Nem s2

This historical hacking document is preserved here for archival purposes.
It has not been revised since its original writing and may be outdated. For an SCHG-equivalent document, see SCHG:Sonic the Hedgehog 2 (16-bit).

Introduction

Here are my hacking notes on the Sonic 2 rom. If you use these notes I'd appreciate it if you would mention it with your hack or your utility, so that other people can find their way here. For hex editing I recommend you use a hex utility called Hex workshop, and for emulators I recommend you get both Gens and Genecyst. Genecyst may be old and kinda crap, but it has a whole heap of debug outputs that really help when it comes to seeing exactly what it's doing and when.

First of all it's very important that you understand the basics. All data stored on a computer is in the form of 1's and 0's. On a CD for example, a laser hits the surface, and if the laser bounces back and hits the lens it's a 1, and if it doesn’t it's a zero. Each 1 or 0 is called a bit, and a bit cannot have any other characters in it other that a 1 or a 0. Now the computer deals with bits in groups of 4. There are 16 possible combinations for a group of four 1's and 0's, so to make it simpler it deals with it as one value, rather than 4 (Eg. 0110 becomes 6). Now as there are 16 possible combinations for a group of 4 bits, this value to represent their values must have 16 values itself, so rather than a simple 0-9, this value is 0-F (0123456789ABCDEF). This value is called a hexadecimal value (hex value for short). Each hex value is dealt with in groups of 2, called a byte, each byte having 128 possible combinations. Now on a final output level the byte may be looked up on an ASCII table, which will convert that value into a recognisable character (Eg. a byte value of 73 becomes a lowercase s on an English ASCII table). You will practically never touch the ASCII version of the code in hacking though.

Now one important thing to realise is that as one character of hex has 16 values and a decimal (real) value only has 10, it may be necessary to convert the numbers between them from time to time. This is done with the use of a base converter (included in hex workshop). Let's say you wanted to give Sonic 50 rings. If you enter 50 as the value, you will in fact end up with 80, because that value you are entering is actually a hex value, but if you use the base converter to convert it first, you merely enter the value of 50 into the decimal box, and it will spit out a hex value of 32, which will in fact give you 50 rings in the game. Another useful utility that you will need is a hex calculator (also included in hex workshop). A hex calculator is the same as a normal calculator, but it deals with hex values rather than decimal values.

Another thing you need to know is that each level in Sonic 2 has a value assigned to it, but this value does not correspond with each level's final position in the game. Here is a list of the level values in Sonic 2:

00	Emerald Hill zone
04	Metropolis zone
05	Metropolis zone act 3
06	Wing fortress zone
07	Hill Top zone
08	Hidden Palace zone
0A	Oil Ocean zone
0B	Mystic Cave zone
0C	Casino Night zone
0D	Chemical Plant zone
0E	Death Egg zone
0F	Aquatic Ruin zone
10	Sky Chase zone

One other thing you need to know is the way that the Mega Drive stores all the art. All the art that is used in the game is stored in the form of 8x8 pixel blocks. These blocks do not actually store colours at all, they actually only have one hex value per pixel. That value specifies what point on the palette line the pixel will get its colour from. The palette has 4 lines, each with 16 colours on them. Now the colours on the palette can be changed at any point during play, and some palette colours may even automatically change colour each couple of frames to make it look like the colour is flashing.

Now these 8x8 blocks are not what make up the level directly. 4 8x8 blocks are grouped together to form a 16x16 block, and it is at this point that the palette line to use for that 16x16 block is specified. The 8x8 patterns can also have their x, y, or x and y values reversed when placing them in a 16x16 block. Also it's at the 16x16 level that the collision definitions are specified. Now finally we get to a 128x128 block, and these are the things that the actual level info loads. It is made up of 64 16x16 blocks, and each block inside them can use a different palette line. You cannot place anything except a sprite or a 128x128 block directly into a level.

Now that you know all the basics, here's my breakdown:

Address listing

For the most part, there are no breaks in this list. If one address follows on directly from another on this list, it does so in the rom, with the exception of data that comes before 40000. All the compiled code that is used in the game is stored in this section, and that's a pain in the ass to sort through, so there will be many breaks there. The column on the left lists it's file location, the column in the middle contains it's name and description, and the column on the right will contain any special notes about it, such as compression format used if applicable, and number of blocks an art tile uses. The right column will also contain a link with the text further info, if that particular block requires it. That link will jump to a detailed explanation of that thing. Anything linked like that will also be listed in the contents at the top of the page.

