Sonic the Hedgehog (8-bit)

This is the Sonic Community Hacking Guide for Sonic the Hedgehog. The Guide is based on notes by David Declerck[1][2] & Kroc Camen. The Music Hacking Guide is based on notes by ValleyBell.

ROM Map

Master System

Offset	Description
$00000 – ?	Main Game Code
$0122D – $01286	Zone Titles: Offset Description $0122D–3B "GREEN HILL" $0123C–4A "BRIDGE" $0124B–59 "JUNGLE" $0125A–68 "LABYRINTH" $01269–77 "SCRAP BRAIN" $01278–86 "SKY BASE"
$0197E – $019AD	Final Score Titles: The text used for the score bonuses given on the completion of the game Offset Description $0197E–8D "CHAOS EMERALD" $0198E–9D "SONIC LEFT" $0199E–AD "SPECIAL BONUS"
$02095 – $02AD5	Credits Text
$0627C – $0628B	Palette Pointers: Offset Description $0627C–D 0: Green Hill $0627E–F 1: Bridge $06280–1 2: Jungle $06282–3 3: Labyrinth $06284–5 4: Scrap Brain $06286–7 5: Sky Base 1/2A $06288–9 6: Sky Base 2B/3 $0628A–B 7: Special Stage
$0628C – $0629E	Palette Cycle Pointers: Offset Description $0628C–D 0: Green Hill $0628E–F 1: Bridge $06290–1 2: Jungle $06292–3 3: Labyrinth $06294–5 4: Scrap Brain $06296–7 5: Sky Base 1 $06298–9 6: Sky Base 2-Interior / 3 $0629A–B 7: Special Stage $0629C–D 8: Sky Base 2
$0629E – $065EE	Palettes
$0C7F0 – $0FFFF	Music, sounds, and other stuff
$10000 – $14CA0	Block Mappings: Offset Description $10000 – $10B7F Green Hill (184 blocks) $10B80 – $1147F Bridge (144 blocks) $11480 – $11E7F Jungle (160 blocks) $11E80 – $1297F Labyrinth (176 blocks) $12980 – $1357F Scrap Brain (192 blocks) $13580 – $142FF Sky Base 1-2 (216 blocks) $14300 – $1497F Sky Base 3 (104 blocks) $14980 – $14C9F Bonus Stage (50 blocks)
$14CA0 – $1517F	UNKNOWN
$15180 – $1557F	Blinking Items Art (Box, etc...)
$15580 – $155C9	Level Header Pointers: Offset Description $15580–81 0: Green Hill Act 1 $15582–83 1: Green Hill Act 2 $15584–85 2: Green Hill Act 3 $15586–87 3: Bridge Act 1 $15588–89 4: Bridge Act 2 $1558A–8B 5: Bridge Act 3 $1558C–8D 6: Jungle Act 1 $1558E–8F 7: Jungle Act 2 $15590–91 8: Jungle Act 3 $15592–93 9: Labyrinth Act 1 $15594–95 10: Labyrinth Act 2 $15596–97 11: Labyrinth Act 3 $15598–99 12: Scrap Brain Act 1 $1559A–9B 13: Scrap Brain Act 2 $1559C–9D 14: Scrap Brain Act 3 $1559E–9F 15: Sky Base Act 1 $155A0–A1 16: Sky Base Act 2 $155A2–A3 17: Sky Base Act 3 $155A4–A5 18: End Sequence $155A6–A7 19: UNUSED (invalid data) $155A8–A9 20: Scrap Brain Act 2 (Emerald Maze), from corridor $155AA–AB 21: Scrap Brain Act 2 (Ballhog Area) $155AC–AD 22: Scrap Brain Act 2, from transporter $155AE–AF 23: Scrap Brain Act 2 (Emerald Maze), from transporter $155B0–B1 24: Scrap Brain Act 2, from Emerald Maze $155B2–B3 25: Scrap Brain Act 2, from Ballhog Area $155B4–B5 26: Sky Base Act 2 (Interior) $155B6–B7 27: Sky Base Act 2 (Interior), this one is identical $155B8–B9 28: Special Stage 1 $155BA–BB 29: Special Stage 2 $155BC–BD 30: Special Stage 3 $155BE–BF 31: Special Stage 4 $155C0–C1 32: Special Stage 5 $155C2–C3 33: Special Stage 6 $155C4–C5 34: Special Stage 7 $155C6–C7 35: Special Stage 8 $155C8–C9 36: UNUSED (invalid data)
$155CA – $15AB3	Level Headers: Offset Description $155CA – $155EE Green Hill Act 1 $155EF – $15613 Green Hill Act 2 $15614 – $15638 Green Hill Act 3 $15639 – $1565D End Sequence $1565E – $15682 Bridge Act 1 $15683 – $156A7 Bridge Act 2 $156A8 – $156CC Bridge Act 3 $156CD – $156F1 Jungle Act 1 $156F2 – $15716 Jungle Act 2 $15717 – $1573B Jungle Act 3 $1573C – $15760 Labyrinth Act 1 $15761 – $15785 Labyrinth Act 2 $15786 – $157AA Labyrinth Act 3 $157AB – $157CF Scrap Brain Act 1 $157D0 – $157F4 Scrap Brain Act 2 $157F5 – $15819 Scrap Brain Act 2 (Emerald Maze), From Corridor $1581A – $1583E Scrap Brain Act 2 (Ballhog Area) $1583F – $15863 Scrap Brain Act 3 $15864 – $15888 Scrap Brain Act 2, From Emerald Maze $15889 – $158AD Scrap Brain Act 2, From Ballhog Area $158AE – $158D2 Scrap Brain Act 2, From Transporter $158D3 – $158F7 Scrap Brain Act 2 (Emerald Maze), From Transporter $158F8 – $1591C Sky Base Act 1 $1591D – $15941 Sky Base Act 2 $15942 – $15966 Sky Base Act 3 $15967 – $1598B Sky Base Act 2 (Interior) $1598C – $159B0 Special Stage 1 $159B1 – $159D5 Special Stage 2 $159D6 – $159FA Special Stage 3 $159FB – $15A1F Special Stage 4 $15A20 – $15A44 Special Stage 5 $15A45 – $15A69 Special Stage 6 $15A6A – $15A8E Special Stage 7 $15A8F – $15AB3 Special Stage 8
$15AB4 – $15FFF	Sprite Layout
$16000 – $16DE9	Misc Mappings: Offset Description $16000 – $1612E? Title Screen $1627E – $163F5 Map 1 Background $163F6 – $1653A Map 1 Foreground $1653B – $166AA Map 2 Background (zoom on Robotnik's 'fortress') $166AB – $167FE? Map 2 Foreground (zoom on Robotnik's 'fortress') $16830 – $169A9? Map 3 (when the game is finished) $16C61 – $16DE9 Credits Screen
$16DEA – $1FFFF	Level Layout: Offset Description $16DEA – $17627 Green Hill Act 1 / End Sequence $17628 – $17C88 Green Hill Act 2 $17C89 – $17FB5 Green Hill Act 3 $17FB6 – $18A61 Jungle Act 1 $18A62 – $1933C Jungle Act 2 / Special Stage 4 / 8 (The tall special stage is packed on top of the Jungle waterfall level! The level headers specify a different starting position, tileset, music &c.) $1933D – $199D6 Scrap Brain Act 1 $199D7 – $1A2DA Scrap Brain Act 2 $1A2DB – $1ABD2 Scrap Brain Act 2 (Emerald Maze) $1ABD3 – $1B281 Scrap Brain Act 2 (Ballhog Area) $1B282 – $1BB33 Scrap Brain Act 3 $1BB34 – $1BED0 Sky Base Act 2 $1BED1 – $1C564 Bridge Act 1 $1C565 – $1CE26 Labyrinth Act 1 $1CE27 – $1DB39 Labyrinth Act 2 $1DB3A – $1E2A7 Sky Base Act 1 $1E2A8 – $1E740 Bridge Act 2 $1E741 – $1EC00 Sky Base Act 2 (Interior) / Sky Base Act 3 $1EC01 – $1EFF5 Jungle Act 3 $1EFF6 – $1F300 Labyrinth Act 3 $1F301 – $1F440 Bridge Act 3 $1F441 – $1FBA0 Special Stage 1 / 2 / 3 / 5 / 6 / 7 $1FBA1 – $1FFFF UNUSED SPACE. Since the levels are compressed, the total space they occupy will vary as they are changed. The final level layouts leave 1'118 bytes of free space. This space contains a repeating copyright message to act as filler.
$20000 – $32FE5	Contains sprite art and/or sprite mappings
$32FE6 – $3DA27	Level Art: Offset Description $32FE6 – $32FED Header For Green Hill Art $32FEE – $330ED Line Pre-load For Green Hill $330EE – $337A7 Line Selection For Green Hill $337A8 – $34576 Art Data For Green Hill $34578 – $3457F Header For Bridge Art $34580 – $3467F Line Pre-load For Bridge $34680 – $34C7F Line Selection For Bridge $34C80 – $35AFF Art Data For Bridge $35B00 – $35B07 Header For Jungle Art $35B08 – $35C07 Line Pre-load For Jungle $35C08 – $36176 Line Selection For Jungle $36177 – $371BE Art Data For Jungle $371BF – $371C6 Header For Labyrinth Art $371C7 – $372C6 Line Pre-load For Labyrinth $372C7 – $37866 Line Selection For Labyrinth $37867 – $3884A Art Data For Labyrinth $3884B – $38852 Header For Scrap Brain Art $38853 – $38952 Line Pre-load For Scrap Brain $38953 – $38FF1 Line Selection For Scrap Brain $38FF2 – $39CED Art Data For Scrap Brain $39CEE – $39CF5 Header For Sky Base 1 / 2 Art $39CF6 – $39DF5 Line Pre-load For Sky Base 1 / 2 $39DF6 – $3A3C8 Line Selection For Sky Base 1 / 2 $3A3C9 – $3B3B4 Art Data For Sky Base 1 / 2 $3B3B5 – $3B3BC Header For Sky Base 3 Art $3B3BD – $3B4BC Line Pre-load For Sky Base 3 $3B4BD – $3BBB5 Line Selection For Sky Base 3 $3BBB6 – $3C7FD Art Data For Sky Base 3 $3C7FE – $3C805 Header For Special Stages Art $3C806 – $3C905 Line Pre-load For Special Stages $3C906 – $3CF53 Line Selection For Special Stages $3CF54 – $3DA27 Art Data For Special Stages
$3DA28 – $40000	Contains sprite art and/or sprite mappings

