SCHG:Sonic Drift

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Bellow are some notes for Sonic Drift. These notes are based on Ravenfreak's research.

Error Handler

This game has a hidden error handler text located at offset $46350. Whenever the game crashes it will display this error text before returning to the Sega screen. The error handler routine is located at offset $462CF.

SRAM

It should be noted that this is the only 8-bit Sonic title to utilize battery back up. The game keeps track of the player's best times in Free Run Mode. The SRAM read/write code is located at offset $8F5 in the ROM.

Object Pointers

Offset	Description
$70AA	Ring
$70AD	Palm Tree Left
$70B0	Palm Tree Right
$70B3	Stone Pillar Left
$70B6	Stone Pillar Right
$70B9	Spring Yard Block Left
$70BC	Spring Yard Block Right
$70BF	Labyrinth Block Left
$70C2	Labyrinth Block Right
$70C5	Lamppost Left
$70C8	Lamppost Right
$70CB	Electrical Sphere Left
$70CE	Electrical Sphere Right
$70D1	Booster
$70D4	Invincible Power Up
$70D7	Spring
$70DA	Spike Ball
$70DD	Heart
$70E0	Sign Left
$70E3	Sign Right

Art

All art in the ROM can be edited in Aspect Edit. The player sprites are uncompressed and each sprite consits of 9 tiles that make up a 3x3 grid. The following diagram shows the tile rearrangement data for their sprites:

[1] [2] [3]  
[4] [5] [6]
[7] [8] [9]

Each map in the game consists of 80 tiles that make up a 8x10 grid. The tile arrangement is as follows:

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
[11][12][13][14][15][16][17][18][19][20]
[21][22][23][24][25][26][27][28][29][30]
[31][32][33][34][35][36][37][38][39][40]
[41][42][43][44][45][46][47][48][49][50]
[51][52][53][54][55][56][57][58][59][60]
[61][62][63][64][65][66][67][68][69][70]
[71][72][73][74][75][76][77][78][79][80]

Level background sprites are compressed. However, the title arrangement is different than the maps. Further research is needed to fully understand the tile arrangement data at this time.

Art Offsets

Offset	Description
$87E7-$9436	Small Text Sprites (Compressed)
$9B3F	Marble Red Map (Compressed)
$A1FA	Spring Yard Red Map (Compressed)
$A954	Labyrinth Red Map (Compressed)
$B072	Star Light Red Map (Compressed)
$B74F	Scrap Brain Red Map (Compressed)
$D33C	Spring Yard Block (Compressed)
$D4B9	Labyrinth Block (Compressed)
$D63F	Star Light Lamppost (Compressed)
$D724	Scrap Brain Electric Orb (Compressed)
$D882	Green Hill Green Map (Compressed)
$DFA1	Marble Green Map (Compressed)
$E660	Spring Yard Green Map (Compressed)
$20000	Sonic Player Select Sprites
$212DF	Tails Player Select Sprites
$22BE0	Eggman Player Select Sprites
$23220	Amy Player Select Sprites
$258C0	Green Hill Yellow Map (Compressed)
$25F56	Marble Yellow Map (Compressed)
$2661F	Spring Yard Yellow Map (Compressed)
$26D6B	Labyrinth Yellow Map (Compressed)
$27484	Star Light Yellow Map (Compressed)
$2C000	Labyrinth Green Map (Compressed)
$2C797	Green Hill Background (Compressed)
$2CFD9	Marble Background (Compressed)
$2D262	Spring Yard Background (Compressed)
$2D6A1	Labyrinth Background (Compressed)
$2DBFA	Star Light Background (Compressed)
$2E244	Scrap Brain Background (Compressed)
$34000	Tails Uncompressed Sprites
$34C40	Amy Uncompressed Sprites
$35880	Eggman Uncompressed Sprites
$36500	Sonic Thumbs Up Sprite
$36660	Eggman Fist Pump Sprite
$367E0	Tails Peace Sign Sprite
$36940	Amy Waving Sprite
$38AC0	VS. Text Sprites (Compressed)
$38BEF	Title Card Text Sprites (Compressed)
$3BC4F	Sega Logo (Compressed)
$4339B	Labyrinth Title Card Text (Compressed)
$45A5C	Cursor on Menu Screen Sprite (Compressed)

Palettes

Offset	Description
$2131	Green Hill
$2151	Sonic/Blue Traffic Light
$2171	Marble
$2191	Spring Yard
$21B1	Labyrinth
$21D1	Star Light
$21F1	Scrap Brain
$3888	Track Rotating Palette
$45ABF	Green Hill Course Select
$45AFF	Marble Course Select

Text

The text used on various screens can be edited via a hex editor. Each letter consists of two hex values, one representing the top of the letter, one representing the bottom. See Green Hill's text bellow for an example, the top of the letter is represented with a 1, while the bottom is represented with a 2 in the example:

 
           GT      RT      ET      ET      NT              HT
.db $00 $00 $46 $00 $4B $00 $2E $00 $2E $00 $47 $00 $00 $00 $41 $00
    IT       LT     LT              GB      RB     EB       EB
.db $42 $00 $45 $00 $45 $00 $00 $00 $50 $00 $5B $00 $3E $00 $3E $00
     NB             HB      IB      LB      LB
.db $57 $00 $00 $00 $51 $00 $52 $00 $55 $00 $55 $00

The text on the title cards follow a slightly different pattern, and the value for the letter "G" different. Each title card text consits of 96 bytes. The first two bytes determine the mappings' x and y positions on the screen. The last two bytes determine the letter mappings, the first byte is the top of the letter while the second byte is the bottom. Labyrinth uses both a compressed sprite and hex array for it's name. However, you can still easily edit the name as it follows the same format as the other track names. Here's Green Hill's Title Card Text:

"ROUND" Mappings, this is always the first line of text that displays and therefore appears before the actual track's name.

