Sonic the Hedgehog 2 (Master System)

(Original content by Hivebrain)

ROM Contents

Offset	Description
A4C-DEA	Level select text (plain ASCII format)
E1C-E4B	Level select level pointers
28F0	Pointers for title card mappings (16-bit, little endian)
2904-2FA7	Mappings for title cards & act results screens
55E5	Pointers for level headers (16-bit, little endian)
5635-585A	Level headers
10000	Uncompressed art - Sonic
13800	Compressed Art - bubble (ALZ)
13930	Compressed Art - jumping pig enemy (GMZ)
13BD0	Compressed Art - small yellow numbers (unused)
13CF8	Compressed Art - spinning turtle enemy (SHZ/SEZ/CEZ)
14000	Uncompressed art - Sonic gliding (SHZ)
15E00	Uncompressed art - Sonic riding a minecart (UGZ/GMZ)
18000	Uncompressed art - Sonic runnning round a loop (ALZ/GHZ)
1BC00	Compressed Art - spinning wheel (CEZ)
1BE60	Compressed Art - block (CEZ)
1C000	Compressed Art - numbers, lives icon
1C2CA	Compressed Art - collected ring sparkles
1C306	Compressed Art - invincibility sparkles
1C332	Compressed Art - explosion
1C462	Compressed Art - monitor (generic)
1C608	Uncompressed art - monitors (specific)
1CCC8	Compressed Art - rings (dropped when Sonic gets hit)
1CE98	Compressed Art - moving platform (GHZ)
1CF92	Compressed Art - glider (SHZ)
1D20E	Uncompressed art - prison capsule
1DACE	Uncompressed art - animals
1E0CE	Compressed Art - cloud & tree from scrolling text on title cards
1E124	Uncompressed art - end of act signpost
1EDA4	Compressed Art - minecart (UGZ/GMZ)
1EE48	Uncompressed art - minecart wheel (UGZ/GMZ)
1EEC8	Compressed Art - minecart wheel (UGZ/GMZ)
1F040	Compressed Art - broken minecart (UGZ/GMZ)
1F0EA	Uncompressed art - Silver Sonic (SEZ)
1F96A	Compressed Art - log (SHZ)
1F9B4	Compressed Art - moving platform (SHZ)
1FAB4	Empty
20000	Compressed Art - circle & floor from intro sequence
206A0	Compressed Art - beta title screen (unused)
213B0	32x32 mappings - intro sequence
216E0	32x32 mappings - title screen
21A32	Level layout - intro sequence (uncompressed)
21AB2	Level layout - title screen (uncompressed)
21B32	Uncompressed art - mini Sonic from intro sequence
22832	Compressed Art - scenery from intro sequence
229A2	Compressed Art - Sonic's hand from title screen
22C32	Compressed Art - Tails' face from title screen
22DA2	Mappings for title screen
2307C	Unknown
23C72	Empty
24000	Code
2411F	Pallete - unknown (4 shades of grey)
2412F	Pallete - unknown
2413F	Pallete - unknown
2414F	Pallete - title cards
2415F	Pallete - Sonic (GHZ)
2416F	Pallete - Sonic (SHZ1/3)
2417F	Pallete - Sonic (SHZ2)
2418F	Pallete - Sonic (ALZ1/3)
2419F	Pallete - Sonic (ALZ2)
241AF	Pallete - Sonic (UGZ)
241BF	Pallete - Sonic (GMZ)
241CF	Pallete - Sonic (SEZ)
241DF	Pallete - Sonic (CEZ1/2)
241EF	Pallete - Sonic (CEZ3)
241FF	Pallete - UGZ
2420F	Pallete - SHZ1/3
2421F	Pallete - SHZ2
2422F	Pallete - ALZ1/3
2423F	Pallete - ALZ2
2424F	Pallete - GHZ
2425F	Pallete - GMZ
2426F	Pallete - SEZ
2427F	Pallete - CEZ1/2
2428F	Pallete - CEZ3
2429F	Pallete - intro sequence
242AF	Pallete - title screen