Game Gear

Offset	Description
$00000 – $10000	Main Game Code, musics, sounds...
$10000 – $14D00	Block Mappings
$14D00 – $15200	Unknown
$15200 – $15600	Blinking Items Art (Box, etc...)
$15600 – $1564A	Level Headers Pointers
$1564A – $15B34	Level Headers
$15B34 – $16200	Sprite Layout
$16200 – $16738	Unknown
$16738 – $17430	Misc Mappings
$17430 – $1F600	Level Layout
$1F600 – $20000	Sega Screen Art / Mappings
$20000 – $23400	Sonic Art / Mappings
$23400 – $23450	Palettes Pointers
$23450 – $23C00	Palettes
$23C00 – $25500	'Sega' sound
$25500 – $30000	Art : Title Screen / Map / Levels / Stats / Sprites
$30000 – $40000	Art : Sega Screen / Map / Levels
$3DA28 – $40000	Contains sprite art and/or sprite mappings

General Compression

Some data (level layouts, misc. mappings) in the ROM is compressed using a form of Run-Length-Encoding, where bytes that repeat themselves many times in a row are reduced by specifying the number of times to repeat a byte.

In practice, the ROM stores this data normally byte-for-byte but indicates a value to repeat by a duplicate byte followed by another byte that specifies the number of successive bytes to repeat.

A bug / oversight made by the developers means that the byte that specifies the number of times to repeat is off-by-one (causing an overflow (255 → 0). This means that a repeating-value of 01 means zero repeats (therefore wasting space!), 02 means 1 repeat, FF means 255 repeats, but 00 actually means 256 repeats! Be keenly aware of this when de/compressing!