0-FF	Vector table
100-200	Header
14DE-????	Programming to do wih decompression of art tiles to VRAM	Compiled
2782-28C1	Palette pointers
28C2	Sega screen palette (00)
2942	Title screen palette (01)
2962	Unknown palette (02)
29E2	"Sonic and Miles" screen palette (03)
2A22	EHZ palette (04, 05, 07, 0D)
2A82	WZ palette (06)
2AE2	MTZ palette (08, 09)
2B42	WFZ palette (0A)
2BA2	HTZ palette (0B)
2C02	HPZ palette (0C)
2C62	HPZ underwater palette (15)
2CE2	OOZ palette (0E)
2D42	MCZ palette (0F)
2DA2	CNZ palette (10)
2E02	CPZ palette (11)
2E62	CPZ underwater palette (16)
2EE2	DEZ palette (12)
2F42	ARZ palette (13)
2FA2	ARZ underwater palette (17)
3022	SCZ palette (14)
3082	Unknown palette (19)
30A2	Unknown palette (1A)
30C2	OOZ boss palette (25)
30E2	Menu palette (26)
3162	Main special stage palette (18)
31C2	Special stage 1 palette (1B)
31E2	Special stage 2 palette (1C)
3202	Special stage 3 palette (1D)
3222	Special stage 4 palette (1E)
3242	Special stage 5 palette (1F)
3262	Special stage 6 palette (20)
3282	Special stage 7 palette (21)
32A2	Unknown palette (22)
32C2	Unknown palette (23)
32E2	Unknown palette (24)
3302	Unknown palette (27)
3EA0-3EB0	Music playlist for levels	Further info
4948-498B	Pointers to demo file to use for levels
49E8-4A70	Pointers to collision indexes	Further info
6FC0	??????? (sprite)
70F0	??????? (sprite)
7356	??????? (sprite)
7A68	Continue text (sprite)
7B82	Sprite of sonic lying down from continue screen (sprite)
80BE	??????? (sprite)
9454-947B	Level select level order	Further info
A1D6	Cut scene at end of game (sprite)
A3C8	??????? (sprite)
A894	??????? (sprite)
A988	??????? (sprite)
A9F2	??????? (sprite)
AAAE	??????? (sprite)
C054-C14F	Level size array	Further info
C150-C1CF	Code for initial character and screen placement	Compiled
C1D0-C257	Character start location array	Further info
C4FC	Offset index of rasterised layer deformation	Further info
C51E	Unknown (possibly title screen)	Further info
C57E	EHZ layer deformation info	Further info
C682	Unknown (splitscreen, EHZ?)	Further info
C7F2	HTZ layer deformation info	Further info
C7BA	Unknown (level value 02)	Further info
C7F2	Unknown (identical to WFZ)	Further info
C82A	WFZ layer deformation info	Further info
C8CA	Unknown (identical to HTZ)	Further info
C964	HTZ layer deformation info	Further info
CA92	Unknown (background fixed)	Further info
CB10	Unknown (splitscreen, background locked, no deformation)	Further info
CBA0	HPZ layer deformation info	Further info
CC66	OOZ layer deformation info	Further info
CD2C	MCZ layer deformation info	Further info
CE6C	Unknown (splitscreen, background free moving, but no alteration)	Further info
D0C6	CNZ layer deformation info	Further info
D194	CNZ 2 player splitscreen	Further info
D27C	CPZ layer deformation info	Further info
D382	DEZ layer deformation info	Further info
D4AE	ARZ layer deformation info	Further info
D5DE	SCZ layer deformation info	Further info
D666	01, 03, 09 layer deformation info	Further info
F66C	??????? (sprite)
FC98	Swinging platform from aquatic ruin zone (sprite)
10310	??????? (sprite)
104AC	Stationary floating platform from aquatic ruin zone (sprite)
108BC	Collapsing platform from either oil ocean zone or mystic cave zone (sprite)
10A08	Collapsing platform from aquatic ruin zone (sprite)
111D4	??????? (sprite)
112F0	??????? (sprite)
115C4	Stomper from MCZ (sprite)
1169A	One way barrier from chemical plant zone and DEZ (sprite)
1188C	An animal and the 100 that are given off from busting a badnick (sprite)
11DC6	100 points (sprite)
11F44	A ring (sprite)
12078	Sprite of rings spraying out from being hit by an enemy (sprite)
125E6	??????? (sprite)
12670	Monitor (sprite)
12854	Image of what was in monitor flying up like it does when you bust it (sprite)
12E18	Flashing stars from intro (sprite)
132F0	??????? (sprite)
13600	??????? (sprite)
13C48	Sprite for level title card (sprite)
13F74	??????? (sprite)
14086	End of level result screen (sprite)
142F8-1433B	Main game level order	Further info
143C0	End of special stage result screen (sprite)
15900	Vertical spikes (sprite)
15CC8	??????? (sprite)
15D44	??????? (sprite)
1600C	Sprite programming pointers (one address per sprite value)	Further info
1637C	??????? (sprite)
18888	Red spring facing upwards (sprite)
191B8	??????? (sprite)
19F50	Sonic (sprite)
1B8A4	Tails (sprite)
1D200	??????? (sprite)
1D320	??????? (sprite)
1D8F2	??????? (sprite)
1D97E	??????? (sprite)
1DD20	??????? (sprite)
1E0F0	??????? (sprite)
1F0B0	Starpoll (sprite)
1F624	??????? (sprite)
1F730	Star ball that sonic bounces off from CNZ (sprite)
1F8A8	??????? (sprite)
1FCDC	??????? (sprite)
2009C	Section of pipe that tips you off from chemical plant zone (sprite)
20210	??????? (sprite)
2031C	Master emerald (sprite)
203AC	??????? (sprite)
208DC	??????? (sprite)
20B9E	??????? (sprite)
20DEC	??????? (sprite)
20FD2	Big pylon thing that's in the foreground in CPZ (sprite)
21088	An explosion of an enemy, giving off an animal and 100 points (sprite)
2115C	??????? (sprite)
21392	??????? (sprite)
214C4	Rotating cylinder in Metropolis zone (sprite)
21928	See saw thingy from hill top zone (sprite)
21DAC	Diagonally moving lift thing from hill top zone (sprite)
22018	??????? (sprite)
222AC	Booster things from chemical plant zone (sprite)
22408	??????? (sprite)
2259C	Spin tube from chemical plant zone (sprite)
22FF8	??????? (sprite)
23300	??????? (sprite)
2351A	??????? (sprite)
238DC	??????? (sprite)
23AF4	Platform that shoots up in the air from oil ocean zone (sprite)
23E40	Spiky ball from OOZ (sprite)
24020	Oil in OOZ (sprite)
240F8	Strange spring from OOZ facing vertically (sprite)
24A16	Ball sitting on strange spring from OOZ (sprite)
24DD0	Block thingy in OOZ that launches you into the round ball things (sprite)
25244	Thingy from oil ocean zone that you enter when flying as a ball, and it turns and throws you another direction (sprite)
25694	Arrow shooter from aquatic ruin (sprite)
2588C	Pillar that drops it's lower part from aquatic ruin zone (sprite)
25A5A	Raising pillar from ARZ (sprite)
26104	Sprite that makes leaves fly off when you hit it from ARZ (sprite)
26370	Weird lever like spring from chemical plant zone and ARZ (sprite)
26634	Thing that pops up and releases steam sideways from metropolis zone (sprite)
26920	Twin stompers from MTZ (sprite)
26AE0	??????? (sprite)
26F58	??????? (sprite)
2715C	One of the spin tubes from MTZ (sprite)
27594	Block that has a spike that comes out of each side sequentially from MTZ (sprite)
27794	Floor spike from MTZ (sprite)
27884	Nut from MTZ (sprite)
27AB0	??????? (sprite)
27D6C	??????? (sprite)
28034	??????? (sprite)
283AC	??????? (sprite)
285C0	??????? (sprite)
28BC8	??????? (sprite)
28DF8	Spike block thing from MCZ (sprite)
28FF8	??????? (sprite)
291CC	Stairs from CPZ that move down to open the way (sprite)
293A0	Platform that moves back and forth on top of chemicals in CPZ (sprite)
29590	??????? (sprite)
297E4	Vine switch that you hang off in MCZ (sprite)
2997C	Vine that you hang off and it moves down from MCZ (sprite)
2A000	??????? (sprite)
2A290	Platform that is usually swinging from ARZ (sprite)
2A4FC	3 adjoined platforms from ARZ that rotate in a circle (sprite)
2A7B0	Fan blowing to the left from OOZ (sprite)
2AB84	Vertical spring from CNZ that you hold down jump on to pull back further (sprite)
2B140	??????? (sprite)
2B528	Flashing blocks in CNZ that appear and disappear in a rectangular shape that you can walk across (sprite)
2B84C	??????? (sprite)
2B8EC	Big block from casino night zone that moves back and forth (sprite)
2BA08	Elevator going up from CNZ (sprite)
2BB6C	Poky that gives out points from CNZ (sprite)
2C448	??????? (sprite)
2C6AC	Block thingy in CNZ that changes colour when you hit it and gives you points, until it disappears) (sprite)
2C92C	A sprite that you can hold on to, like the rails in WFZ (sprite)
2CA14	Octopus badnick from OOZ (sprite)
2CCC8	Seahorse badnick from OOZ (sprite)
2D068	Buzz bomber from EHZ (sprite)
2D394	Shark badnick from EHZ (sprite)
2D494	??????? (sprite)
2D734	??????? (sprite)
2EF18	??????? (sprite)
2FC50	??????? (sprite)
2FCF4	Button (sprite)
30480	??????? (sprite)
30FA4	Earthquake (sprite)
318F0	??????? (sprite)
32288	??????? (sprite)
32940	??????? (sprite)
32F90	??????? (sprite)
338EC	??????? (sprite)
340A4	??????? (sprite)
347EC	??????? (sprite)
34A5C	??????? (sprite)
34EB0	??????? (sprite)
34FA0	??????? (sprite)
353FE	??????? (sprite)
35554	??????? (sprite)
35FBC	??????? (sprite)
36924	Blowfly enemy from ARZ (sprite)
36A76	??????? (sprite)
36B88	??????? (sprite)
36BD4	??????? (sprite)
36DAC	??????? (sprite)
36F0E	??????? (sprite)
36FE6	??????? (sprite)
370FE	Red circle dude from hill top zone that has lava balls circling around him that he throws at you. (sprite)
37322	Lava snake (sprite)
373D0	??????? (sprite)
376E8	??????? (sprite)
377C8	Bomber badnick from SCZ (sprite)
37936	Turtle thing on the back of another larger turtle thing from SCZ (sprite)
37A06	??????? (sprite)
37A82	??????? (sprite)
37BFA	Coconuts (the badnick) from EHZ (sprite)
37E16	Snake like badnick from MCZ that flies out from wall at you (sprite)
3800C	Crab like badnick from MTZ (sprite)
3815C	??????? (sprite)
383B4	Praying Mantis dude from MTZ (sprite)
384A2	??????? (sprite)
3873E	Glowbug from MCZ (sprite)
3899C	Exploding star from MTZ (sprite)
38AEA	??????? (sprite)
38B86	??????? (sprite)
38DBA	Spider badnick from CPZ (sprite)
38F66	??????? (sprite)
39032	??????? (sprite)
39066	??????? (sprite)
390A2	??????? (sprite)
3937A	Jet badnick from SCZ (sprite)
3941C	??????? (sprite)
39452	??????? (sprite)
3972C	??????? (sprite)
3A1DC	??????? (sprite)
3A3F8	??????? (sprite)
3A790	??????? (sprite)
3B2DE	??????? (sprite)
3B36A	??????? (sprite)
3B3FA	??????? (sprite)
3B5D0	Tilting platform from WFZ (sprite)
3B8A6	??????? (sprite)
3B968	Wall turret from WFZ (sprite)
3BABA	??????? (sprite)
3BB4C	??????? (sprite)
3BB7C	??????? (sprite)
3BBBC	??????? (sprite)
3BC1C	??????? (sprite)
3BD7A	Platform that pops in and out from WFZ (sprite)
3BEAA	??????? (sprite)
3BF04	??????? (sprite)
3C0AC	Sprite that sonic hangs onto on the back of Robotnic's getaway ship at the end of WFZ (sprite)
3C328	Rivet thing that you bust to get into the ship at the end of WFZ (sprite)
3C3D6	??????? (sprite)
3C442	Laser boss guy at end of WFZ (sprite)
3CED0	??????? (sprite)
3D23E	Bad guy in CNZ (sprite)
3D4C8	Death Egg (sprite)
3EAC8	??????? (sprite)
3F1E4	Egg prison (sprite)
3FCE0	Offset index of dynamic pattern and animated pattern load cue's	Double offset Further info
3FD24	Dynamic pattern reloading for 01, 02, 03, WFZ, 09, MCZ, SCZ (empty)	Further info
3FD26	Dynamic pattern reloading for HTZ	Further info
3FF08	Dynamic pattern reloading for CNZ	Further info
3FF10	Dynamic pattern reloading for CNZ (not linked in offset index, but loads CNZ stuff)	Further info
3FF24	Dynamic pattern reloading for ARZ	Further info
3FF2C	Dynamic pattern reloading for EHZ, MTZ, MTZ3, HPZ, OOZ, CPZ, DEZ	Further info
3FF94	EHZ animated pattern load cue (4)	Further info
3FFF8	MTZ animated pattern load cue (3)	Further info
40038	HTZ animated pattern load cue (4)	Further info
4009C	HPZ animated pattern load cue (2)	Further info
400C8	OOZ animated pattern load cue (4)	Further info
4010E	CNZ animated pattern load cue (1)	Further info
40160	Unknown animated pattern load cue (1) (mirror of CNZ load cue, but not linked in offset index)	Further info
401B2	CPZ animated pattern load cue (0)	Further info
401C4	DEZ animated pattern load cue (0)	Further info
401D6-?????	ARZ animated pattern load cue (3)	Further info
40350	Offset index for mappings and properties of misc spites using animated patterns	Further info
40372	EHZ, HTZ misc sprite mappings	Further info
403EE	MTZ misc sprite mappings	Further info
404C2	HPZ misc sprite mappings	Further info
405B6	OOZ misc sprite mappings	Further info
4061A	CNZ misc sprite mappings	Further info
406BE	Unknown misc sprite mappings (that damn phantom group again)	Further info
40762	CPZ misc sprite mappings	Further info
4076E	DEZ misc sprite mappings	Further info
4077A	ARZ misc sprite mappings	Further info
407BE-407C0	01, 02, 03, WFZ, 09, MCZ, SCZ misc sprite mappings (empty)	Further info
4134C-41A6B	Big and small numbers used on counters in game	Uncompressed Further info
41D0C-41D2D	Offset index of object debug lists	Further info
41D2E	EHZ/01/02/03/DEZ object debug lists	Further info
42AA8	MTZ/MTZ act 3 object debug lists	Further info
41EEC	WFZ object debug lists	Further info
41FEE	HTZ object debug lists	Further info
420E8	HPZ/OOZ object debug lists	Further info
421F2	MCZ object debug lists	Further info
422B4	CNZ object debug list	Further info
42376	CPZ object debug list	Further info
42438	ARZ object debug list	Further info
42522	SCZ object debug list	Further info
42594-4265F	Main level load block pointers	Further info
42660-426E5	Offset index of pattern load cue's	Further info
426E6-42D4F	Pattern load cues	Further info
42D50	Curve and resistance mapping	Further info
42E50	Collision array	Further info
44E50	EHZ and HTZ primary 16x16 collision index	Compressed Further info
44F40	EHZ and HTZ secondary 16x16 collision index	Compressed Further info
45040	MTZ primary 16x16 collision index	Compressed Further info
45100	OOZ primary 16x16 collision index	Compressed Further info
45200	MCZ primary 16x16 collision index	Compressed Further info
452A0	CNZ primary 16x16 collision index	Compressed Further info
45330	CNZ secondary 16x16 collision index	Compressed Further info
453C0	CPZ and DEZ primary 16x16 collision index	Compressed Further info
454E0	CPZ and DEZ secondary 16x16 collision index	Compressed Further info
45610	ARZ primary 16x16 collision index	Compressed Further info
45760	ARZ secondary 16x16 collision index	Compressed Further info
458C0	WFZ/SCZ primary 16x16 collision index	Compressed Further info
459A0	WFZ/SCZ secondary 16x16 collision index	Compressed Further info
45A80	Offset index of level layout's	Further info
45AC4	EHZ act 1 level layout	Level compression Further info
45C84	EHZ act 2 level layout	Level compression Further info
45E74	MTZ act 1 level layout	Level compression Further info
462A4	MTZ act 2 level layout	Level compression Further info
46684	MTZ act 3 level layout	Level compression Further info
46B04	WFZ level layout	Level compression Further info
46DD4	HTZ act 1 level layout	Level compression Further info
47044	HTZ act 2 level layout	Level compression Further info
47404	OOZ act 1 level layout	Level compression Further info
47784	OOZ act 2 level layout	Level compression Further info
47B24	MCZ act 1 level layout	Level compression Further info
47D24	MCZ act 2 level layout	Level compression Further info
47FF4	CNZ act 1 level layout	Level compression Further info
483C4	CNZ act 1 level layout	Level compression Further info
48774	CPZ act 1 level layout	Level compression Further info
48A84	CPZ act 2 level layout	Level compression Further info
48DE4	DEZ level layout	Level compression Further info
48E94	ARZ act 1 level layout	Level compression Further info
49264	ARZ act 2 level layout	Level compression Further info
49634	SCZ level layout	Level compression Further info
49714	Animated flowers in EHZ and HTZ	Uncompressed Further info
49914	Pulsing thing against chequered backing from EHZ	Uncompressed Further info
49A14	Dynamically reloaded cliffs in background from HTZ	Art compression 192 blocks
4A33E	Dynamically reloaded clouds in background from HTZ	Uncompressed Further info
4A73E	Spinning metal cylinder patterns in MTZ	Uncompressed Further info
4B73E	Lava patterns in MTZ and HTZ	Uncompressed Further info
4BD3E	Animated section of MTZ background	Uncompressed Further info
4BF7E	Pulsing ball in OOZ	Uncompressed Further info
4C0FE	Square rotating around ball in OOZ	Uncompressed Further info
4C4FE	Oil in OOZ	Uncompressed Further info
4D4FE	Flipping foreground section in CNZ	Uncompressed Further info
4EEFE	Bonus pictures for pokie in CNZ	Uncompressed Further info
4FAFE	Animated background section in CPZ and DEZ	Uncompressed Further info
4FCFE	Waterfall patterns from ARZ	Uncompressed Further info
50000	Patterns for Sonic	Uncompressed Further info
64180	Patterns for Tails	Uncompressed Further info
6FBE0	Unknown offset index	Further info
6FD8C	Data that offset index is linking to (something to do with mappings for sonic)	Further info
714E0	Unknown offset index	Further info
7168C	Data that offset index is linking to (something to do with mappings for sonic)	Further info
71D8E	Bubble	Art compression 32 blocks
71F14	Invincibility stars	Art compression 00, 34 blocks
71FFC	Splash in water	Uncompressed Further info
7287C	Smoke from dashing	Uncompressed Further info
7393C	Supersonic stars	Art compression 00, 14 blocks
739E2	Unknown offset index	Further info
73AF8	Data that offset index is linking to (something to do with mappings for tails)	Further info
7446C	Unknown offset index	Further info
74582	Data that offset index is linking to (something to do with mappings for tails)	Further info
74876	SEGA patterns	Art compression 127 blocks