242BF	Pallete - intro sequence
242CF	Pallete - title screen
242DF	Pallete - unknown
242EF	Palletes (7) - title cards (UGZ, SHZ, ALZ, GHZ, GMZ, SEZ, CEZ)
2435F	Pallete - unknown
2436F	Pallete - Sonic (ending)
2437F	Pallete - unknown
2438F	Palletes (8) - ending (gradually changes from day to night)
2440F	Pallete - blank
2441F	Loop XY motion data (ALZ/GHZ)
24D1E	Compressed Art - level select text
2511E	Compressed Art - text from title cards
260A4	Compressed Art - UGZ level tiles
270DC	Compressed Art - yellow bird (SHZ)
2715C	Compressed Art - spears (ALZ)
2724E	Compressed Art - moving platform (GMZ)
2724E	Compressed Art - moving platform (SEZ)
273E8	Compressed Art - newtron (UGZ/SHZ/GHZ)
27654	Compressed Art - bomb enemy (GMZ/SEZ)
27782	Compressed Art - moving platform (UGZ)
27870	Compressed Art - crab (UGZ/ALZ/GHZ/SEZ/CEZ)
27B90	Compressed Art - splash (ALZ)
27BE0	Compressed Art - water (ALZ)
27C10	Compressed Art - falling spike (UGZ)
27CB0	Compressed Art - falling spike (GMZ)
27D50	Compressed Art - bubble (ALZ)
27E51	Empty
28000	Compressed Art - SHZ1/3 level tiles
291DC	Compressed Art - SHZ2 level tiles
2A2AC	Compressed Art - GMZ level tiles
2B2E2	Compressed Art - text from ending sequence
2B812	Empty
2C000	32x32 mappings - SHZ1/3
2D8C0	32x32 mappings - SHZ2
2F080	Level layout - CEZ1 (compressed)
2FA48	Empty
30000	32x32 mappings - GHZ
316C0	32x32 mappings - ALZ
32F80	Level layout - SHZ3 (compressed)
33716	Level layout - GMZ3 (compressed)
34000	Level layout - GHZ1 (compressed)
34C6B	Level layout - GHZ2 (compressed)
35884	Level layout - GHZ3 (compressed)
36429	Level layout - SHZ1 (compressed)
370D8	Level layout - SHZ2 (compressed)
38000	Level layout - ALZ1 (compressed)
38F68	Level layout - ALZ2 (compressed)
39DFF	Level layout - ALZ3 (compressed)
3ADBB	Level layout - UGZ1 (compressed)
3C000	32x32 mappings - ALZ2
3DD80	32x32 mappings - UGZ
3F380	Compressed Art - "Game Over" text
3F75A	Compressed Art - numbers 0 to 9 in scrolling GHZ style
3FB7E	Mappings for Game Over screen
3FC5E	Empty
40000	Uncompressed art - Sonic (mirrored)
43680	Compressed Art - little wavy lines (?)
436C0	Compressed Art - fireball (UGZ)
43722	Compressed Art - mini flame (UGZ)
43740	Compressed Art - lobster enemy (ALZ)
439A0	Compressed Art - burrobot (GMZ)
43C82	Compressed Art - moto bug (UGZ/GHZ)
43E28	Compressed Art - fish enemy (CEZ)
43EDA	Empty
44000	Uncompressed art - Sonic gliding (mirrored)
45E00	Uncompressed art - Sonic riding a minecart (mirrored)
48000	Uncompressed art - Sonic running round a loop (mirrored)
4BC00	Compressed Art - block (SEZ)
4BC80	Compressed Art - moving platform (CEZ)
4BD46	Compressed Art - levitating spiked rock (CEZ)
4C000	Level layout - GMZ1 (compressed)
4CC68	Level layout - GMZ2 (compressed)
4DA91	Level layout - SEZ1 (compressed)
4E9C9	Level layout - SEZ2 (compressed)
4F76E	Compressed Art - Tails & Eggman from intro sequence
4FC2A	Empty
50000	Compressed Art - SEZ1/3 level tiles
5102A	Compressed Art - SEZ2 level tiles
52102	Uncompressed art - UGZ boss