Example:

06 7C 32 2E 2E 01 D1 → 06 7C 32 2E 2E D1

6B 3F 3F 02 51 2D C0 → 6B 3F 3F 3F 51 2D C0

05 07 5A 5A 03 08 B2 → 05 07 5A 5A 5A 5A 08 B2

05 07 5A 5A 03 08 B2 → 05 07 5A 5A 5A 5A 08 B2

29 EF 00 00 00 BB D2 → 29 EF 00 00 […00x256…] BB D2

As you can see, this format wastes one byte of space if a byte is only repeated twice. If a byte is repeated three times, there is no size difference. Only when a byte is repeated four or more times is there a savings in space usage.

Level Headers / Level Headers Pointers

Each level has a header. This header contains several pieces of data, such as starting position, music location, etc. Each level header is 37 bytes long and is referenced by a pointer.

Level Headers Pointers

A level header pointer is comprised of two bytes. The data for the pointer is stored in little endian format. The value stored by this pointer specifies the number of bytes away a level header is from $15580 (Master System) / $15600 (Game Gear).

There are 37 Level Headers in total. Some levels have more than one pointer because they can be accessed from different locations, and Sonic's starting position differs at level start. Also two invalid level pointers exist that act as dividers between the main sequence of levels and the out-of-sequence levels (Scrap Brain's sub-maps and the Special Stages). See the ROM map for the full list of level pointers.

Example: The two first bytes are 4A 00. So, at $15580 + $004A, we will find the level header for the first level (Green Hill 1).

Level Headers

Each level header use 37 bytes. Here is the format (partial):

SP TW TW TH TH ?? ?? LW LH ?? ?? ?? ??

SX SY FL FL FS FS ML ML LA LA ?? SA SA

IP ?? CC CP SL SL ?? UW ?? ?? MU

• SP (Solidity Pointer): Indicates what solidity data to use
• TW,TH (Tiles Wide, Tiles High): Rough level dimension, in other words how many tiles wide and high the level is (low byte is first)
• LW,LH (Level Width, Level Height): Real level dimensions (divided by 32), used to limit the view
• SX,SY (Start X, Start Y): Player starting position (divided by 32)
• FL (Floor Location): How many bytes away from $14000 the floor is located (Little Endian)
• FS (Floor Size): Floor data size, in bytes (Little Endian)
• ML (Mappings Location): How many bytes away from $10000 the mappings for this level are located (Little Endian)
• LA (Level Art): How many bytes away from $30002 the level art is located (Little Endian)
• SA (Sprite Art): How many bytes away from ????? is the sprite art located (Little Endian)
• IP (Initial Palette): Indicates the number of the initial palette pointer to use, in the pointer order of appearance (so, indicates the palette)
• CC (Color Cycles): Indicates the number of color cycles for the level palette
• CP (Cyclic Palette): Indicates the number of the cyclic palette pointer to use, in the pointer order of appearance (so, indicates the palette)
• SL (Sprites Location): How many bytes away from $15580 (SMS) / $15600 (GG) the sprite layout is located (Little Endian)
• UW (Under Water flag): Controls the under-water effect (slow movement / water colour / drowning) in Labyrinth (80 is on, 00 is off). Appears to only affect Labyrinth, using this flag on other levels does not create the water effect so either it's hard-coded to Labyrinth or some other undocumented data controls it
• MU (Music):
  • 00–05: Level Music
  • 06: Title Music
  • 08: Invincibility
  • 09: Level Complete
  • 0A: Dead
  • 0B/0C/0D: Boss
  • 0E: Credits
  • 10: Bonus Stage

In the Game Gear version, Level Headers are located at $1564A – $15B34
In the Master System version, Level Headers are located at $155CA

This is a table of the complete level headers for the Master System version: (all values are hexadecimal)