74F6C	Main patterns from title screen	Art compression 336 blocks
7667A	Sonic and tails from title screen	Art compression 674 blocks
78CBC	A few menu patterns	Art compression 10 blocks
78DAC	Button	Art compression 16 blocks
78E84	Vertical spring	Art compression 00, 20 blocks
78FA0	Horizontal spring	Art compression 12 blocks
7906A	Diagonal spring	Art compression 00, 32 blocks
7923E	Score, Rings, Time patterns	Art compression 24 blocks
79346	Sonic lives counter	Art compression 12 blocks
7945C	Ring	Art compression 00, 14 blocks
79550	Monitors and contents	Art compression 60 blocks
7995C	Spikes	Art compression 8 blocks
799AC	Numbers	Art compression 18 blocks
79A86	Starpoll	Art compression 00, 16 blocks
79BDE	Signpost	Art compression 78 blocks
7A1A0	Signpost	Uncompressed Further info
7AB4A	Strange spring from CPZ and ARZ	Art compression 00, 28 blocks
7AC9A	Long horizontal spike	Art compression 00, 8 blocks
7AD16	Bubble man, and bubbles from bubble man	Art compression 00, 37 blocks
7AEE2	Bubbles from character	Art compression 00, 10 blocks
7AF82	Countdown text for drowning	Uncompressed Further info
7B400	Game/Time over text	Art compression 34 blocks
7B592	Explosion	Art compression 68 blocks
7B946	Miles life counter	Art compression 00, 12 blocks
7BA32	Egg prison	Art compression 49 blocks
7BDBE	Image of tails in plane when sonic is on final frame at end of game	Art compression 00, 36 blocks
7C0AA	Sonic continue	Art compression 12 blocks
7C20C	Tails life counter	Art compression 12 blocks
7C2F2	Tails continue	Art compression 12 blocks
7C43E	ASCII characters used in game	Art compression 00, 88 blocks
7C9AE	1P/2P wins text from 2P mode	Art compression 38 blocks
7CD30	Sonic/Miles animated background from options and level select menu	Uncompressed Further info
7D22C	Title card patterns	Art compression 94 blocks
7D58A	Alphabet for font using large broken letters	Art compression 92 blocks
7D990	A menu box with a shadow	Art compression 21 blocks
7DA10	Pictures in level preview box in level select	Art compression 170 blocks
7E86A	Unknown. More text I haven't figured out	Art compression 68 blocks
7EB58	Text for end of special stage, along with patterns for 3 emeralds.	Art compression 72 blocks
7EEBE	"Perfect" text	Art compression 00, 14 blocks
7EF60	Bluebird	Art compression 00, 16 blocks
7F0A2	Squirrel	Art compression 00, 20 blocks
7F206	Mouse	Art compression 00, 20 blocks
7F340	Chicken	Art compression 16 blocks
7F4A2	Beaver	Art compression 00, 20 blocks
7F5E2	Penguin	Art compression 16 blocks
7F710	Pig	Art compression 18 blocks
7F846	Seal	Art compression 00, 14 blocks
7F962	Penguin	Art compression 00, 18 blocks
7FADE	Turtle	Art compression 00, 20 blocks
7FC90	Bears	Art compression 00, 20 blocks
7FDD2	Rabbit	Art compression 00, 18 blocks
7FF2A	Rivet thing that you bust to get inside ship at the end of WFZ	Art compression 00, 4 blocks
7FF98	Breakaway panels in WFZ	Art compression 15 blocks
8007C	Spiked ball from OOZ	Art compression 00, 32 blocks
80274	Unknown. HPZ style shaded blocks	Art compression 6 blocks
8030A	Striped blocks from CPZ	Art compression 4 blocks
80376	Oil falling into oil in OOZ	Art compression 16 blocks
804F2	Cascading oil from OOZ	Art compression 13 blocks
805C0	Ball on spring from OOZ	Art compression 20 blocks
806E0	Spinball from OOZ	Art compression 53 blocks
809D0	Collapsing platform form OOZ	Art compression 40 blocks
80C64	Diagonal and vertical weird spring from OOZ	Art compression 30 blocks
80E26	Swinging platform from OOZ	Art compression 28 blocks
81048	4 stripy blocks from OOZ	Art compression 4 blocks
810B8	Raising platform in OOZ	Art compression 24 blocks
81254	Fan in OOZ	Art compression 30 blocks
81514	Green flame from thingy that shots platform up in OOZ	Art compression 18 blocks
81600	Patterns for appearing and disappearing string of platforms in CNZ	Art compression 00, 4 blocks
81668	Spiky ball from poky in CNZ	Art compression 00, 04 blocks
816C8	Moving cube from either CNZ and CPZ	Art compression 16 blocks
817B4	Elevator in CNZ	Art compression 4 blocks
81826	Bars from pokies in CNZ	Art compression 12 blocks
81894	Hexagonal bumper in CNZ	Art compression 6 blocks
8191E	Normal round bumper from CNZ	Art compression 24 blocks
81AB0	Diagonal spring from CNZ that you pull back on	Art compression 32 blocks
81C96	Vertical red spring	Art compression 18 blocks
81DCC	Weird blocks in CNZ that you hit 3 times to get rid of them	Art compression 28 blocks
81EF2	Flippers from CNZ	Art compression 52 blocks
82216	Large moving platform from CPZ	Art compression 00, 10 blocks
82364	Top of water in HPZ and CPZ	Art compression 00, 24 blocks
824D4	Booster things in CPZ	Art compression 00, 4 blocks
8253C	CPZ enemy made up of a string of balls	Art compression 00, 04 blocks
825AE	CPZ metal things	Art compression 33 blocks
827B8	CPZ large moving platform blocks	Art compression 4 blocks
827F8	Stripy blocks from CPZ	Art compression 8 blocks
82864	Small yellow moving platform from CPZ	Art compression 48 blocks
82A46	Moving block from CPZ	Art compression 24 blocks
82C06	Spring that covers tube in CPZ	Art compression 32 blocks
82E02	Top of water in ARZ	Art compression 00, 16 blocks
82EE8	Leaves from ARZ	Art compression 00, 07 blocks
82F74	Arrow shooter and arrow from ARZ	Art compression 00, 17 blocks
830D2	One way barrier from ARZ	Art compression 8 blocks
8316A	Buzz bomber	Art compression 00, 28 blocks
8336A	Octopus badnick from OOZ	Art compression 00, 58 blocks
8368A	Flying badnick from OOZ	Art compression 00, 56 blocks
839EA	Fish badnick from EHZ	Art compression 22 blocks
83BF6	Robotnic's main ship	Art compression 96 blocks
84332	Robotnic enemy from CPZ	Art compression 111 blocks
84890	Large explosion	Art compression 00, 100 blocks
84F18	Mini explosion	Art compression 00, 8 blocks
84F96	Unknown. Trail of smoke of some kind.	Art compression 00, 16 blocks
8507C	EHZ boss	Art compression 128 blocks
85868	Chopper blades for EHZ boss	Art compression 00, 20 blocks
8595C	HTZ boss	Art compression 00, 107 blocks
86128	ARZ boss	Art compression 166 blocks
86B6E	MCZ boss	Art compression 204 blocks
87AAC	CNZ boss	Art compression 00, 133 blocks
882D6	OOZ boss	Art compression 181 blocks
88DA6	MTZ boss	Art compression 124 blocks
894E4	Unknown	Uncompressed Further info
89534	Blowfly from ARZ	Art compression 00, 9 blocks
8970E	Grounder from ARZ	Art compression 00, 50 blocks
89B9A	Fish from ARZ	Art compression 24 blocks
89DEC	Lava snake from HTZ	Art compression 00, 19 blocks
89FAA	Enemy with spike cone on top form HTZ	Art compression 20 blocks
8A142	Circular badnick from SCZ	Art compression 28 blocks
8A362	Turtle badnick from SCZ	Art compression 00, 57 blocks
8A87A	Coconuts from EHZ	Art compression 38 blocks
8AB36	Snake like badnick from MCZ	Art compression 10 blocks
8AC5E	Firefly from MCZ	Art compression 00, 20 blocks
8AD80	Praying mantis badnick from MTZ	Art compression 00, 32 blocks
8B058	Crab badnick from MCZ	Art compression 00, 36 blocks
8B300	Exploding star badnick from MTZ	Art compression 00, 15 blocks
8B430	Weird crawling badnick from CPZ	Art compression 00, 32 blocks
8B6B4	Spider badnick in CPZ	Art compression 00, 45 blocks
8B9DC	Scratch from WFZ	Art compression 26 blocks
8BC16	Jet like badnick from SCZ	Art compression 00, 25 blocks
8BE12	Robotic sonic	Art compression 00, 217 blocks
8CC44	The tornado	Art compression 79 blocks
8D1A0	Wall turret from WFZ	Art compression 24 blocks
8D388	Hook on chain in WFZ	Art compression 00, 20 blocks
8D540	Retracting platform from WFZ	Art compression 54 blocks
8D7D8	Wheel for belt in WFZ	Art compression 00, 16 blocks
8D96E	Moving platform in WFZ	Art compression 00, 12 blocks
8DA6E	Giant unused vertical red laser in WFZ	Art compression 12 blocks
8DAFC	Clouds	Art compression 00, 18 blocks
8DC42	Red horizontal laser in WFZ	Art compression 10 blocks
8DCA2	Platform that shoots sonic across from WFZ	Art compression 00, 05 blocks
8DD0C	Rising platforms on belt from WFZ	Art compression 00, 12 blocks
8DDF6	Unused badnick in WFZ	Artcompression 12 blocks
8DEB8	Vertical spinning blades from WFZ	Art compression 4 blocks
8DEE8	Horizontal spinning blades from WFZ	Art compression 00, 29 blocks
8E010	Platforms that tilt in WFZ	Art compression 12 blocks
8E0C4	Thrust from Robotnic's getaway ship in WFZ	Art compression 00, 8 blocks
8E138	Laser boss from WFZ	Art compression 00, 117 blocks
8E886	Robotnic's head	Art compression 00, 24 blocks
8EA5A	Robotnic	Art compression 76 blocks
8EE52	Robotnic's lower half	Art compression 28 blocks
8EF96	Window in back that Robotnic looks through in DEZ	Art compression 00, 08 blocks
8F024	Death egg	Art compression 327 blocks
901A4	Bouncer badnik from CPZ	Art compression 42 blocks
90520	Rocket thruster for tornado	Art compression 00, 26 blocks
906DE-90973	Mappings for movie sequence and images at end of game
90974	Movie sequence at end of game	Art compression 00, 193 blocks
91F3C	Final image of tornado with it and sonic facing screen	Art compression 00, 127 blocks
927E0	Mini pictures of tornado in final ending sequence	Art compression 00, 109 blocks
92F0A	Mini pictures of sonic and final image of sonic	Art compression 135 blocks
93848	Mini pictures of sonic and final image of sonic in supersonic mode	Art compression 117 blocks
93F3C	Final image of Tails	Art compression 181 blocks
94B28	Sonic the hedgehog 2 image at end of credits	Art compression 72 blocks
94E74	EHZ 16x16 block mappings	Compressed Further info
95C24	EHZ/HTZ main level patterns	Compressed Further info
985A4	HTZ 16x16 block mappings	Compressed Further info
98AB4	HTZ pattern suppliment to EHZ level patterns	Compressed Further info
99D34	EHZ/HTZ 128x128 block mappings	Compressed Further info
9CFD4	MTZ 16x16 block mappings	Compressed Further info
9DB64	MTZ main level patterns	Compressed Further info
A06C4	MTZ 128x128 block mappings	Compressed Further info
A3364	OOZ 16x16 block mappings	Compressed Further info
A4204	OOZ main level patterns	Compressed Further info
A6834	OOZ 128x128 block mappings	Compressed Further info
A8D04	MCZ 16x16 block mappings	Compressed Further info
A9D74	MCZ main level patterns	Compressed Further info
AD454	MCZ 128x128 block mappings	Compressed Further info
AFFC4	CNZ 16x16 block mappings	Compressed Further info
B0894	CNZ main level patterns	Compressed Further info
B2CF4	CNZ 128x128 block mappings	Compressed Further info
B5234	CPZ/DEZ 16x16 block mappings	Compressed Further info
B6174	CPZ/DEZ main level patterns	Compressed Further info
B90F4	CPZ/DEZ 128x128 block mappings	Compressed Further info
BB944	ARZ 16x16 block mappings	Compressed Further info
BCC24	ARZ main level patterns	Compressed Further info
C1434	ARZ 128x128 block mappings	Compressed Further info
C4074	WFZ/SCZ 16x16 block mappings	Compressed Further info
C5004	WFZ/SCZ main level patterns	Compressed Further info
C7EC4	WFZ pattern suppliment to SCZ tiles	Compressed Further info
C85E4	WFZ/SCZ 128x128 block mappings	Compressed Further info
CA906-?????	????????? (Special stage stuff)
DCD68	Background patterns for special stage	Art compression 127 blocks
DD48A	Sonic/Miles and number text from special stage	Art compression 62 blocks
DD790	"Start" patterns in special stage	Art compression 48 blocks
DD8CE	Stars in special stage	Art compression 00, 37 blocks
DD9C8	Unknown. Text in special stage.	Art compression 13 blocks
DDA7E	Ring patterns in special stage	Art compression 104 blocks
DDFA4	Horizontal shadow patterns in special stage	Art compression 38 blocks
DE05A	Diagonal shadow patterns from special stage	Art compression 58 blocks
DE120	Vertical shadow patterns in special stage	Art compression 25 blocks
DE188	Explosion patterns in special stage	Art compression 00, 48 blocks
DE4BC	Bomb patterns in special stage	Art compression 80 blocks
DE8AC	Emerald patterns in special stage	Art compression 46 blocks
DEAF4	Unknown. Text in special stage.	Art compression 99 blocks
DEEAE	Sonic and tails' animation frames from special stage	Art compression 851 blocks
E247E	"Tails" Patterns from special stage	Art compression 5 blocks
E24FE	?????????? (affected sprites in special stage)
E34EE	Unknown. looks like a really crap falling water thing.	Art compression 00, 12 blocks
E35F2	?????????? (compressed block of data to do with sprites for special stage)	Compressed
E427C	Data block acting as seperator
E4300	Offset index of ring location lists	Further info
E4344	EHZ act 1 ring locations	Further info
E456A	EHZ act 2 ring locations	Further info
E47AC	Unknown (empty)	Further info
E47AE	Unknown (empty)	Further info
E47B0	Unknown (empty)	Further info
E47B2	Unknown (empty)	Further info
E47B4	Unknown (empty)	Further info
E47B6	Unknown (empty)	Further info
E47B8	MTZ act 1 ring locations	Further info
E4A2A	MTZ act 2 ring locations	Further info
E4C48	MTZ act 3 ring locations	Further info
E4D72	MTZ act 4 ring locations (empty)	Further info
E4D74	HTZ act 1 ring locations	Further info
E4EAA	HTZ act 2 ring locations	Further info
E50E8	HPZ act 1 ring locations	Further info
E525A	HPZ act 2 ring locations (empty)	Further info
E525C	Unknown (empty)	Further info
E525E	Unknown (empty)	Further info
E5260	OOZ act 1 ring locations	Further info
E5422	OOZ act 2 ring locations	Further info
E5598	MCZ act 1 ring locations	Further info
E570E	MCZ act 2 ring locations	Further info
E5944	CNZ act 1 ring locations	Further info
E5B7E	CNZ act 2 ring locations	Further info
E5E2C	CPZ act 1 ring locations	Further info
E5F9E	CPZ act 2 ring locations	Further info
E6150	DEZ act 1 ring locations (empty)	Further info
E6152	DEZ act 2 ring locations (empty)	Further info
E6154	WFZ act 1 ring locations	Further info
E6292	WFZ act 2 ring locations	Further info
E6294	ARZ act 1 ring locations	Further info
E642E	ARZ act 2 ring locations	Further info
E6684	SCZ act 1 ring locations	Further info
E66D6	SCZ act 2 ring locations (empty)	Further info
E66D8	Data block acting as seperator	Further info
E6800	Offset index of sprite lists	Further info
E684A	EHZ act 1 sprite locations	Further info
E6B7A	EHZ act 2 sprite locations	Further info
E6F34	MTZ act 1 sprite locations	Further info
E73C0	MTZ act 2 sprite locations	Further info
E78EE	MTZ act 3 sprite locations	Further info
E7F48	WFZ act 1 sprite locations	Further info
E82FC	WFZ act 2 sprite locations (empty)	Further info
E8302	HTZ act 1 sprite locations	Further info
E8668	HTZ act 2 sprite locations	Further info
E8C80	HPZ act 1 sprite locations	Further info
E8D94	HPZ act 2 sprite locations (empty)	Further info
E8D9A	Unknown (empty)	Further info
E8DA0	OOZ act 1 sprite locations	Further info
E9214	OOZ act 2 sprite locations	Further info
E968E	MCZ act 1 sprite locations	Further info
E99A0	MCZ act 2 sprite locations	Further info
E9D1E	CNZ act 1 sprite locations	Further info
EA3D8	CNZ act 2 sprite locations	Further info
EA9D2	CPZ act 1 sprite locations	Further info
EAD6E	CPZ act 2 sprite locations	Further info
EB230	DEZ act 1 sprite locations	Further info
EB254	DEZ act 2 sprite locations (empty)	Further info
EB25A	ARZ act 1 sprite locations	Further info
EB6A4	ARZ act 2 sprite locations	Further info
EBBDE	SCZ act 1 sprite locations	Further info
EBD4C	SCZ act 2 sprite locations (empty)	Further info
EBD52	Unknown (empty)	Further info
EBD58	Unknown (empty)	Further info
EBD5E	Unknown (empty)	Further info
EBD64	Unknown (empty)	Further info
EBD66	Data block acting as seperator
EC000	????????? (Sound effects?)
ED100	????????? (instruments?)
F0160	Fireball from EHZ	Art compression 20 blocks
F02D6	Waterfall tiles	Art compression 24 blocks
F03DC	Another fireball	Art compression 00,16 blocks
F052A	Bridge in EHZ	Art compression 00, 8 blocks
F0602	Diagonally moving lift in HTZ	Art compression 48 blocks
F08F6	One way barrier from HTZ	Art compression 4 blocks
F096E	See-saw in HTZ	Art compression 24 blocks
F0B06	Fireball from HTZ	Art compression 24 blocks
F0C14	Rock from HTZ	Art compression 20 blocks
F0D4A	Badnick from HTZ with 4 balls of fire spinning around him	Art compression 4 blocks
F0DB6	Large spinning wheel from MTZ	Art compression 120 blocks
F120E	Indent in large spinning wheel from MTZ	Art compression 9 blocks
F12B6	Unknown. MTZ block.	Art compression 00, 8 blocks
F1384	Steam from MTZ	Art compression 00, 15 blocks
F148E	Spike from MTZ	Art compression 00, 8 blocks
F1550	Similarly shaded blocks from MTZ	Art compression 54 blocks
F15C6	Lava bubble from MTZ	Art compression 00, 9 blocks
F167C	Unknown. A steam vent?	Art compression 00, 4 blocks
F16EC	Things from MTZ	Art compression 8 blocks
F178E	Small cog from MTZ	Art compression 00, 12 blocks
F1870	Unknown. Four blocks that are all the same colour from MTZ.	Art compression 00, 4 blocks
F187E	Large wooden box from MCZ	Art compression 32 blocks
F1ABA	Collapsing platform from MCZ	Art compression 00, 26 blocks
F1C64	Switch that you pull on from MCZ	Art compression 16 blocks
F1D5C	Vine that lowers in MCZ	Art compression 10 blocks
F1E06	Unknown. A log viewed from the end	Art compression 00, 4 blocks
F1E8C	SEGA intro sound	16000Hz mono 8-bit unsigned PCM raw audio Further info
F8000-?????	????????? (Midis?)

