Sonic the Hedgehog (Genesis)/RAM map

The following article is a RAM map for Sonic the Hedgehog (Genesis).

Note: This page has been copied from http://info.sonicretro.org/SCHG:Sonic_the_Hedgehog/RAM_Editing with minor modifications.

Main System Memory Locations

This is a map of 68k memory as used by the main gameplay engine.

RAM offset	Description
$FF0000 - $FFA3FF	Chunk mappings
$FFA400 - $FFA7FF	Level layout (1 byte per chunk, $40 bytes per line, lines for planes A and B interlaced)
$FFA800 - $FFA9FF	Something to do with background scrolling in Star Light Zone
$FFAA00 - $FFABFF	Something to do with the pattern load queue
$FFAC00 - $FFAFFF	Sprite table (2 bytes per sprite + 2 bytes per priority level, each priority level takes $80 bytes.)
$FFB000 - $FFC7FF	Block mappings
$FFC800 - $FFCAFF	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.
$FFCB00 - $FFCBFF	Sonic previous position array (used for invincibility stars and such)
$FFCC00 - $FFCFFF	Scroll RAM cache (sent to VRAM every frame via DMA)
$FFD000 - $FFD7FF	Object table—Reserved
$FFD800 - $FFEFFF	Object table—Main
$FFF600	Game Mode $00 - Sega Screen $04 - Title Screen $08 - Demo Mode $0C - Normal Level $10 - Special Stage $14 - Continue Screen $18 - Ending Sequence $1C - Credits
$FFF616-$FFF617	Plane A y-position for previous frame
$FFF618-$FFF619	Plane B y-position for previous frame
$FFF61A-$FFF61B	Plane A x-position for previous frame
$FFF61C-$FFF61D	Plane B x-position for previous frame
$FFF61E-$FFF61F	Value of $F71C for previous frame
$FFF620-$FFF621	Value of $F718 for previous frame
$FFF680	Address queue for Pattern Load Cues
$FFF700-$FFF701	Plane A x-position
$FFF704-$FFF705	Plane A y-position
$FFF708-$FFF709	Plane B x-position
$FFF70C-$FFF70D	Plane B y-position
$FFF744	Scroll Hold Flag (set to stop all scrolling dead)
$FFF754-$FFF755 $FFF756-$FFF757 $FFF758-$FFF759 $FFF75A-$FFF75B	Bitfields used by the scrolling engine
$FFF760-$FFF761	Sonic's Top Speed
$FFF762-$FFF763	Sonic's Acceleration
$FFF764-$FFF765	Sonic's Deceleration
$FFF7AA	Screen Lock Flag (Set to lock screen)
$FFF7C8	Control Lock Flag (Set to lock controls)
$FFFA00-$FFFA7F	Target underwater palette
$FFFA80-$FFFAFF	Normal underwater palette
$FFFB00-$FFFB7F	Normal palette
$FFFB80-$FFFBFF	Target palette (calling Pal_FadeTo will brighten the normal palette until it is the same as the target palette)
$FFFC00-$FFFD7F	Destroyable object state table
$FFFE02	Restart Level Flag (Set to restart level)
$FFFE04-$FFFE05	Level Timer (in frames)
$FFFE10-$FFFE11	Zone/Act
$FFFE12	Lives
$FFFE14-$FFFE15	Remaining Air (When underwater)
$FFFE16	Last Special Stage Entered
$FFFE18	Continues
$FFFE1C	Lives Counter Update Flag (Set to update lives total on screen)
$FFFE1D	Ring Counter Update Flag (Set to update rings total on screen)
$FFFE1E	Time Counter Update Flag (Set to update current time on screen)
$FFFE1F	Score Counter Update Flag (Set to update score total on screen)
$FFFE20-$FFFE21	Ring Count
$FFFE23	Level Timer (Minutes)
$FFFE24	Level Timer (Seconds)
$FFFE23	Level Timer (Frames since last second)
$FFFE26-$FFFE29	Score
$FFFE2C	Shield Flag (Set to take a single hit without losing rings)
$FFFE2D	Invincibility Flag (Set to make Sonic invincible)
$FFFE2E	Speed Shoes Flag (Set to activate speed shoe physics)
$FFFE30	Lamppost Counter
$FFFE57	Emeralds
$FFFFF4	Credits Index Number

Object Table Format

The Object Status Table is located at RAM address $FFD000. 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 0 is the horizontal mirror flag. If set, the object will be flipped on its horizontal axis. Bit 1 is the vertical mirror flag. Bits 2 and 3 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). Bit 4 is a flag indicating whether to check the y-position of the object. If clear, it will be drawn even if it isn't inside the screen. Bit 5 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. Bit 6 is Sonic's "Ride B" flag Bit 7 is the draw object flag. It will be set if the object was onscreen when it came time to draw things. Otherwise, it is clear. There should be no reason to edit this flag, but it's good to know what it does.
$02	word	Starting art block. First nybble: Palette line the sprite will use. Latter 3 nybbles: Starting art block in VRAM for which the object reads from. Multiply by $20 to get VRAM address; divide VRAM address by $20 to get this value.
$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	Height/2
$17	byte	Width/2
$18	byte	Sprite priority (00 = front).
$19	byte	Width of the object, in pixels
$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 seems to be a special thing for the starpole. 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.
$2B	byte	Unused
$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).
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