Name	SP	TW	TH	??	??	LW	LH	??	??	??	??	SX	SY	FL	FS	ML	LA	??	SA	IP	??	CC	CP	SL	??	UW	??	??	MU
Green Hill Act 1	0	0100	0010	40	0	C0	18	20	0	40	01	08	0B	2DEA	083E	0000	2FE6	9	612A	0	A	3	0	0534	04	0	20	0	0
Green Hill Act 2		0080	0020	01		A0	0C	01	0	40	03	02	03	3628	0661									0580	04		20
Green Hill Act 3		0080	0020	01		00	0A	E8	0	40	03	07	16	3C89	032D									05C3	00		00
Bridge Act 1	1	0100	0010	01	0	00	1F	01	0	40	01	03	0C	7ED1	0694	0B80	4578	9	6C3D	1	8	3	1	05D6	04	0	20	0	1
Bridge 2		0080	0020				0F	01	0	40	03	02	1C	A2A8	0499									061C	0C		20
Bridge 3		0080	0020				0F	00	3	40	03	06	1B	B301	0140									0656	00		00
Name	SP	TW	TH	??	??	LW	LH	??	??	??	??	SX	SY	FL	FS	ML	LA	??	SA	IP	??	CC	CP	SL	??	UW	??	??	MU
Jungle Act 1	2	0100	0010	01	0	00	1F	01	0	20	01	02	0B	3FB6	0AAC	1480	5B00	9	77CD	2	5	3	2	066C	04	0	20	0	2
Jungle Act 2		0010	0100				01	01	0	20	1F	02	FA	4A62	08DB									06C4	84		20
Jungle Act 3		0080	0020				0F	00	3	40	03	06	1B	B301	0140									0656	00		00
Labyrinth Act 1	3	0040	0040	01	0	00	07	01	0	40	07	02	05	8565	08C2	1E80	71BF	9	83B6	3	5	3	3	0723	04	80	20	0	3
Labyrinth Act 2		0040	0040									03	09	8E27	0D13									0772	04		20
Labyrinth Act 3		0040	0040									03	25	AFF6	030B									07CA	00		10
Name	SP	TW	TH	??	??	LW	LH	??	??	??	??	SX	SY	FL	FS	ML	LA	??	SA	IP	??	CC	CP	SL	??	UW	??	??	MU
Scrap Brain Act 1	4	0100	0010	01	0	00	1E	01	0	20	01	03	0B	533D	069A	2980	884B	9	8F75	4	6	4	4	07E9	04	0	20	0	4
Scrap Brain Act 2		0080	0020				0F	01	0	40	03	04	16	59D7	0904									082C			20
Scrap Brain Act 3		0040	0040				07	20	2	40	07	03	36	7282	08B2									08CE			00
Sky Base Act 1	5	0080	0020	01	0	00	0F	01	0	40	03	02	1D	9B3A	076E	3580	9CEE	9	99E0	5	6	4	5	091A	04	0	22	0	4
Sky Base Act 2	5	0040	0040				07			40	06	0A	17	7B34	039D	3580	9CEE			5	6		8	0954	40		20		5
Sky Base Act 3	7	0040	0040				07			20	01	02	01	A741	04C0	4300	B3B5			6	8		6	0997	00		00		4
End Sequence	0	0100	0010	40	1A	00	1D	E0	0	E8	00	D9	0A	2DEA	083E	0000	2FE6	9	AEB1	0	A	3	0	0A3A	00	0	00	0	FF
Name	SP	TW	TH	??	??	LW	LH	??	??	??	??	SX	SY	FL	FS	ML	LA	??	SA	IP	??	CC	CP	SL	??	UW	??	??	MU
Scrap Brain Act 2 (Emerald Maze), From Corridor	4	0040	0040	01	0	00	07	01	0	40	07	03	3D	62DB	08F8	2980	884B	9	8F75	4	6	4	4	0869	04	0	20	0	4
Scrap Brain Act 2 (Ballhog Area)		0020	0080				03			A0	0A	03	03	6BD3	06AF									08B5
Scrap Brain Act 2, From Transporter		0080	0020				0F			40	03	50	10	59D7	0904									082C
Scrap Brain Act 2 (Emerald Maze), From Transporter		0040	0040				07			40	07	27	1C	62DB	08F8									0869
Scrap Brain Act 2, From Emerald Maze		0080	0020				0F			40	03	7B	03	59D7	0904									082C
Scrap Brain Act 2, From Ballhog Area		0080	0020				0F			40	03	7B	1B	59D7	0904									082C
Sky Base Act 2 (Interior)	7	0040	0040	01	0	00	07	01	0	40	07	03	3B	A741	04C0	4300	B3B5	9	99E0	6	8	4	6	09A1	00	0	00	0	4
Name	SP	TW	TH	??	??	LW	LH	??	??	??	??	SX	SY	FL	FS	ML	LA	??	SA	IP	??	CC	CP	SL	??	UW	??	??	MU
Special Stage 1	6	0040	0040	01	0	00	07	01	0	40	02	02	06	B441	0760	4890	C7FE	9	A511	7	1	1	7	09A5	4	0	21	0	10
Special Stage 2		0040	0040				07	20	3	C0	05	02	1E	B441	0760									09AC
Special Stage 3		0040	0040				07	80	6	40	07	03	3B	B441	0760									09B6
Special Stage 4		0010	0100				01	01	0	20	1F	06	40	4A62	08DB									09C0
Special Stage 5		0040	0040				07	01	0	40	02	02	06	B441	0760									09D3
Special Stage 6		0040	0040				07	20	3	C0	05	02	1E	B441	0760									09E6
Special Stage 7		0040	0040				07	80	6	40	07	03	3B	B441	0760									09FF
Special Stage 8		0010	0100				01	01	0	20	1F	06	04	4A62	08DB									0A15

Level Palettes / Level Palettes Pointers

Level Palettes Pointers

This is an easy section, but reading the Level Headers section may help you a bit. Well, you probably noticed that levels generally have the same palettes for the three acts (the exception is Sky Base). If you use MEKA, you can see that a palette is composed of 32 colors. We can divide this palette into two 16 colors sub-palettes (the first one for colors 1–16, the second one for colors 17–32). When the GG loads a level, it loads the initial palette, which contains 16 colors (colors are applied to the second sub-palette). Then, while you are playing, you can see that there are color cycles. In fact, the GG switch between several palettes, so you think that there is a cycle. These palettes contains only 16 colors, and are applied on the first sub-palette only. All these palettes are all referenced by a pointer, which is 2 bytes long, and little endian. There are 8 pointers (16 bytes, little endian). They point to the level color cycles. The 16 next pointers are still unknown. The 8 next are blank pointers. Finally, The 8 next pointers point to the level initial palette. Pointers represent how many bytes away from $23400 are the palettes located at.

Palettes Pointers are located at: $24000 – $24350

Initial level palettes pointers:

Offset	Description
$23440	Green Hill
$23442	Bridge
$23444	Jungle
$23446	Labyrinth
$23448	Scrap Brain
$2344A	Sky Base 1/2A
$2344C	Sky Base 2B/3
$2344E	Special Stage

Level palettes color cycle pointers:

Offset	Description
$23400	Green Hill
$23402	Bridge
$23404	Jungle
$23406	Labyrinth
$23408	Scrap Brain
$2340A	Sky Base 1/2A
$2340C	Sky Base 2B/3
$2340E	Special Stage

Level Palettes

Now that you know everything about palettes pointers, you can start the palettes. There is just one thing you must remember : initial palettes always contain an half-palette (16 colors). Color cycles can contain a variable quantity of half-palettes (16 colors applied to the first sub-palette). This variable quantity corresponds to the CC parameter in the level header. OK, now, let's start.

The GG uses 9-bit colors, so, each color use two bytes (the first nibble of the second byte is 0, and values for a nibble are set 2 by 2). This color can be defined like that : R(0–7), G(0–7), B(0–7). So, there are only 8 possible values for each primary color. Here are the RGB correspondence : 0=1=0, 2=3=36, 4=5=72, 6=7=109, 8=9=145, A=B=182, C=D=218, E=F=255. So, you can have a maximum of 512 colors.

The format for the two bytes is : GR 0B

Block Mappings

The floor layout for each level is comprised of "blocks": prearranged 4×4 tile (32×32 px) patterns. This reduces the amount of memory required for storing levels ("Floor Layouts") as levels are constructed out of these larger blocks rather than storing every tile seen.

Each block is composed of 16 smaller 8×8 px tiles. The block is 16 bytes, each byte is an index to which tile to display in the Level Art. The bytes are arranged on screen in this order:

And the corresponding data is:

00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F

Block Mappings are stored uncompressed in the ROM. The number of blocks in each mapping is apparently not stored in the ROM, so to determine the number of blocks in each mapping you need to compare the distance between "Mapping Location" Level Headers. For your convenience the size of the mappings are charted below.