Offset indexes

An offset index is a handy way of keeping a block of data grouped together, and also replacing a whole heap of pointers. The way it works is there is a list of two byte values, and depending on the block of data the game wants to load, it will take one of those values and add it to the starting address of the offset index. The most common usage is for every act of every level value, there is an offset that acts as the pointer to the block of data to use for it. Lets look at an example:

E6800: 004A 037A 5552 5552 5552 5552 5552 5552 0734 0BC0 10EE 10EE 1748 1AFC

Now this is the beginning of the offset index for the sprite locations. Now, can you see which values are the offset for MTZ act 2? They're 0BC0. All you had to do to get that was to take the level value for MTZ, which is 04, and count across four bytes for every value it is above 00, then across another two because it's the second act. Now we take that value and using a hex calculator, add it to the address of the offset table, which is E6800. That will give us a value of E73C0, which is the address in the rom of the MTZ act 2 sprite locations.

Also notice those 5552 values in there? Well those are for the unused level values in the game. You'll see some damn weird values put in for them in many places, and that's what causes the game to lockup in the final build of sonic 2 if you try and enter one of those levels.

There are other forms of an offset index such as the one where there's just one offset per level value, and another type that actually uses a double offset index. The first is easy enough, but a double offset index will specify two values per level, but that's not for act one and two, but rather two blocks of data to use for both acts.

