible sprite.
$1A byte Current animation frame to display. i.e, the frame currently being displayed on-screen, according to the animation script (currently parsed mappings frame, in other words).
$1B byte Current frame in animation script. i.e, the frame entry IN the animation script being currently processed.
$1C byte Animation number. When an animation ID is moved here, it's processed and displayed.
$1D byte Restart animation flag (when $1D is not equal to $1C, animation restarts)
$1E byte Animation frame duration (time until next frame).
$20 byte Collision response bitfield. Tells what the object will do if hit by the character. The bitfield is in the format TTSS SSSS. TT is the type of collision - 00 is enemy, 01 increments the routine counter, 10 is harm, and 11 is "special", used for the starpole, caterkiller, yadrin and some other objects. SSSSSS is the size, lifted from a lookup table in the collision response routine.
$21 byte Custom collision property, for special interaction with Sonic. This is used by bosses, badniks, bumpers and other objects. The way in which this byte is used is different for each object. Bosses use this byte as a hit counter.
$22 byte Status bitfield.

Counting from the least significant bit:

Bit Hex Description
0 $01 X Orientation. Clear is left and set is right.
1 $02 Y Orientation. Clear is right-side up, and set is upside-down
2 $04 Unknown or unused
3 $08 Set if Sonic is standing on this object.
4 $10 Unknown or unused
5 $20 Set if Sonic is pushing on this object.
6 $40 Unknown or unused
7 $80 Unknown or unused
Note that these bits have different meanings for Sonic (see below).
$23 byte Respawn index reference number, used by badniks, rings and monitors. Each destroyable object is assigned an index number (01, 02, 03 etc.) which refers to a list at $FC00 in the RAM.
$24 byte Routine counter.
$25 byte Second routine counter. This is used for some of the more complicated objects.
$26 word Angle.
$28 byte Object subtype. For example, the current monitor selected. See the Object List above for values. Has a different meaning for Sonic.
$29-$3F Object's scratch RAM
Sonic-specific variables
Offset Size Description
$22 byte Special bitfield. Counting from the least significant bit:
Bit Hex Description
0 $01 Orientation. Clear is left and set is right.
1 $02 Set if Sonic is in the air (jump counts).
2 $04 Set if jumping or rolling.
3 $08 Set if Sonic isn't on the ground but shouldn't fall. (Usually when he is on a object that should stop him falling, like a platform or a bridge.)
4 $10 Set if jumping after rolling.
5 $20 Set if pushing something.
6 $40 Set if underwater.
7 $80 Unused.

You can add the hex values together to use multiple settings at once. Also notice that the first 3 bits (0-2) are used in the character object as a second routine counter.

$28 byte Seconds of air left. Usually $1E; it decrements every second while the player is underwater. Beeps on $18, $13, and $0E. Countdown starts on $0B.
$29-$2F Unused (scratch RAM)
$30-$31 word Remaining invulnerability time. Starts at $0078 after Sonic is hit, and seems to decrement every frame until it reaches $0000.
$32-$33 word Remaining time of invincibility.
$34-$35 word Remaining time of Speed Shoes.
$36 byte Angle of ground at Sonic's front collision hot spot ($03 is "edge of ground")
$37 byte Angle of ground at Sonic's rear collision hot spot
$39 byte Unused.
$3A-$3B word Unused.
$3C byte Set if jumping.
$3D byte OST index of object currently being stood on. (multiply by $40 and add $D000 to get the object's address).
$3E-$3F word Control lock timer. Set to lock left and right controls after hitting a spring or running down a slope.
Boss Variables
Offset Size Description
$21 byte Hit counter. Number of hits it takes to defeat a boss.
$30 word X position
$38 word Y position
$3E byte Number of times to flash when hit
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