Master System Version

Offset	Description
$10000 – $10B80	Green Hill (184 blocks)
$10B80 – $11480	Bridge (144 blocks)
$11480 – $11E80	Jungle (160 blocks)
$11E80 – $12980	Labyrinth (176 blocks)
$12980 – $13580	Scrap Brain (192 blocks)
$13580 – $14300	Sky Base 1-2 (216 blocks)
$14300 – $14980	Sky Base 3 (104 blocks)
$14980 – $14C9F	Bonus Stage (50 blocks)

Game Gear Version

Offset	Description
$10000 – $10BB0	Green Hill (187 blocks)
$10BB0 – $11510	Bridge (150 blocks)
$11510 – $11FE0	Jungle (128 blocks)
$11FE0 – $12A10	Labyrinth (208 blocks)
$12A10 – $135B0	Scrap Brain (186 blocks)
$135B0 – $14330	Sky Base 1-2 (216 blocks)
$14330 – $149B0	Sky Base 3 (104 blocks)
$149B0 – $14D00	Bonus Stage (53 blocks)

Floor Layout

The floor layout is composed of multiple 4×4 tile blocks (See Block Mappings). Each byte indicates the index of the tile to which to use. However, the data is compressed, so you will have to decompress it first (see The compression format). Be careful when compressing and decompressing a level, because it's easy to make it corrupt.

Floor layouts are located at: $16DEA – $1FFFF (Master System) / $17430 – $1F600 (Game Gear) You'll have to use the pointers (Level Headers), in order to determine their starting address.

Sprite Layout

Sprites are special parts of a level. They can move, attack, give you rings, and do a lot of other things, even if they are not visible.

Sprite Layout Data begins with one byte, which is the total number of sprites in the zone plus one. Immediately following is the sprite data itself. Each sprite uses 3 bytes. Here is the format:

OO XX YY

XX and YY are the object's coordinates divided by 32. OO is one of the object listed below. (Both the GG and SMS version share the same Sprite Layout Data)

ID	Description
00	Empty (no object)
01	Super Ring monitor
02	Power Sneakers monitor
03	One-Up monitor
04	Shield monitor
05	Invincibility monitor
06	Chaos Emerald
07	End sign
08	Badnik "Crabmeat" (GH)
09	Wooden platform - Swinging (GH)
0B	Wooden platform (GH)
0C	Wooden platform - Falls when touched (GH)
0E	Badnik "Buzz Bomber" (GH/B)
0F	Wooden platform - Sliding left-right (GH)
10	Badnik "Moto Bug" (GH)
11	Badnik "Newtron" (GH)
12	Robotnik - Green Hill Boss (GH)
16	Flame Thrower (SB)
17	Door - Opens from left only (SB)
18	Door - Opens from right only (SB)
19	Door - Two ways (SB)
1A	Electric sphere (SB)
1B	Badnik "Ball Hog" (SB)
1C	Unknown - Ball from the "Ball Hog" ?
1D	Switch (SB, L, others ?)
1E	Switch Activated Door (SB)
1F	Badnik "Caterkiller" (SB)
21	Bumper - Sliding left-right (Bonus Stage ?)
22	Robotnik - Scrap Brain Boss (SB)
23	Free animal - Rabbit
24	Free animal - Bird
25	Animal Cell
26	Badnik "Chopper" (J, B)
27	Vertical Step - Falling from a waterfall (J)
28	Horizontal Step - Falling from a waterfall (J)
29	Floating Step - Sonic can travel with it (J)
2C	Robotnik - Jungle Boss (J)
2D	Badnik "Yadrin" (B)
30	Passing Clouds (SKYB)
31	Propeller (SKYB)
32	Badnik "Bomb" (SKYB)
33	Large Cannons in Sky Base Act 2 (SKYB)
35	Badnik "Unidus" (SKYB)
37	Rotating cannon (SKYB)
38	Flying platform (SKYB)
39	Spiked wall slowly moving right (SKYB)
3A	Small cannon in Sky Base Act 1 (SKYB)
3B	Flying platform moving up-down (SKYB)
3C	Badnik "Jaws" (L)
3D	Rotating spiked ball (L)
3E	Spear, shifting up-down (L)
3F	Fire ball thrower (L)
40	Water Level Object (L)
41	Bubble Maker (L)
44	Badnik "Burrobot" (L)
45	Platform, move up when touched (L)
46	Electrical Hazard for the Sky Base Boss (SKYB)
48	Robotnik - Bridge Boss (SB)
49	Robotnik - Labyrinth Boss (L)
4A	Robotnik - Sky Base Boss (SKYB)
4B	Zone that makes you fall (like in GH2)
4C	Flipper (Bonus Stage)
4D	Reset!
4E	Balance (B)
4F	Reset!
50	Flower (GH)
51	Box - Starpost
52	Box - Continue
53	Final animation in GH, when Sonic falls on Robotnik (then, goes to the next level)
54	Emeralds animation (on the map), when Sonic has them all (and goes to the next level)
55	Makes Sonic blink for a short time

It seems that no more sprites are defined.

Sprite Layout Data Locations

You can use the Level Headers to determine where are the sprites located at. Or you can use this table.

Zone	Offset (SMS)	Offset (GG)
Green Hill Act 1	$15AB4	$15B34
Green Hill Act 2	$15B00	$15B80
Green Hill Act 3	$15B43	$15BC3
Bridge Act 1	$15B56	$15BD6
Bridge Act 2	$15B9C	$15C1C
Bridge Act 3	$15BD6	$15C50
Jungle Act 1	$15BEC	$15C66
Jungle Act 2	$15C44	$15CC1
Jungle Act 3	$15C8D	$15D04
Labyrinth Act 1	$15CA3	$15D1A
Labyrinth Act 2	$15CF2	$15D5A
Labyrinth Act 3	$15D4A	$15DB2
Scrap Brain Act 1	$15D69	$15DD1
Scrap Brain Act 2	$15DAC	$15E20
Scrap Brain Act 2 (Emerald Maze)	$15DE9	$15E60
Scrap Brain Act 2 (Ballhog Area)	$15E35	$15EAC
Scrap Brain Act 3	$15E4E	$15ECE
Sky Base Act 1	$15E9A	$15F1A
Sky Base Act 2	$15ED4	$15F54
Sky Base Act 2 (Interior)	$15F21	$15FAD
Sky Base Act 3	$15F17	$15FA3
Special Stage 1	$15F26	$15FB1
Special Stage 2	$15F2C	$15FC1
Special Stage 3	$15F36	$15FCB
Special Stage 4	$15F40	$15FD2
Special Stage 5	$15F53	$15FE5
Special Stage 6	$15F66	$15FFE
Special Stage 7	$15F7F	$16014
Special Stage 8	$15F95	$1602A
Final "Cutscene"	$15FBA	$16046

Misc Mappings

This section contains the mappings for some screens. It looks like the Blocks Mappings, except that the 8×8 tiles are written to the screen, so there is more than 16 tiles. Another thing is that they are compressed (see The compression format).