52E42	Compressed Art - "END" from ending sequence
53002	Compressed Art - Tails running in ending sequence
53850	Compressed Art - Tails looking up in ending sequence
53960	Uncompressed art - chaos emeralds
54000	Compressed Art - ALZ1/3 level tiles
550F0	Compressed Art - ALZ2 level tiles
56116	Compressed Art - CEZ1/2 level tiles
56E7E	Compressed Art - CEZ3 level tiles
58000	32x32 mappings - GMZ
59960	32x32 mappings - SEZ1/3
5AF20	32x32 mappings - ending sequence / original GHZ
5B900	Empty
5C000	32x32 mappings - SEZ2
5D760	32x32 mappings - CEZ3
5E000	Compressed Art - GHZ level tiles
5F11B	Level layout - UGZ2 (compressed)
60000	32x32 mappings - CEZ1/2
61600	Compressed Art - title screen
6299A	Compressed Art - Eggman final boss (CEZ3)
6339C	Compressed Art - cloud (CEZ3)
633CC	Empty
64000	Compressed Art - UGZ title card pic
649A0	Compressed Art - SHZ title card pic
65410	Compressed Art - ALZ title card pic
65DC0	Compressed Art - GHZ title card pic
66446	Compressed Art - GMZ title card pic
66C26	Compressed Art - SEZ title card pic
670A6	Compressed Art - CEZ title card pic
67806	Mappings for title card pics
67F06	Empty
68000	Uncompressed art - GMZ boss
68980	Uncompressed art - SHZ boss
69740	Uncompressed art - ALZ boss
6A0C0	Uncompressed art - GHZ boss
6B340	Compressed Art - rock (SHZ)
6B460	Compressed Art - fragment of a smashed block (all zones)
6B4EA	Uncompressed art - Tails (CEZ3)
6C000	Level layout - UGZ3 (compressed)
6C6D3	Level layout - SEZ3 (compressed)
6D1F5	Level layout - CEZ2 (compressed)
6DE81	Level layout - CEZ3 (compressed)
6DFD7	Level layout - good ending sequence (compressed)
6E151	Level layout - bad ending sequence (compressed)
6E2C2	Compressed Art - original GHZ level tiles from ending sequence
6F719	Empty
70000	Unknown
75C00	Uncompressed art - rings (UGZ)
75F00	Uncompressed art - rings (SHZ1/3)
76200	Uncompressed art - rings (SHZ2)
76500	Uncompressed art - rings (ALZ)
76800	Uncompressed art - rings (GHZ)
76B00	Uncompressed art - rings (GMZ)
76E00	Uncompressed art - rings (SEZ)
77100	Uncompressed art - rings (CEZ)
77400	Unknown
77552	Credits (plain ASCII format)
77732	Empty
78000	Unknown
78135	Pointers for sprite layouts (16-bit, little endian)
78173	Sprite layout - UGZ1
782C1	Sprite layout - UGZ2
7840F	Sprite layout - UGZ3
7848E	Sprite layout - SHZ1
784E9	Sprite layout - SHZ2
78568	Sprite layout - SHZ3
78584	Sprite layout - ALZ1
78666	Sprite layout - ALZ2
787BD	Sprite layout - ALZ3
787F4	Sprite layout - GHZ1
788C4	Sprite layout - GHZ2
78970	Sprite layout - GHZ3
78983	Sprite layout - GMZ1
78A4A	Sprite layout - GMZ2
78AFF	Sprite layout - GMZ3
78B24	Sprite layout - SEZ1
78BE2	Sprite layout - SEZ2
78C7C	Sprite layout - SEZ3
78C8F	Sprite layout - CEZ1
78D20	Sprite layout - CEZ2
78D8D	Sprite layout - CEZ3
78D97	Unknown
7AF41-7AF49	Cycling palette - UGZ lava
7AF4A-7AF52	Cycling palette - SHZ2 rain
7AF53-7AF5B	Cycling palette - ALZ water
7AF5C-7AF64	Cycling palette - GMZ wheels and conveyor belts
7AF9F-7AFCE	Cycling palette - CEZ3 lightning