Main level load block

This pointer table is what is used to load up the block mappings and main level patterns for every level in the game. Here's a breakdown of the pointer table in the S2 rom:

42594-42660: Indexed main level load block
(patterns/16x16/128x128)

42594-425A0: Emerald hill zone (00)

42594: EHZ/HTZ main level patterns (95C24) [04]
42598: EHZ/HTZ 16x16 block mappings (94E74) [05]
4259C: EHZ/HTZ 128x128 block mappings (99D34) [04]
425A0-425AC: Unknown (01)

425A0: EHZ/HTZ main level patterns (95C24) [06]
425A4: EHZ/HTZ 16x16 block mappings (94E74) [07]
425A8: EHZ/HTZ 128x128 block mappings (99D34) [05]
425AC-425B8: Unknown (02)

425AC: EHZ/HTZ main level patterns (95C24) [08]
425B0: EHZ/HTZ 16x16 block mappings (94E74) [09]
425B4: EHZ/HTZ 128x128 block mappings (99D34) [06]
425B8-425C4: Unknown (03)

425B8: EHZ/HTZ main level patterns (95C24) [0A]
425BC: EHZ/HTZ 16x16 block mappings (94E74) [0B]
425C0: EHZ/HTZ 128x128 block mappings (99D34) [07]
425C4-425D0: Metropolis zone (04)

425C4: MTZ main level patterns (9DB64) [0C]
425C8: MTZ 16x16 block mappings (9CFD4) [0D]
425CC: MTZ 128x128 block mappings (A06C4) [08]
425D0-425DC: Metropolis zone act 3 (05)

425E0: MTZ main level patterns (9DB64) [0C]
425E4: MTZ 16x16 block mappings (9CFD4) [0D]
425E8: MTZ 128x128 block mappings (A06C4) [08]
425DC-425E8: Wing fortress zone (06)

425DC: WFZ/SCZ main level patterns [10]
425E0: WFZ/SCZ 16x16 block mappings [11]
425E4: WFZ/SCZ 128x128 block mappings [0A]
425E8-425F4: Hill top zone (07)

425E8: EHZ/HTZ main level patterns (95C24) [12]
425EC: EHZ/HTZ 16x16 block mappings (94E74) [13]
425F0: EHZ/HTZ 128x128 block mappings (99D34) [0B]
425F4-42600: Hidden Palace zone (08)

425F4: OOZ 16x16 block mappings (A3364) [14]
425F8: OOZ 16x16 block mappings (A3364) [15]
425FC: OOZ 16x16 block mappings (A3364) [0C]
42600-4260C: Unknown (09)

42600: EHZ/HTZ main level patterns (95C24) [16]
42604:
EHZ/HTZ 16x16 block mappings (94E74) [17]
42608: EHZ/HTZ 128x128 block
mappings (99D34) [0D]
4260C-42618: Oil ocean zone (0A)

4260C: OOZ main level patterns (A4204) [18]
42610: OOZ
16x16 block mappings (A3364) [19]
42614: OOZ 128x128 block mappings
(A6834) [0E]
42618-42624: Mystic cave zone (0B)

42618: MCZ main level patterns (A9D74) [1A]
4261C: MCZ
16x16 block mappings (A8D04) [1B]
42620: MCZ 128x128 block mappings
(AD454) [0F]
42624-42630: Casino night zone (0C)

42624: CNZ main level patterns (B0894) [1C]
42628: CNZ
16x16 block mappings (AFFC4) [1D]
4262C: CNZ 128x128 block mappings
(B2CF4) [10]
42630-4263C: Chemical plant zone (0D)

42630: CPZ/DEZ main level patterns (B6174) [1E]
42634:
CPZ/DEZ 16x16 block mappings (B5234) [1F]
42638: CPZ/DEZ 128x128 block
mappings (B90F4) [11]
4263C-42648: Death egg zone (0E)

4263C: CPZ/DEZ main level patterns (B6174) [20]
42640:
CPZ/DEZ 16x16 block mappings (B5234) [21]
42644: CPZ/DEZ 128x128 block
mappings (B90F4) [12]
42648-42654: Aquatic ruin zone (0F)

42648: CPZ/DEZ main level patterns (BCC24) [22]
4264C:
CPZ/DEZ 16x16 block mappings (BB944) [23]
42650: CPZ/DEZ 128x128 block
mappings (C1434) [13]
42654-42660: Sky chase zone (10)

42654: WFZ/SCZ main level patterns (C5004) [24]
42658:
WFZ/SCZ 16x16 block mappings (C4074) [25]
4265C: WFZ/SCZ 128x128 block
mappings (C85E4) [14]

Now as you can see, these pointers specify the loading addresses in the rom for the 16x16 and 128x128 block mappings, as well as the main pattern block for that level. The main pattern block will always be loaded into the VRAM at address 0000. These loading addresses are all preceeded by a one byte value, so the address pointer only uses three bytes rather than the standard four. The value that proceeds the 128x128 block mappings specifies a value on the palette index to load for that level. For more information on the palette system, go here. The values preceeding the main level pattern and 16x16 block mapping pointers give the index numbers of the pattern load cue's to use for that level. For more information on that go here.

Pattern load cues

The pattern load cues are what loads all the pieces of art into the VRAM that are not in the main level load block. Here's a breakdown of the pattern load cues in the S2 rom:

00	426E6	Standard block 1 (score\time\rings/lives/ring/numbers)	(3)
01	42700	Standard block 2 (starpoll/monitor/stars/bubbles)	(3)
02	4271A	pattern block for level with water (explosion/stars/bubbles)	(2)
03	4272E	game/time over (game\time over text)	(0)
04	42736	EHZ (waterfall/bridge/fireball/buzz bomber/coconuts/fish)	(6)
05	4275C	EHZ (spikes/diagonal spring/horizontal\vertical spring)	(3)
06	42776	Miles 1up patch (Miles lives counter)	(0)
07	4277E	Miles life counter (Miles lives counter)	(0)
08	42786	Tails 1up patch (Tails lives counter)	(0)
09	4278E	Tails life counter (Tails lives counter)	(0)
0A	42796	MTZ (Wheel/Unknown/Unknown/MSC from MTZ/steam/Unknown/MTZ spike/Badnick/starbomb)	(8)
0B	427CE	MTZ (Button/spikes/Mantis/vertical spring/horizontal spring/Unknown/Lava bubble/Cog/Unknown)	(8)
0C	42806	WFZ (Tornado/clouds/vertical blades/horizontal blades/jet badnick/unknown/scratch/tilting platforms/tornado/clouds)	(9)
0D	42844	WFZ (vertical blades/ horizontal blades/ unknown/ turret/ unknown/ unknown/ unknown/ unknown/ belt/ platforms/ retracting platform/ unknown/ trolley/ rivet/ unknown)	(D)
0E	4289A	HTZ (fireball/rock/conveyor/orbit badnick/unknown badnick/unknown badnick/spikes/diagonal spring/vertical spring/horizontal spring)	(9)
0F	428D8	HTZ (lift/fireball/unknown)	(2)
10	428EC	OOZ (green flame/raising platform/spikeball/unknown/unknown/cascading oil/unknown/unknown)	(8)
11	42924	OOZ (unknown/ unknown/ swinging platform/ unknown/ fan/ button/ spikes/ diagonal spring/ vertical spring/ horizontal spring/ seahorse/ octopus)	(B)
12	4296E	MCZ (box/collapsing platform/vine switch/lowering vine/firefly/unknown badnick)	(5)
13	42994	MCZ (long horizontal spike/ spikes/ unknown/ other spring/ vertical spring/ horizontal spring)	(5)
14	429BA	CNZ (unknown badnick/ moving cube/ unknown/ unknown/ unknown/ unknown/ unknown/ unknown/ unknown/ destroyable blocks)	(9)
15	429F8	CNZ (poky spring/vertical red spring/spikes/diagonal spring/vertical spring/horizontal spring)	(5)
16	42A1E	CPZ (CPZ stuff/CPZ stuff/booster/moving platform/small moving platform/spring cap/water surface/moving block/CPZ platform blocks)	(8)
17	42A56	CPZ (unknown badnick/unknown badnick/spikes/unknown/other spring/vertical spring/horizontal spring)	(6)
18	42A82	DEZ (stripy blocks from CPZ)	(0)
19	42A8A	DEZ (robotic sonic/unknown/Robotnic/Robotnic's head/Robotnic's lower half)	(4)
1A	42AAA	ARZ (one way barrier/water surface/leaves/arrow shooter)	(3)
1B	42AC4	ARZ (shark/blowfly/grounder/bubbles/spikes/other spring/vertical spring/horizontal spring)	(7)
1C	42AF6	SCZ (tornado)	(0)
1D	42AFE	SCZ (clouds/vertical blades/horizontal blades/jet badnick/turtle badnick/bomber)	(5)
1E	42B24	Sonic end of level results screen (title card patterns/unknown/sonic continue/perfect text)	(3)
1F	42B3E	End of level signpost (signpost)	(0)
20	42B46	CPZ boss (Robotnic's main ship/CPZ boss/mini explosion/unknown/large explosion)	(4)
21	42B66	EHZ boss (Robotnic's main ship/EHZ boss/chopper blades/large explosion)	(3)
22	42B80	HTZ boss (Robotnic's main ship/HTZ boss/large explosion/unknown)	(3)
23	42B9A	ARZ boss (Robotnic's main ship/ARZ boss/large explosion)	(2)
24	42BAE	MCZ boss (Robtnic's main ship/MCZ boss/large explosion)	(2)
25	42BC2	CNZ boss (Robotnic's main ship/CNZ boss/large explosion)	(2)
26	42BD6	MTZ boss (Robotnic's main ship/MTZ boss/mini explosion/large explosion)	(3)
27	42BF0	OOZ boss (OOZ boss/large explosion)	(1)
28	42BFE	Unknown (large explosion)	(0)
29	42C06	Death egg (Death egg)	(0)
2A	42C0E	EHZ animals (squirrel/blue bird)	(1)
2B	42C1C	MCZ animals (rat/chicken)	(1)
2C	42C2A	HTZ/MTZ/WFZ animals (beaver/penguin)	(1)
2D	42C38	DEZ animals (pig/chicken)	(1)
2E	42C46	Unknown (HPZ animals) (rat/seal)	(1)
2F	42C54	OOZ animals (penguin/seal)	(1)
30	42C62	SCZ animals (turtle/chicken)	(1)
31	42C70	CNZ animals (bears/blue bird)	(1)
32	42C7E	CPZ animals (rabbit/penguin)	(1)
33	42C8C	ARZ animals (penguin/blue bird)	(1)
34	42C9A	Special stage (emerald/unknown/special stage text/horizontal shadow/diagonal shadow/vertical shadow/explosion patterns/special stage ring/"Start"/unknown/special stage background/stars/"Tails")	(C)
35	42CDE	Unknown (Unknown)	(0)
36	42CF4	Boss at end of WFZ (Laser boss from WFZ/Robotnic/Robotnic's head/Robotnic's lower half/explosion)	(4)
37	42D12	Tornado at end of WFZ (tornado/rocket thruster from tornado/clouds)	(2)
38	42D26	Egg prison (egg prison)	(0)
39	42D2E	Normal explosion (explosion)	(0)
3A	42D36	Tails end of level results screen (title card patterns/unknown/tails continue/perfect text)	(3)

This pointer list has an offset index to locate the correct cue. Go here for an explanation of how this works. Now, each load cue has a two byte value before it that specifies how many addresses are in the cue, to avoid it reading too far, and going into the next cue. This value is dead simple, just enter the number of patterns on the cue in hex between the first two bytes, but you have to count 00 as 1, so if there was 16 loading addresses in the cue, you'd have 000F preceding it. After that, there are 6 bytes per load request. The first four bytes are a pointer to the art block to use, and the two bytes after that are the location in the VRAM to load them into.To change the pattern load cue's being loaded for a level, you need to change the values in the main level load block. For more information on that go here.