     X   Y   RT  X   Y   RB  X   Y   OT  X   Y   OB  X   Y   UT  X 
.db $3E $F0 $4B $46 $F0 $5B $3E $F8 $48 $46 $F8 $58 $3E $00 $4E $46
     Y   UB  X   Y   NT  X   Y   NB  X   Y   DT  X   Y   DB
.db $00 $5E $3E $08 $47 $46 $08 $57 $3E $10 $2D $46 $10 $3D 

Round Number Mappings (in this case it's 1)
     X   Y   1T   X   Y  1B
.db $3E $20 $82 $46 $20 $92

Unlike the mappings used on the results screen, the top and bottom of each letter appear in two bytes, each next to each other. See bellow:

     X   Y       X   Y   
.db $5E $00 $40 $66 $00 $50 G
.db $5E $08 $4B $66 $08 $5B R
.db $5E $10 $2E $66 $10 $3E E
.db $5E $18 $2E $66 $18 $3E E 
.db $5E $20 $47 $66 $20 $57 N
 
.db $6E $10 $41 $76 $10 $51 H 
.db $6E $18 $42 $76 $18 $52 I 
.db $6E $20 $45 $76 $20 $55 L
.db $6E $28 $45 $76 $28 $55 L 
.db $6E $30 $00 $76 $30 $00

Text Offsets

Offset	Description
$15FEF	SONIC Text on GP Results Screen
$160B3	TAILS Text on GP Results Screen
$16177	AMY Text on GP Results Screen
$1623B	EGGMAN Text on GP Results Screen
$43445	Green Hill Title Card
$434A5	Marble Title Card
$43505	Spring Yard Title Card
$43565	Labyrinth Title Card
$435C5	Star Light Title Card
$43625	Scrap Brain Title Card
$4BCA1	Green Hill Grand Prix Results
$4BCCD	Marble Grand Prix Results
$4BCF9	Spring Yard Grand Prix Results
$4BD25	Labyrinth Grand Prix Results
$4BD51	Star Light Grand Prix Results
$4BD7D	Scrap Brain Grand Prix Results
$4AF3C	Green Hill Free Run Results
$4AF68	Marble Free Run Results
$4AF94	Spring Yard Free Run Results
$4AFC0	Labyrinth Free Run Results
$4AFEC	Star Light Free Run Results
$4B018	Scrap Brain Free Run Results
$4B170	Green Results Text
$4B18C	Yellow Results Text
$4B1A8	Red Results Text

Letter Values

Offset	Description
$00	Space/End of Line
$2A	Top of A
$3A	Bottom of A
$2B	Top of B
$3B	Bottom of B
$2C	Top of C
$3C	Bottom of C
$2D	Top of D
$3D	Bottom of D
$2E	Top of E
$3E	Bottom of E
$2F	Top of F
$3F	Bottom of F
$40	Top of G (Title Card Text)
$46	Top of G (Results Text)
$50	Bottom of G
$41	Top of H
$51	Bottom of H
$42	Top of I
$52	Bottom of I
$43	Top of J
$53	Bottom of J
$44	Top of K
$54	Bottom of K
$45	Top of L
$55	Bottom of L
$46	Top of M
$56	Bottom of M
$47	Top of N
$57	Bottom of N
$48	Top of O
$58	Bottom of O
$49	Top of P
$59	Bottom of P
$4A	Top of Q
$5A	Bottom of Q
$4B	Top of R
$5B	Bottom of R
$4C	Top of S
$5C	Bottom of S
$4D	Top of T
$5D	Bottom of T
$4E	Top of U
$5E	Bottom of U
$4F	Top of V
$5F	Bottom of V
$60	Top of W
$70	Bottom of W
$61	Top of X
$71	Bottom of X
$62	Top of Y
$72	Bottom of Y
$63	Top of Z
$73	Bottom of Z

Track Layouts

The track layout pointers are located at offset $361C. The first track is Green Hill Green which is located at offset $1D6A0, followed by Marble Green and so forth. Note that you'll need to edit each track map as well as the object layout for them to match your updated tracks. Each part of the track consists of three bytes. The first byte controls the background movement, while the second and third bytes are the actual track layout. Further research is needed in order to make edits to the track layouts.

Offset	Description
$1D6A0	Green Hill Green
$1D6D0	Marble Green
$1D71A	Spring Yard Green
$1D76E	Labyrinth Green
$1D7C4	Star Light Green
$1D80E	Scrap Brain Green
$1D859	Green Hill Red
$1D8AA	Marble Red
$1D908	Spring Yard Red
$1D944	Labyrinth Red
$1D99A	Star Light Red
$1D9DC	Scrap Brain Red
$1DA47	Green Hill Yellow
$1DA8B	Marble Yellow
$1DACF	Spring Yard Yellow
$1DB0E	Labyrinth Yellow
$1DB72	Star Light Yellow
$1DBC9	Scrap Brain Yellow

Music

Music Pointers are located at offset $7D473 in the ROM. The sound effect pointers are located at $7D491.

Music Locations

Offset	Description
$7D4D1	Options
$7D625	Title Screen
$7D6EE	Green Hill
$7D940	Course Select With Intro
$7DB6E	Spring Yard
$7DD79	Labyrinth
$7DF7D	Star Light
$7E15A	Scrap Brain
$7E494	Race Results
$7E511	GP Final Results
$7E5CC	Invincibility Theme
$7E6B4	Credits
$7E838	Marble
$7E9F7	Course Select With No Intro