Sonic the Hedgehog:RAM map

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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