Collision definitions

Ok, I’m going to explain how collision works on objects that are not sprites in the sonic games. First of all forget the image that the block contains. The image has absolutely nothing to do with the collision. There's basically in invisible collision box on top of every 16x16 block that sets what's solid and what's not. This collision box has two parts to it. The main part of it is the collision array (an array is basically a table of values). The collision array stores the actual data that says that is a block is solid in certain places. It does that through 16 bytes per collision box. To understand how those 16 bytes set what's solid, imagine a 16x16 block. Now imagine that 16x16 block with the numbers 1-10 (hex) down the side starting from the bottom and going up to the top, and starting from the left and going up as it moves right along the top, so that you can give an exact location of each pixel. (eg, the top left pixel would be 10,00) Now, in this array the first two bytes define what's solid for the row of pixels on the left side of that 16x16 block. Basically the first of those two bytes says where to start making things solid, and the second byte says where to stop making them solid. So if you entered the value 0010, the first row would be solid from the very bottom of the block all the way up to the top. If you were to enter the value 020E, there would be two pixels on either side of the block that are not solid, and everything in the middle would be. The next two bytes after that are for the next row, etc. After 16 bytes the definitions for the next collision box begin.

Now, to specify which blocks use which collision boxes, there are collision indexes, which give an array location for each 16x16 block. If you look at the space used by the collision array, you can divide that by 16 (the number of bytes used by each entry into the array) to give a one byte value, which represents the maximum number of collision boxes that the array can hold in the S2 platform, which is FF. Now, for each level there is at least one 16x16 collision index, and that index consists of a whole heap of array locations, one for each 16x16 block. If for example you wanted the 9th 16x16 block to use the collision box with an array location of 3E, you would give the 9th byte of the collision index for that level a value of 3E.

It is through collision indexes that the loops work in the game. It's commonly believed that there are layers, and that the sprite represented by 4 rings changes between them. This is partly true. For any levels that use multiple layers like that, there is a secondary 16x16 collision index. That object switches the 16x16 collision index in use. The pointers to these 16x16 collision indexes are stored at 49E8-4A70. The first set of pointers in this group specify the location of the primary collision index to use for each level, and the second set specify the location of the secondary collision index to use for each level.

Now just setting something as solid, doesn't give it the effect of having a slope. There's an index in the rom, which has one byte in it for each collision array location. To understand how this value works, think of two lines running parallel to each other, both with the value 0-F along them. Now, the first value of the byte gives a point on the right line, and the second value gives a point on the left line. Now imagine drawing a line between these to points. If the points are different, the resulting gradient will be taken to create the effect of a slope and the resistance going up it on the box in the corresponding collision array location.

Level layout

The level data in the S2 rom is compressed. Editing compressed data is extremely impractical, so you can either extract it first using an extraction program such as ChaoSaX-Extract, and once it's in that form refer to my savestate hacking info for information on how to edit it. Once you've finished editing it you can then use the soon to be added compression routine in ChaoSax-Extract to recompress it, and then you can place it back into the rom. If it is a larger size than the original, you will need to fiddle around with the locations and the offset index to get it back in.

Ring placement

The data for ring placement has an offset index to locate the correct group. Go here for an explanation of how this works. Now, in the rom each ring does not have to be stored individually. Instead you can place one ring, and specify that a certain number of rings come after it. The definitions for one ring takes up 4 bytes in the rom. Let's look at an example:
1234 5678
Now, in this example the ring will be placed at an x location of 1234, and a y location of 678. With that value where the 5 was entered, if the value entered is below 8, an additional ring will be placed next to the one before it for every unit that the value entered is above 0. Entering a value above 8 will place an additional ring below the one before it for each unit that the value entered is above 8. When specifying a position as the start of a group of rings, if the group is horizontal, the rings will go across to the right of that point, and if the group is vertical the rings will go down from that point. The value FFFF closes the group of ring locations, and it is essential that this is at the end of the ring locations list, or else the game will keep on reading past that point until it hits that value.

Sprite placement

Sprites are quite simple to define. It takes six bytes to define one sprite. The first two bytes are the x location of the sprite, and the next two bytes are the y location of the sprite. The 5th byte is the number on the sprite array to lookup to get the location for the function to use for the programming of that sprite, and the 6th byte is an optional declaration to use with the function for that sprite. Go here for more info on what exactly that means, and a list of all the sprites in the S2 rom.

Main level block mappings

The block mappings in the S2 final rom are compressed. If you want to edit them you need to extract them first, edit them, and then recompress them afterwards. For more information on exactly what the block mappings do, and how to edit them, refer to the sonic 2 savestate breakdown. Note that the 16x16 block mappings in the S2 beta rom are not compressed, so if any port from the beta to the final is to be successful, those mappings will need to be compressed first.

Main level pattern blocks

Unlike the beta, the main pattern block for the levels do not use the standard art compression format, but a less complex one to allow the patterns to be loaded faster into the VRAM, and hence cut down on loading times for levels significantly. Also unlike the standard art compression format, this one has been cracked. A decompression utility such as ChaoSaX-Extract is capable of decompressing these patterns. If a port is to be done from the beta, it will be necessary to either allow the game to decompress the main pattern block into the VRAM, and then recompressing the appropriate blocks manually and putting them into the rom, or just cheat and stick these patterns on the pattern load cue. That is rather sloppy though. For information on how to edit extracted patterns, refer to the Sonic 2 savestate breakdown.

Uncompressed art

Any art that needs to be updated on the fly will not be compressed in the rom. This is because the game can't work with compressed data, and needs to extract any data it wants to use to the ram. This decompression process takes a fair bit of computation, and because of that it wouldn't be possible to create the appearance of a smooth animation using compressed data. The same reason is true as to why any data that is not loaded into the ram will always be uncompressed, because otherwise they would have to load it into the ram just to read it.

Now, for example all of the blocks for Sonic and Tails are uncompressed, because they of course need to be updated quickly. You can use basically any editing utility to edit them, but you can only change what each block looks like through this. In order to edit what blocks are placed where in each of Sonic's animation frames, you need to edit the mappings for them, and for more information on that go here. Now in their uncompressed form, you can edit them just like you would in the VRAM, so for more information on that check the sonic 2 savestate breakdown.

Mappings for Sonic/Tails

What the mappings do is they tell the game what patterns to display, and where to put them for each of the frames that the sprite has. The mappings for most sprites are contained within their function (go here for more details), but with the Sonic and Tails sprites, to allow the programmers to add a new frame easily without having to recompile the function, and hence the whole game because of the size difference, they created a separate array containing the mapping data for each frame. I haven't examined this to see how it works yet, but when I do I’ll explain the format here.

Sega intro sound

The Sega intro sound is just a 16000HZ wav sound. Opening up the rom in any sound editing program that supports raw audio with the settings listed next to the location on the list will enable you to playback the sound effect. You could also insert your own sound in the same place as long as it's at the same bitrate. The bitrate is to do with the quality of the sound. The lower it is the crappier it will sound, because that's to do with how often the output is modified. Just think of the difference between long play and short play on a VCR. Stuff recorded in long play does save space, but the quality of the recording is worse.

Art compression format

The art compression format is a very dense compression format. It's only used for art because the format relies on the data being in blocks of 64 bytes. This format is used for all the compressed art in sonic 1 and sonic 2 beta, and all the compressed art except for the main level pattern blocks in Sonic 2 and S3&K. As I said this format is very dense, and as such it requires a hell of a lot of computation to decompress. Because of this it also requires a hell of a lot of time and effort to crack. I've had a go at it, but the lightning has yet to strike. When I crack it I’ll post the format breakdown here, and create a simple program to decompress it, but until then it is impossible to directly edit any art using this format. The porting of a compressed block of art is simple though, so if you want to transfer beta art to the final, you can just tack it onto the end and enter the pointer into the pattern load cue of the level or event you want it to appear at, along with a VRAM location to load it into. Go here for more information on that.

Dynamic pattern reloading

Dynamic pattern reloading is a method of loading patterns into the VRAM, but unlike the other methods, this one has no limitations, because it uses actual programmed code. Basically this is here for any effects to do with patterns changing in game that isn't posible using the normal load cue system, like the hills in the background of HTZ where the patterns to display are determined by the position of the screen. Because the programming for it is in the form of compiled code, direct editing of it is not possible.

Animated pattern load cue's

Animated patterns require the art they are working with to be in an uncompressed form, so if you are attempting to make an animated pattern, make sure the art is uncompressed or it won't work. Now, the length of each load request on the cue varies depending on the animation. The first byte specifies a value that will determine how often to switch frames in the animation. If you use this value you will not be able to specify a frame as having a different duration to another. Setting this to FF will disable automatic frame control and allow the user to input a manual duration for each frame. This will mean that for every frame the user wants in the animation, they have to allow another byte at the end of the load request. If you want automatic pattern control, which you would use if you want all the animations frames to have the same duration. Just enter how many screen refreshes to wait until the next frame of the animation is displayed. Bytes 2,3, and 4 are the address in the rom to load the patterns from. Bytes 5 and 6 are the location in VRAM to load the patterns into. Byte 7 is the number of frames the animation has, and byte 8 is the number of spaces in VRAM to use for each frame. The function of the bytes beyond this point depend on whether or not automatic frame control is enabled, so refer to appropriate section below.

With automatic frame control:
One byte per frame of animation. This byte simply specifies the image to use for each frame of the animation. Based on the loading address entered, this value is the offset in 8x8 blocks to load each frame from in the rom. So if for example you were dealing with an animation that had 4 8x8 blocks per frame, and there were 4 frames of that, and you wanted it to display each frame one by one, you would enter the value 0004 080C.

Without automatic frame control:
Two bytes per frame of animation. The first byte is the offset of blocks in the file to load the animation frame from. This byte simply specifies the image to use for each frame of the animation. Based on the loading address entered, this value is the offset in 8x8 blocks to load each frame from in the rom. The second byte is the number of screen refreshes to keep that image loaded for, before going on to the next animation frame.

Misc sprite definitions

It looks like there are some sprites that get thier mappings and whatnot from this index, so they can reuse just a couple of sprites for some of the the miscellaneous objects in a level that the character doesn't actually interact with, but those objects can be completely different from one level to the next. A handy way of avioding programming each one individually, and thus saving a hell of a lot of space. As for how to edit them, well I haven't looked at that yet, but when I do i'll post an explination of the format here.

Rasterised layer deformation

Raster effects are simply effects to do with deformation of an image based on lines of pixels. In Sonic 2, these effects are used to create the movement effects for the backgrounds of all the levels, as well as the 2 player splitscreen effect. Because the programming for these effects are stored in the form of compiled code, you'll have to learn 68K ASM in order to edit them. Switching between levels is easy enough though, just change the corresponding value on the preceeding offset index to link to the effect you wish to use.