Level headers

Each level header consists of 22 bytes:

AA BBBB CC DDDD EEEE FFFF GGGG HHHH IIII JJJJ KKKK LLLL

• AA = 32x32 mapping bank number
• BBBB = 32x32 mappings pointer
• CC = Level layout bank number
• DDDD = Level layout pointer
• EEEE = Level width in mapping blocks
• FFFF = 2's complement level width in blocks
• GGGG = Vertical offset into layout data. This is always 7 * EEEE.
• HHHH = Minimum camera x coordinate
• IIII = Minimum camera y coordinate
• JJJJ = Maximum camera x coordinate
• KKKK = Maximum camera y coordinate
• LLLL = Pointer to table of multiples of level width.

The EEEE and FFFF words are used by collision detection routines to move backwards and forwards in the level data array, whole rows at a time (e.g. for checking collisions above and below an object). The LLLL field is used to calculate the address of any given row of mappings in the level data array.

ROM addresses for the level headers:

Offset	Level
5635, 564B, 5661	GHZ1/2/3
5677, 568D, 56A3	SHZ1/2/3
56B9, 56CF, 56E5	ALZ1/2/3
56FB, 5711, 5727	UGZ1/2/3
573D, 5753, 5769	GMZ1/2/3
577F, 5795, 57AB	SEZ1/2/3
57C1, 57D7, 57ED	CEZ1/2/3
5803	Bad Ending
5819	Good Ending
582F	Intro
5845	Title Screen

Level select pointers

The pointers define which level the game goes to for each item on the level select. Each pointer consists of two bytes: one for the zone number, and one for the act number. The level numbers are as follows:

Value	Zone
00??	Underground
01??	Sky High
02??	Aqua Lake
03??	Green Hills
04??	Gimmick Mt.
05??	Scrambled Egg
06??	Crystal Egg
0700	Bad Ending Sequence
0701	Good Ending Sequence
0900	Intro Sequence
0901	Title Screen

The act numbers are just 00, 01 and 02. There are a total of 18 entries in the pointer array.

Title card mappings

The title cards are built up with rows of 8x8 tiles. Mappings define which tiles are pulled from the VRAM and displayed on the screen. Each 8x8 tile is given a 2-byte, little endian value based on where it appears in the VRAM:

So, the letter A would mapped like so:

0111	0211
0C11	0D11

Spaces not containing any letters are 7011. Here are the ROM addresses for each title card:

Offset	Level
2904	GREEN HILLS
298C	ZONE
29C4	AQUA LAKE
2A34	UNDERGROUND
2AA4	SKY HIGH
2B14	GIMMICK MT.
2B84	MOUNTAIN
2BF4	SCRAMBLED EGG
2C64	CRYSTAL EGG
2CE4	SONIC HAS
2D54	PASSED
2D98	RING
2DAC	TIME
2DC0	SCORE
2DD6	100X
2DE5	00
2DF6	00
2E04	ACT 1
2E90	ACT 2
2F1C	ACT 3

Palletes

Master System palettes contain 16 colours, and each colour is defined by a single byte. Although you might think this means the colour format is 8-bit, it's actually only 6-bit, which gives a total of 64 possible colours. The colours are stored in the ROM as a value between 00 and 3F:

32x32 Mappings

First of all, the mappings begin with an offset index containing 256 16-bit, little endian pointers. The pointers are relative to a fixed position in the ROM, which varies depending on which set of mappings you're looking at. Immediately after the offset index is the mappings data itself. Each 32x32 tile is composed of 16 8x8 tiles. A 16-bit, little endian value is used to define which 8x8 tile is pulled from the VRAM, and also how that 8x8 tile is displayed. Adding certain amounts to the value cause the 8x8 tile to be flipped/mirrored:

Value added	Effect
+0000	None
+0002	Flip horizontally
+0004	Flip vertically
+0006	Flip both ways

Level layout and compression

Each byte in the level layout represents a 32x32 tile. The level is composed of many 32x32 tiles. In a savestate, the level is easy to edit because it's simple and uncompressed. However, in the ROM it's usually compressed.

Uncompressed: 2020 2020 2020

--->

Compressed: FD06 20

FD means compression has occurred. 06 is the number of times a byte has been repeated. 20 is the byte which has been repeated.

Strangely, 1515 (for example) appears in the ROM as FD02 15, which wastes space. Repeated strings of bytes can be used in the ROM, and will appear correctly in the game. Compression is entirely optional, given that there is enough space in the ROM for the data.

