Sonic the Hedgehog (16-bit)/RAM Editing

SCHG: Sonic the Hedgehog
Main Article Art Editing Object Editing Level Editing Text Editing RAM Editing Miscellaneous Editing

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 DMA'd to VRAM every V-int.
$CB00 - $CBFF	Sonic previous position array (used for invincibility stars and such)
$CC00 - $CF7F	H-Scroll buffer (sent to VRAM every frame via DMA)
$CF80 - $CFFF	Unused (However, this space is used for the H-Scroll buffer to overflow to, so using this is not recommended)
$D000 - $D7FF	Reserved object SST space (Various important objects, such as Sonic, shields, invincibility stars, and title cards, are loaded here.)
$D800 - $EFFF	Dynamic SST space (Level objects are loaded here.)
$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, exists because it's from a standard Mega Drive function)
$F607	Player 2's pressed buttons (unused, exists because it's from a standard Mega Drive function)
$F608-$F60B	Unused
$F60C-$F60D	VDP command for mode register 2
$F60E-$F613	Unused
$F614-$F615	General countdown timer
$F616-$F617	Plane A y-position for VSRAM
$F618-$F619	Plane B y-position for VSRAM
$F61A-$F61B	Plane A x-position for HScroll (unused)
$F61C-$F61D	Plane B x-position for HScroll (unused)
$F61E-$F61F	Copy of $F71C
$F620-$F621	Copy of $F718
$F622-$F623	Unused
$F624-$F625	VDP register $A (H-INT counter) cache (used for water height on screen)
$F626-$F627	Variables used in palette fade routines
$F628	Counter that increments if the V-INT routine is $E, which is unused
$F629	Unused
$F62A	V-INT routine ID
$F62B	Unused
$F62C	Number of sprites on screen
$F62D-$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	Pattern load queue Nemesis decompression 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 unused otherwise
$F732-$F733	Upper level boundary +$240, but unused otherwise
$F734-$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	Possibly 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	Unused
$F76A	Angle value at Sonic's right sensor
$F76B	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 the special stages
$F79C-$F79D	Palette cycle delay for the special stages
$F79E-$F79F	Palette cycle offset for the special stages
$F7A0-$F7A1	Special stage background scroll mode
$F7A2-$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
$FEF4-$FEFF	Unused
$FF00-$FF01	Used by the collision code to determine collision response for chunk $00. While this is technically unused, editing this is not a good idea.
$FF02-$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. 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 Mega Drive sprite. This format is used for objects such as breakable wall fragments. 6 $40 Set if Sonic is behind a loop, clear if above. This is used to change loop collision, based on the chunk Sonic is standing in. (Unused for any other object.) 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 Palette line used by the object. (Note that object mappings may alter this.) 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 right and set is left. 1 $02 Y Orientation. Clear is right-side up, and set is upside-down 2 $04 Unused 3 $08 Set if Sonic is standing on this object. 4 $10 Unused 5 $20 Set if Sonic is pushing on this object. 6 $40 Unused 7 $80 Object specific flag 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. Set is left and clear is right. 1 $02 Set if Sonic is in the air 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 Sonic is jumping after rolling on the ground. (Used mainly to lock horizontal controls.) 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
$38	byte	Is set to make Sonic stick to the SBZ wheel
$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