Palettes

All the main palettes used in the game are linked in a pointer table at 294E. In this pointer table there are 8 bytes per palette. The first four simply give a location in the rom for the data to load. The fifth and sixth bytes give the location in the system ram to load the data into, and the seventh and eighth bytes give the number of bytes being loaded. Here's a palette pointer:

0000 2942 FB20 0007

Now in this case the palette is being loaded from 00002942 in the rom into the beginning of the second above water palette row (FB20), and it's loading 4 colours (0007). Here's a listing of the palette pointers in the S2 rom:

00	2782	SEGA screen palette (28C2)
01	278A	Title screen palette (2942)
02	2792	Unknown palette (2962)
03	279A	"Sonic and Miles" screen palette (29E2)
04	27A2	EHZ palete (2A22)
05	27AA	Level value 01 palette (2A22)
06	27B2	WZ palette (2A82)
07	27BA	Level value 03 palette (2A22)
08	27C2	MTZ palette (2AE2)
09	27CA	MTZ act 3 palette (2AE2)
0A	27D2	WFZ palette (2B42)
0B	27DA	HTZ palette (2BA2)
0C	27E2	HPZ palette (2C02)
0D	27EA	Level value 09 (2A22)
0E	27F2	OOZ palette (2CE2)
0F	27FA	MCZ palette (2D42)
10	2802	CNZ palette (2DA2)
11	280A	CPZ palette (2E02)
12	2812	DEZ palette (2EE2)
13	281A	ARZ palette (2F42)
14	2822	SCZ palette (3022)
15	282A	HPZ underwater palette (2C62)
16	2832	CPZ underwater palette (2E62)
17	283A	ARZ underwater palette (2FA2)
18	2842	Main special stage palette (3162)
19	284A	Unknown palette (3082)
1A	2852	Unknown palette (30A2)
1B	285A	Special stage 1 palette (31C2)
1C	2862	Special stage 2 palette (31E2)
1D	286A	Special stage 3 palette (3202)
1E	2872	Special stage 4 palette (3222)
1F	287A	Special stage 5 palette (3242)
20	2882	Special stage 6 palette (3262)
21	288A	Special stage 7 palette (3282)
22	2892	Unknown palette (32A2)
23	289A	Unknown palette (32C2)
24	28A2	Unknown palette (32E2)
25	28AA	OOZ boss palette (30C2)
26	28B2	Menu palette (30E2)
27	28BA	Unknown palette (3302)

To change the palette being loaded for each level, you need to change the palette index number in the main level load block. For more information on that go here. The palettes themeslves are very easy to modify as they are all uncompressed in the rom. Refer to the savestate hacking documents for information on how to edit palettes.

Level size array

All the sizes of the levels are stored in a simple array, and can be easily modified to change the boundaries of the level. This array is located at C054, and for each act there are four values. The first gives the X start location of the level, and the second gives the X end location of the level. Likewise the third value gives the Y start location of the level, and the fourth gives the Y end location of the level.

Character start location array

The start location for the characers and the camera for each level are stored in this array at address C1D0. It couldn't be any simpler to modify, there's just two values per act, the first being the X location, and the second being the Y location.

Music playlist for levels

This is actually part of a sub in the code, but it's editable all the same. Pretty simple, just one byte per level. That one byte is the value of the song you wish to play. You must enter the value in the form in which it is displayed in the beta version, which is with 80 as the starting value, not 00 as it is displayed in the final. You'll notice with the value for death egg, it's 08 not 88, which is why there's no sound in the level.

Level order

There are several different sections in the game to do with the order of levels. The first one is the one that determins which level the game loads when someone finishes a level. The second one is the one which determins which level to load when you select a level from the level select menu. The level select one is easy to modify, just enter the level value followed by an act number for each of the levels you want to load, in the order you want them to be loaded. For a list of the level values for all the different stages, look at the beginning of the introduction to this page. Note that if you load the same level into the level order twice, when you link to it via the level select, it will continue the sequence from the last one in the list, not the first.

The second system for which level to load when one ends works slightly differently. There is a list of values, with two bytes allocated to each act of each level. That two byte value is simply the value of the level you wish to load when that level ends. So, if you wanted ARZ act 1 to load up when you finish MTZ act 2, you would take the location of the table, which in this case is 142F8, and you would add 18 bytes to that location, and then you would replace the next two bytes from that point with 0F00. That's becuase you add four bytes to the address for every level that comes before it, and in this case there are 4, and then you would add another 2 bytes becuase you want to alter the level to load when you finish the second act.

Also, note that when you want a level to be the last level in the game, you must enter the value FFFF as the stage to load when it finishes. This tells the game to end and return to the title screen.

Object debug list

This list is what determins what sprites you can place in debug mode, and in what levels. An offset index is used to locate which object debug list to use for which level, so if you want further info on that go here. Now, there is a 2 byte value at the start of each debug list. That 2 byte value is simply the number of sprites in the list. After that there are 8 bytes per sprite. Here's an object that's present in an object debug list from sonic 2:

2601		2D36		0800		0680
1	2	3	4	5	6	7	8

And here's a quick referance sheet of what each byte does. For further info on each one, refer to information below.

1	Object number
2-4	Sprite mappings
5	Declaration
6	Frame to display
7 and 8	flip/mirror/palette/VRAM location

Object number:
This value is what determins which object to use. For a list of all the sprites in the game, check out the sprite programming section of this document here. In the case of the above example the object number is 26, which is a monitor.

Sprite mappings:
This is a pointer to the location in the rom to load the mappings for that sprite. This will be located in the programming for the sprite itself. Do not change this unless porting between levels, or you're absolutely sure you know what you're doing, or the game will most likely hang. This only affects the preview picture, not the placed object itself. In this case, the sprite mappings are being loaded from the address 012D36 in the rom.

Declaration:
This is the declaration for the loading of the sprite. For information on exactly what that is, go here. In this example the declaration is 08, which switches the type of monitor it is in the case of this sprite.

Frame to display:
This is simply the frame of the sprite that will be displayed on the preview picture before you place an object. Right now it's set to 00, which is the first frame.

Flip/mirror/VRAM location:
The first hex value of the 7th byte is to do with flipping and mirroring of the sprite, as well as the palette line to use for it. Refer to the following table for an explination of what responce each value will create.

Value	palette line	flipped horizntally	flipped vertically
0	1	n	n
1	1	n	y
2	2	n	n
3	2	n	y
4	3	n	n
5	3	n	y
6	4	n	n
7	4	n	y
8	1	n	n
9	1	y	n
A	2	n	n
B	2	y	n
C	3	n	n
D	3	y	n
E	4	n	n
F	4	y	n

In the case of our example, the value of 0 is used, so the preview image will use the first palette line, and will not be flipped or mirrored.

The last hex value of the 7th byte, and all of the 8th byte in this block combine to give a starting location in the VRAM to find the patterns to use for that sprite when displaying the preview. This is not an actual location, but merely the number of blocks after which to load the patterns from. After the value of 800 is passed, the block number resets to zero but the image is mirrored. In this case, that block number is 680.

And that's about it. I've just realised that this is a rediculous amount of detail for such a small thing, but at least you're sure to get it.

Sprite programming

The programming for a sprite in the sonic games are stored as a function, and that function can take one declaration. People often confuse this declaration with a subtype value, but it's not really. What that basically means for the non programmers out there is that when the game places a sprite, you can enter a value along with the sprite number that may alter something about that sprite, but what it does varies depending on what sprite you're working with. Now because these functions are now compiled, the only way you can effectively edit them is to find the code for it and decompile it into assembly, alter it as necessary, and then recompile it and enter it back into the rom. This document will contain all the info necessary to enter a new sprite or replace an old one, but when it comes to the ASM you're on your own.

Now, the value assigned to a sprite in the game is done by a pointer index at 1600C-1637C. It’s Pretty simple, just one pointer per address value. Here's the pointer list in Sonic 2. I've named all the ones I could at the time. I made this list quite a while ago though, and I think I could do a better job if I redid it now, but I can't be stuffed doing it at this point in time.