Mappings Offset	Description
$16738 – $16865	Title Screen
$16962 – $163F6	Map 1 Background
$163F6 – $16AFD	Map 1 Foreground

Blinking Items Art

In the game, there are some blinking items. The most common are the box items. You can easily edit them. Each of these items is 16×16, but you must understand that it is composed of four 8×8 tiles. The tiles are displayed from up to down and from left to right. The format is the same as the art in savestates (32 bytes for each 8×8 tile, 128 bytes for the item). For more information, refer to the S1 SMS Savestates Hacking Guide.

Blinking items are located at: $15200 – $15600

Here are the items you can edit :

ID	Description
1	Super Rings
2	Power Sneakers
3	One-Up
4	Shield
5	Invincible
6	Chaos Emerald
7	Starpost
8	Continue

Level Art

This is a quite difficult section, because the level art is compressed in a very strange way. The compression format itself is not so difficult to apply, but learning it is not a piece of cake. First, I'll describe each part of the level art, then I'll give more details.

The level art starts at: $32FE6 (Master System) / $32ED5 (Game Gear)

The level art is composed of 8×8 tiles, which are used by the block mappings. These 8×8 tiles are composed of 8 lines of art data. In other terms, the compression format relies on the fact that these lines can be used by several different tiles. So, these lines will be written to the art data only once. During decompression, the game will use two methods to write the lines to the tile. One is bit-based, but doesn't allow to choose what line of art data to write to the tile; the second one is byte-based, and it allows to choose the line of art data to write to the tile. The game use both methods. It is very important that you understand this paragraph, so read it again.

From what I said, you can guess that there will be 3 sections for each level. But, there is also an 8 bytes header at the beginning (for each level too). Each header is in the format of:

FE FN LS LS AD AD X0 X8

(Note that dual-byte values "LS" and "AD" are Little-Endian, the lowest byte first. e.g. "08 01" = "0108")

• FE: "Forty-Eight". Unknown purpose, but in all headers this value is 48
• FN: "Fifty-Nine". Unknown purpose, but in all headers this value is 59
• LS: "Line Selection". An offset from the start of the level art header, to the where the Line Selection data begins. Since the Line Pre-Load is always 256 bytes, this value is always "0108" (256 + 8 in hexadecimal). Perhaps the programmers provided the ability to use less than 256 tiles, but didn't make use of it
• AD: "Art Data". An offset, from the beginning of the level art header (e.g. $32FE6), to the start of the Art Data section. This is critically important because the length of the Line Selection can vary
• X0: Unknown, but in all headers, this value is "0"
• X8: Unknown, but in all headers, this value is "8"

Here is a table of each of the level art headers together for the Master System:

Zone / Level	Address	FE	FN	LS	AD	X0	X8
Green Hill	$32FE6	48	59	0108	07C2	0	8
Bridge	$3457A	48	59	0108	0708	0	8
Jungle	$35B02	48	59	0108	0677	0	8
Labyrinth	$371C1	48	59	0108	06A8	0	8
Scrap Brain	$3884D	48	59	0108	07A7	0	8
Sky Base 1 / 2	$39CF0	48	59	0108	06DB	0	8
Sky Base 3	$3B3B7	48	59	0108	0801	0	8
Special Stages	$3C800	48	59	0108	0756	0	8

The second section is what I call "Line Pre-load" (the most complex section, containing bit-based compressed data), the third one is "Line Selection" (containing the byte-based compressed data), and the last one is "Art Data".

The Art Data section simply contains the lines of art data. Each line is 8 pixels wide, and uses 4 bytes. Note that there won't be two identical lines of art data here (it's evident if you followed what I said).

Now, the most complex section, the Line Pre-load. It's 256 bytes (one byte for each tile). A 1 byte number can be converted to a 8-bit number. Yeah, 8-bit, and there are 8 lines in a tile. So, each bit will be associated with a line of the tile. The rightmost bit will be associated to the topmost line of the tile. So, the byte will be read from right to left. Note that before starting to load the art, the GG puts a pointer at the begining of the Art Data. While reading the byte, each time the GG meets a 0, it will write the pointed line of art data to the tile, at the position indicated by the position of the 0, and move the pointer to the next line of art data (don't forget that 1 line of art data = 4 bytes). The process repeats until all the bytes were read. As for now, we have 256 partial tiles.

The last section is the Line Selection. As you know, we have partial tiles. We need to put something in the blank lines. For each tile, there is a certain quantity of lines that were not defined. We will call that quantity L. To calculate L, simply count the number of 1's there is in the corresponding byte in the Line Pre-Load section. Well, now you can start reading the data in Line Selection. This section contains line numbers. You must read L line numbers for each tile (remember that L depends on the tile). Each line number indicates a line of art data to write to the tile. These lines must be written were no lines were defined. To determine where to write the lines in the tile, use the positions of the 1's in the corresponding byte in the Line Pre-Load section. The last thing you must know is the format for each line number. This is a bit special. For line numbers 00 to EF (0 to 239), there is only one byte, indicating the line number. However, for tiles after EF, there are two bytes. The first one starts with an F, so the actual line number is the two bytes number minus F0 00. Ex: F3 A2 is for art line number 3A2. Note that you can also write line numbers 00 to EF using two bytes numbers, but this is a waste of space (that's why they are stored with one byte!).

Game Text

Strings of text in the game are stored in a simple format that begins with an X / Y location on the screen to begin printing, are terminated with FF and may contain special control-codes, such as FE to move the cursor.

Note that the character codes used for the letters are dependent on the tile map currently being used! There are two tile maps used, one for each of the map screens in the game.