Object Layout

The properties of each object is determined by 9 bytes, which follow this format:

TT XXXX YYYY ZZ SS VVWW (FF)

• TT = object type
• XXXX = horizontal position
• YYYY = vertical position
• ZZ = per-object flags
• SS = object parameters
• VV = tiles to read from the VRAM
• WW = tiles to read from the VRAM when the object is reversed
• FF = end of sprite list (only appears once)

Object types and parameters:

TT SS	Description
00	Nothing
01	Sonic
02	Collected ring sparkle
03	Power Sneakers stars
04	Fragment of a smashed block
05	Invincibility stars
06	Special object to hide ring counter & timer
07	Fragment of smashed block (different logic to 04)
08	Dropped ring
09	Air timer countdown
0A	Bubbles (ALZ)
0B	Splash of water (ALZ)
0D-0E	Fireball (UGZ)
0F	Explosion
10	Monitors: 00 - Blank 01 - Blank 02 - Rings 03 - Extra life 04 - Speed shoes 05 - Invincibility 06 - Running Sonic (extra continue) 07 - Question mark (does nothing) 08 - Lead shoes (does nothing) 09 - Arrow (does nothing)
11-14	Chaos emerald
15-18	End level signpost
19	Falling spike with wrong mappings
1A	Unknown
1B	Falling spike (UGZ)
1C	Clouds (intro sequence)
1D	Clouds & cycling palette (intro sequence)
1E	Tree (intro sequence)
1F	Invalid object
20	Platforms: 00 - Moves side-to-side 01 - Falls when you stand on it 02 - Swinging
21	Platforms: 00 - Rises when you stand on it 01 - Rises when you stand on it, then reappears in its original position 02 - Moves side-to-side 03 - Moves right when you stand on it 04 - Doesn't move
22 XX	Platform: moves side-to-side, XX = distance moved
23	Crab enemy
24	Fireballs that the crab enemy spits out
25	Yellow bird (SHZ)
26	Spears (ALZ): 00 - Moves upwards 01 - Moves upwards 02 - Moves downwards 03 - Moves left 04 - Moves right
27	Glider (SHZ)
28	Flywheel (GMZ)
29	Minecart: 00 - Doesn't move 01 - Doesn't move 02 - Moves right 03 - Moves left 04 - Explodes when you touch it
2A XX	Flywheel: moves side-to-side, XX = distance moved (CEZ)
2B	Spiked crystal (CEZ)
2C	Falling block (CEZ)
2D	Falling log (SHZ)
2E	Unknown
2F	Unknown
30	Makes Sonic flash
31	Makes Sonic flash
32	Spinning turtle enemy (SHZ)
33	Lobster enemy (ALZ)
34	Moto bug enemy
35	Newtron enemy
36	SHZ boss - mini robot birds
37	Fish enemy (CEZ)
38	Burrobot enemy (GMZ)
39	Bomb enemy
3A	Pig enemy (GMZ)
3B-3F	Unknown
40	Prison capsule
41	Animals
42	GHZ boss - sumo robot
43	Unknown
44	GMZ boss - pig robot
45	GMZ boss - falling rocks
46	SHZ boss - mini robot birds
47	SHZ boss - eggs
48	SHZ boss - big bird robot
49	SHZ boss - fireball
4A	ALZ boss - seal robot
4B	UGZ boss - Eggman
4C	UGZ boss - cannonball
4D	UGZ boss - pincers thing
4E	SEZ boss - Silver Sonic
4F	ALZ boss - orange bouncing ball
50	Sonic's hand (title screen)
51	Tails' face (title screen)
52	Unknown
53	CEZ boss - floating cluster of yellow balls
54	CEZ boss - zigzag thing that crawls across the floor
55	CEZ boss - Eggman
56-5C	Unknown
5D	Tails (CEZ)
5E	Unknown
5F	Tails & Eggman (intro sequence)

Loop XY motion data

When Sonic hits a loop in either Aqua Lake or Green Hills Zone, you're momentarily unable to control him. The loop motion data determines how Sonic moves vertically and horizontally when he reaches a loop. By default, he moves in a large circle. Modifying the loop data could allow Sonic to move around virtually any shape.

Little endian things

This means 1234 becomes 3412, and 5500 becomes 0055. It's a feature of Master System ROMs which makes hacking them slightly more confusing. Pointers have to be read backwards.