The first map screen shows the overview of the island and is used for the first three zones (Green Hill, Bridge & Jungle). These are the character codes used on this screen:

A	B	C	D	E	F	G	H	I	J	K	L	M	N	O	P	Q	R	S	T	U	V	W	X	Y	Z	©	␣
34	35	36	37	44	45	46	47	40	41	42	43	50	51	52	60	61	62	70	80	81	54	3C	3D	3E	3F	CF	EB

The second map screen shows the close-up of the mountain top with Robotnik's fortress, and uses the following character codes:

A	B	C	D	E	F	G	H	I	J	K	L	M	N	O	P	Q	R	S	T	U	V	W	X	Y	Z	©	␣
1E	1F	2E	2F	3E	3F	4E	4F	5E	5F	6E	6F	7E	7F	8E	8F	9E	9F	AE	AF	BE	BF	CE	CF	DE	DF	AB	EB

Zone Titles

The zone titles ("GREEN HILL", "BRIDGE", …) shown on the map screen are stored at $0122D – $01286 (Master System). Each title is padded with spaces so that all titles are always 12 bytes long.

The first three zones use the character codes from the first map screen, therefore the data looks like this:

Address	Bytes	Text
$0122D	10 13 46 62 44 44 51 EB 47 40 43 43 EB EB FF	GREEN HILL
$0123C	10 13 35 62 40 37 46 44 EB EB EB EB EB EB FF	BRIDGE
$0124B	10 13 41 81 51 46 43 44 EB EB EB EB EB EB FF	JUNGLE

The next three zones use the second map screen, therefore the letters appear differently:

Address	Bytes	Text
$0125A	10 13 6F 1E 1F DE 9F 5E 7F AF 4F EB EB EB FF	LABYRINTH
$01269	10 13 AE 2E 9F 1E 8F EB 1F 9F 1E 5E 7F EB FF	SCRAP BRAIN
$01278	10 13 AE 6E DE EB 1F 1E AE 3E EB EB EB EB FF	SKY BASE

Final Score Titles

Upon completion of the game, a final score screen is shown and bonuses are awarded for the number of emeralds, lives remaining and a "Special Bonus". These titles are stored at $0197E – $019AD (Master System).

Each string is padded with spaces to be 13 characters in length. The first tile set is used.

Address	Bytes	Text
$0197E	08 0A 36 47 34 61 70 EB 44 50 44 62 34 43 37 FF	CHAOS EMERALD
$0198E	08 0A 70 52 51 40 36 EB 43 44 45 80 EB EB EB FF	SONIC LEFT
$0199E	08 0A 70 60 44 36 40 34 43 EB 35 52 51 81 70 FF	SPECIAL BONUS

Credits Text

The text for the credits that roll at the end of the game is stored at $02095 – $02AD5 (Master System).

Additional control codes appear in the credits:

FE XX YY: Moves the cursor to XX, YY on the screen

FD XX 00: Pauses for XX length. Values 3C, 4B & F0 are used in the credits roll

FC XX: Number of lines to scroll the credits screen

Note that the credits use the second tile map.

Here is a full decode of the credits roll:

$02905	14 03 AE 9E 7F 5E 2E				SONIC
	FE 15 04 AF 4F 3E				THE
	FE 13 05 4F 3E 2F 4E 3E 4F 9E 4E		HEDGEHOG
	FD 3C 00
	FE 12 0C 7E 1E AE AF 3E 9F			MASTER
	FE 13 0D AE DE AE AF 3E 7E			SYSTEM
	FE 14 0E BF 3E 9F AE 5E 9E 7F			VERSION
	FD 3C 00					
	FC 09					
	FE 14 0B AE 9E 7F 5E 2E				SONIC
	FE 15 0C AF 4F 3E				THE
	FE 13 0D 4F 3E 2F 4E 3E 4F 9E 4E		HEDGEHOG
	FD 3C 00					
	FE 12 0F 8E 9F 5E 4E 5E 7F 1E 6F		ORIGINAL
	FE 13 10 2E 4F 1E 9F 1E 2E AF 3E 9F		CHARACTER
	FE 14 11 2F 3E AE 5E 4E 7F			DESIGN
	FD 3C 00					
	FC 04					
	FE 14 10 AB AE 3E 4E 1E				©SEGA
	FD B4 00					
	FC 09
	FE 14 0E AE AF 1E 3F 3F				STAFF
	FD B4 00					
	FC 09
	FE 12 0B 4E 1E 7E 3E				GAME
	FE 13 0C 8F 9F 9E 4E 9F 1E 7E			PROGRAM
	FD 3C 00					
	FE 13 0E AE 4F 5E 7F 9E 1F BE			SHINOBU
	FE 14 0F 4F 1E DE 1E AE 4F 5E			HYASHI
	FD F0 00					
	FC 09					
	FE 12 0B 4E 9F 1E 8F 4F 5E 2E			GRAPHIC
	FE 14 0C 2F 3E AE 5E 4E 7F			DESIGN
	FD 3C 00					
	FE 13 0E 1E DE 1E 7F 9E				AYANO
	FE 14 0F 6E 9E AE 4F 5E 9F 9E			KOSHIRO
	FD 3C 00					
	FE 13 11 AF 1E CF 3E 3F BE 7F 5E		TAKEFUNI
	FE 14 12 DE BE 7F 9E BE 3E			YUNOUE
	FD F0 00					
	FC 09					
	FE 12 0B AE 9E BE 7F 2F				SOUND
	FE 13 0C 8F 9F 9E 2F BE 2E 3E			PRODUCE
	FD 3C 00					
	FE 13 0E 7E 1E AE 1E AF 9E			MASATO
	FE 14 0F 7F 1E CF 1E 7E BE 9F 1E		NAKAMURA
	FD F0 00					
	FC 09					
	FE 12 0B 9F 3E 1E 9F 9F 1E 7F 4E 3E		REARRANGE
	FE 15 0C 1E 7F 2F				AND
	FE 12 0D 9E 9F 5E 4E 5E 7F 1E 6F		ORIGINAL
	FE 16 0E 7E BE AE 5E 2E				MUSIC
	FD 3C 00					
	FE 13 10 DE BE DF 9E				YUZO
	FE 14 11 6E 9E AE 4F 5E 9F 9E			KOSHIRO
	FD F0 00					
	FC 09					
	FE 13 0D AE 8F 3E 2E 5E 1E 6F			SPECIAL
	FE 15 0E AF 4F 1E 7F 6E AE			THANKS
	FD B4 00					
	FC 02					
	FE 13 0E DE 8E AE 4F 5E 8E EB DE		YOSHIO Y
	FD 3C 00					
	FE 13 11 6F BE 7F 1E 9F 5E 1E 7F		LUNARIAN
	FE 1A 12 AE 4E					SG
	FD B4 00					
	FC 09					
	FE 12 0C 8F 9F 3E AE 3E 7F AF 3E 2F		PRESENTED
	FE 16 0E 1F DE					BY
	FE 15 10 AE 3E 4E 1E				SEGA
	FD B4 00					
	FE 19 13 3E 7F 2F				END
$02AD5	FF

RAM Editing

To convert offset values to Gens Plus 0.0.9.61 savestate values, add $00A4.

Offset	Description
$0000	Tile Layout Data
$1400	Object Status Table (?)
$1403	Sonic's X speed
$1406	Sonic's Y speed
$202C	Palette
$206C	Tile Graphics Data

Tile Layout Data

Unlike in the ROM, the data here is uncompressed. Each byte directly corresponds to one 32×32 tile.

The number of tiles in a horizontal line before they wrap around differs depending on the width of the Zone. Here is table of the values.

Zone	Tile Line Data Size in Bytes
Green Hill Act 1	256
Green Hill Act 2	128
Green Hill Act 3	128
Bridge Act 1	256
Bridge Act 2	128
Bridge Act 3	128
Jungle Act 1	256
Jungle Act 2	16
Jungle Act 3	64
Labyrinth Act 1	64
Labyrinth Act 2	64
Labyrinth Act 3	64
Scrap Brain Act 1	256
Scrap Brain Act 2	128
Scrap Brain Act 2 (Emerald Maze)	64
Scrap Brain Act 2 (Ballhog Area)	32
Scrap Brain Act 3	64
Sky Base Act 1	128
Sky Base Act 2	64
Sky Base Act 2 (Interior)	64
Sky Base Act 3	64

Music Hacking

Music Locations

• Music Pointers (These add to $8000): $C716
  • Specifics...

Song ID	Location (SMS)	Location (GG)	Song
00	C7D0	C7D0	Green Hill Zone
01	D74A	D74A	Bridge Zone
02	D24A	D24A	Jungle Zone
03	F60C	F60C	Labyrinth Zone
04	DB4F	DB4F	Scrap Brain Zone
05	E1A7	E1A7	Sky Base Zone
06	E4C3	E4C3	Title Screen
07	E63C	E63C	Map Screen (Act Intro)
08	E704	E704	Invincible
09	E8B4	E8B4	Act Complete
0A	E991	E991	Death
0B	EAC0	EAC0	Boss Theme
0C	EAC0	EAC0	Boss Theme
0D	EAC0	EAC0	Boss Theme
0E	ED54	ED54	Ending
0F	C7D0	C7D0	Green Hill Zone
10	F12C	F12C	Bonus Zone
11	C7D0	C7D0	Green Hill Zone
12	C7D0	C7D0	Green Hill Zone
13	F98C	F98C	Unused (some SFX)
14	FA26	FA32	Chaos Emerald

The only difference in the music section of the SMS and GG version is, that the GG version of song 13 has an additional 83 (vibrato) command.

Song Format

All pointers in a song are absolute (i.e. relative to the beginning of the song) 16-bit pointers stored in Little Endian (Intel) format.

Song Header

Offset	Size	Description
00	Word	Pointer to Channel 1 Data (usually points to 000A)
02	Word	Pointer to Channel 2 Data
04	Word	Pointer to Channel 3 Data
06	Word	Pointer to Channel 4/Noise Data
08	Word	unused (always zero)

Channel Data

There's no header for channel data. Channel setup is done via commands.
The commands start directly at the offset specified in the header.

Command	Description
00–7F xx	Play Note with length xx if dd is 0, the value set by 8A is used Note Values: Tone: first nibble is the octave, second nibble is the key Key is a signed value with A = -3 .. C = -0, C = +0 .. B = +11 (-0 is the same C as +0) Noise: 70 - Bass Drum, 71 - Snare Drum, others are maybe used for SFX 7F - Rest
80 aaaa bbbb	Set song tempo (only present in channel 1) aa = Tempo Divider bb = Tempo Multipler aa gets copied to $DC08 and bb gets copied to $DC0A and $DC0C collecting a Speed Shoe increases the Tempo Multipler at $DC0A by 1 BPM = 120 * aa / bb (120 for 30 Ticks per Quarter)
81 aa	Set volume (0F = max, 00 = min)
82 aa bb cc dd ee ff	Envelope Setup (only present in channels 1-3) aa = attack rate (usually FF) bb = decay 1 rate (00 - slow, FF - fast) cc = decay 1 level (00 - min, FF - max) dd = decay 2 rate ee = decay 2 level ff = decay 3 rate
83 aa bb cc dd ee	Vibrato/Pitch Bend (only present in channels 1-3) aa = Delay bb = Speed cc = Step Count + 1 Note: Step 0 (dd=01) stops instead of reversing the counter's direction dd = Change per Step ee = Vibrato Type (only the 4 lower bits are used) Note: some vibrato types sound really weird, so it seems to be intended for SFX only and it's best to leave it at zero
84 aaaa	Set Detune value to aaaa (aaaa is a signed byte that modifies the FNum) positive value = higher FNum = lower note
85 aa	Unknown (only present in channel 1) aa is always FF (with very few excpetions)
86	Loop Start Marker increases Loop Stack Pointer
87 aa bbbb	Loop back aa - number of times to repeat bbbb - Position to loop back to (usually points to the byte after the 86 command) Note: When leaving the loop, the Loop Stack Pointer is decreased.
88	Master Loop Start sets the master loop address for this channel
8A aa	Set default note length to aa
8B	Increase volume by 1
8C	Decrease volume by 1
8D	no attack for next note
FF	Track End When this command is enountered and the master loop address is set, the driver jumps back to the address set by the 88 command.

Note on 86/87

The 86 command is used the make nested loops possible.
Example: 86 86 ... 87 04 aaaa ... 87 02 bbbb

Note on 88/FF

There's no need for an 88 command in non-looping songs.
However every jingle (with exception of the Death-Jingle) has a master loop, consisting of a rest.