1	Sonic [19F50]
2	Tails [1B8A4]
3	??????? [1FCDC]
4	??????? [208DC]
5	??????? [1D200]
6	Rotating cylinder in Metropolis zone [214C4]
7	Oil (stretches across entire map) [24020]
8	Unknown (some kind of vertical barrier, but barrier is not visible) [1DD20]
9	??????? [338EC]
A	??????? [1D320]
B	Section of pipe that tips you off from chemical plant zone [2009C]
C	Unknown (some strange thing that bobs up and down. Possibly top of water in HPZ.) [20210]
D	Unknown (hit it from left or right and get thrown back faster than should be possible) [191B8]
E	Flashing stars from intro [12E18]
F	??????? [13600]
10	Unknown (tails won't move) [347EC]
11	??????? [F66C]
12	Master emerald [2031C]
13	??????? [203AC]
14	See saw thingy from hill top zone [21928]
15	Swinging platform from ARZ [FC9C]
16	Diagonally moving lift thing from HTZ [21DAC]
17	Unknown (looks like some sort of steam burst like from MTZ) [10310]
18	Stationary floating platform from ARZ [104AC]
19	Unknown (some kind of floating platform that moves back and forth, but not very far) [22018]
1A	Collapsing platform from either OOZ or MCZ [108BC]
1B	Booster things from CPZ [222AC]
1C	??????? [111D4]
1D	Unknown (seemed to have a random area within a set distance from the sprite in which you could get hurt.) [22408]
1E	Spin tube from CPZ [2259C]
1F	Collapsing platform from OOZ [10A08]
20	Unknown (lava bomb that Robotnic drops in HTZ?) [22FF8]
21	Unknown (sprite just changes between two palette lines) [80BE]
22	Arrow shooter from ARZ [25694]
23	Pillar that drops it's lower part from ARZ [2588C]
24	??????? [1F8A8]
25	A ring [11F44]
26	Monitor [12670]
27	An explosion of an enemy, giving off an animal and 100 points [21088]
28	An animal and the 100 that are given off from busting a badnick [1188C]
29	The 100 [11DC6]
2A	Stomper from MCZ [115C4]
2B	Raising pillar from ARZ [25A5A]
2C	Sprite that makes leaves fly off when you hit it from ARZ [26104]
2D	One way barrier from CPZ and DEZ [1169A]
2E	Image of what was in monitor flying up like it does when you bust it [12854]
2F	Unknown (a small block) [23300]
30	Unknown (something you can't see with a collision box the size of the whole damn screen) [238DC]
31	Lava collision marker [20DEC]
32	Unknown (A block that when you hit it from the top you get 100 points, and it then splits into six pieces which fly down off the screen) [2351A]
33	Platform that shoots up in the air from oil ocean zone [23AF4]
34	Sprite for level title card [13C48]
35	??????? [1D97E]
36	Vertical spikes [15900]
37	Sprite of rings spraying out from being hit by an enemy [12078]
38	Shield that surrounds character [1D8F2]
39	Unknown (sprite comes from the left, and sits on the screen near the centre. If you press jump it will reset to Sega logo. Possibly game over sprite.) [13F74]
3A	End of level result screen [14086]
3B	Unknown (a block) [15CC8]
3C	Unknown (a barrier you can smash by dashing into. Like the rock barriers in Sonic 3.) [15D44]
3D	Block thingy in OOZ that launches you into the round ball things [24DD0]
3E	Egg prison [3F1E4]
3F	Fan blowing to the left from OOZ [2A7B0]
40	Weird lever like spring from CPZ and ARZ [26370]
41	Red spring facing upwards [18888]
42	Thing that pops up and releases steam sideways from metropolis zone [26634]
43	Spiky ball from OOZ [23E40]
44	Star ball that sonic bounces off from CNZ [1F730]
45	Strange spring from OOZ facing vertically [240F8]
46	Ball sitting on strange spring from OOZ [24A16]
47	Button [2FCF4]
48	Thingy from OOZ that you enter when flying as a ball, and it turns and throws you another direction [25244]
49	Unknown (Huge vertical laser from WFZ?) [20B9E]
4A	Octopus badnick from OOZ [2CA14]
4B	Buzz bomber from EHZ [2D068]
4C	??????? [1637C]
4D	??????? [1637C]
4E	??????? [1637C]
4F	??????? [1637C]
50	Seahorse badnick from OOZ [2CCC8]
51	CNZ boss [318F0]
52	HTZ boss [2FC50]
53	Things spinning around MTZ boss [32940]
54	MTZ boss [32288]
55	OOZ boss [32F90]
56	EHZ boss [2EF18]
57	MCZ boss and Earthquake [30FA4]
58	Unknown (some kind of really big explosion?) [2D494]
59	Unknown (alters the palette to put in some colours that look really awesome in ARZ, then teleports you back to x0 y0 and doesn't allow you to move) [35FBC]
5A	??????? [35554]
5B	??????? [353FE]
5C	Shark badnick from EHZ [2D394]
5D	CPZ boss [2D734]
5E	??????? [6FC0]
5F	??????? [70F0]
60	??????? [34FA0]
61	??????? [34EB0]
62	??????? [1637C]
63	??????? [340A4]
64	Twin stompers from MTZ [26920]
65	Unknown (a long platform) [26AE0]
66	Unknown (a tall bumper from CNZ, that I don't think was ever implemented in a level) [26F58]
67	One of the spin tubes from MTZ [2715C]
68	Block that has a spike that comes out of each side sequentially from MTZ [27594]
69	Nut from MTZ [27884]
6A	Unknown (jump on it and it's fine. Jump off it and it moves down) [27AB0]
6B	Unknown (Same type of block as above, but it doesn't move) [27D6C]
6C	Unknown (a block that moves up and to the left slowly)[28034]
6D	Floor spike from MTZ [27794]
6E	Unknown (a block that rotates round in a circle) [283AC]
6F	End of special stage result screen [143C0]
70	Unknown (Big circle of sprites rotating around, and changing their tiles depending on their position) [285C0]
71	Unknown (six vertical blocks that change patterns) [112F0]
72	??????? [2893C]
73	Unknown (a small thing with a picture of a ring on it) [289D4]
74	Unknown (a small block with no patterns on it) [20EE0]
75	??????? [28BC8]
76	Spike block thing from MCZ [28DF8]
77	Unknown (long block) [28F88]
78	Stairs from CPZ that move down to open the way [291CC]
79	Starpoll [1F0B0]
7A	Platform that moves back and forth on top of chemicals in CPZ [293A0]
7B	Unknown (acts exactly like a spring, but tries to use level specific patterns) [29590]
7C	Big pylon thing that's in the foreground in CPZ [20FD2]
7D	??????? [1F624]
7E	??????? [1E0F0]
7F	Vine switch that you hang off in MCZ [297E4]
80	Vine that you hang off and it moves down from MCZ [2997C]
81	Unknown (Long invisible vertical barrier. Suspect it's pasted over the force fields used in the boss at the end of WFZ.) [2A000]
82	Platform that is usually swinging from ARZ [2A290]
83	3 adjoined platforms from ARZ that rotate in a circle [2A4FC]
84	??????? [2115C]
85	Vertical spring from CNZ that you hold down jump on to pull back further [2AB84]
86	Flipper from CNZ [2B140]
87	??????? [7356]
88	??????? [34A5C]
89	ARZ boss [30480]
8A	Unknown (a line of text on the centre of the screen. Will have to put in correct tiles to see what it says.) [3EAC8]
8B	??????? [21392]
8C	Blowfly enemy from ARZ [36924]
8D	Unknown (ummm, a blowfly behind a wall like grounder is, but not exactly behind it. When it busts out weird things happen.) [36A76]
8E	Unknown (Same as above) [36A76]
8F	Unknown (a thing that sits in one place and does nothing. You can bust it like a badnick.) [36B88]
90	Unknown (a sprite that launches up and to the left, then curves back down. You can bust it like a badnick.) [36BD4]
91	Unknown (heads towards you. Can be busted like a badnick.) [36DAC]
92	Unknown (when you pass it, it sends a large burst up which hurts you.) [36F0E]
93	Unknown (sprite rapidly reverses direction and spirals down.) [36FE6]
94	Lava snake [37322]
95	Red circle dude from hill top zone that has lava balls circling around him that he throws at you. [370FE]
96	Lava snake... again [37322]
97	Unknown (sprite accelerates down and to the right) [373D0]
98	??????? [376E8]
99	Bomber badnick from SCZ [377C8]
9A	Turtle thing on the back of another larger turtle thing from SCZ [37936]
9B	??????? [37A06]
9C	??????? [37A82]
9D	Coconuts (the badnick) from EHZ [37BFA]
9E	Snake like badnick from MCZ that flies out from wall at you [37E16]
9F	Crab like badnick from MTZ [3800C]
A0	Unknown (sprite falls down) [3815C]
A1	Praying Mantis dude from MTZ [383B4]
A2	Unknown (sprite falls down, then locks up emulator) [384A2]
A3	Glowbug from MCZ [3873E]
A4	Exploding star from MTZ [3899C]
A5	Unknown (sprite moves back and forth, throwing something at you that curves up and back down) [38AEA]
A6	Unknown (sprite moves up and down slowly, shooting a fast moving projectile down and to the left) [38B86]
A7	Spider badnick from CPZ [38DBA]
A8	??????? [38F66]
A9	??????? [39032]
AA	??????? [39066]
AB	Unknown (sprite sits there and does nothing. You can bust it like a badnick) [390A2]
AC	Jet badnick from SCZ [3937A]
AD	Unknown (appears to be a block and has no image, but if you hit it in the corner it busts like a badnick) [3941C]
AE	??????? [39452]
AF	Unknown (block that laser beams shoot from to form barrier at end of WFZ? Just a hunch.) [3972C]
B0	Unknown (weird sums it up. It alters the palette, 128x128 block mappings, and quickly moves to the left as a killable badnick. Destroying it interrupts the process of altering the block mappings data (among other things).) [3A1DC]
B1	Unknown (appears across from it's actual position and does nothing. Can be busted like a badnick.) [3A3F8]
B2	??????? [3A790]
B3	??????? [3B2DE]
B4	Unknown (sprite flashes between frames. Can be busted like a badnick.) [3B36A]
B5	??????? [3B3FA]
B6	Tilting platform from WFZ [3B5D0]
B7	Unknown (sprite flashes between there and not there really fast, then disappears completely) [3B8A6]
B8	Wall turret from WFZ [3B968]
B9	Unknown (sprite flies really fast to the left. I suspect it's the laser shot from wing fortress zone.) [3BABA]
BA	Unknown (sprite sits there and does nothing. Can be busted like a badnick.) [3BB4C]
BB	Unknown (refer to above) [3BB7C]
BC	Unknown (sprite flashes present and not present really fast) [3BBBC]
BD	Unknown (sprite keeps on dropping things that fall straight down) [3BC1C]
BE	Platform that pops in and out from WFZ [3BD7A]
BF	Unknown (sprite flashes between frames) [3BEAA]
C0	Unknown (it appears to be a spring sort of thing that was cut from the final. It makes you bounce off to the left without gaining much height.) [3BF04]
C1	Sprite that sonic hangs onto on the back of Robotnic's getaway ship at the end of WFZ [3C0AC]
C2	Rivet thing that you bust to get into the ship at the end of WFZ [3C328]
C3	Unknown (sprite moves up and to the left changing frames as it goes, then disappears completely.) [3C3D6]
C4	Unknown (same as above) [3C3D6]
C5	Laser boss guy at end of WFZ [3C442]
C6	Unknown (sprite sits there and does nothing. Can be busted like a badnick) [3CED0]
C7	Death Egg [3D4C8]
C8	Bad guy in CNZ [3D23E]
C9	Unknown (sprite changes palette) [132F0]
CA	Movie sequence at end of game (will display the pictures, alter the block mappings, and after sequence is over will teleport Sonic to appropriate location on map.) [A1D6]
CB	??????? [A9F2]
CC	Unknown (appears to be later trigger of above one. It will alter block mappings and teleport sonic.) [A3C8]
CD	Unknown (sprite bobs up and down, then moves off to the left) [AAAE]
CE	??????? [A894]
CF	??????? [A988]
D0	??????? [1637C]
D1	??????? [1637C]
D2	Flashing blocks in CNZ that appear and disappear in a rectangular shape that you can walk across [2B528]
D3	??????? [2B84C]
D4	Big block from casino night zone that moves back and forth [2B8EC]
D5	Elevator going up from CNZ [2BA08]
D6	Poky that gives out points from CNZ [2BB6C]
D7	Unknown (some kind of bumper) [2C448]
D8	Block thingy in CNZ that changes colour when you hit it and gives you points, until it disappears) [2C6AC]
D9	A sprite that you can hold on to, like the rails in WFZ [2C92C]
DA	Continue text (kinda funny. It uses the rings variables to store the number of seconds you have left, so it's weird to see it in a level.) [7A68]
DB	Sprite of sonic lying down from continue screen [7B82]
DC	??????? [125E6]

References

Sonic Community Hacking Guide
General
SonED2 Manual \| Subroutine Equivalency List
Game-Specific
Sonic the Hedgehog (16-bit) \| Sonic the Hedgehog (8-bit) \| Sonic CD (prototype 510) \| Sonic CD \| Sonic CD (PC) \| Sonic CD (2011) \| Sonic 2 (Simon Wai prototype) \| Sonic 2 (16-bit) \| Sonic 2 (Master System) \| Sonic 3 \| Sonic 3 & Knuckles \| Chaotix \| Sonic Jam \| Sonic Jam 6 \| Sonic Adventure \| Sonic Adventure DX: Director's Cut \| Sonic Adventure DX: PC \| Sonic Adventure (2010) \| Sonic Adventure 2 \| Sonic Adventure 2: Battle \| Sonic Adventure 2 (PC) \| Sonic Heroes \| Sonic Riders \| Sonic the Hedgehog (2006) \| Sonic & Sega All-Stars Racing \| Sonic Unleashed (Xbox 360/PS3) \| Sonic Colours \| Sonic Generations \| Sonic Forces
Technical information
Sonic Eraser \| Sonic 2 (Nick Arcade prototype) \| Sonic CD (prototype; 1992-12-04) \| Dr. Robotnik's Mean Bean Machine \| Sonic Triple Trouble \| Tails Adventures \| Sonic Crackers \| Sonic 3D: Flickies' Island \| Sonic & Knuckles Collection \| Sonic R \| Sonic Shuffle \| Sonic Advance \| Sonic Advance 3 \| Sonic Battle \| Shadow the Hedgehog \| Sonic Rush \| Sonic Classic Collection \| Sonic Free Riders \| Sonic Lost World
Legacy Guides
The Nemesis Hacking Guides	The Esrael Hacking Guides
ROM: Sonic 1 \| Sonic 2 \| Sonic 2 Beta \| Sonic 3 Savestate: Sonic 1 \| Sonic 2 Beta/Final \| Sonic 3	Sonic 1 (English / Portuguese) \| Sonic 2 Beta (English / Portuguese) \| Sonic 2 and Knuckles (English / Portuguese)
Move to Sega Retro
Number Systems (or scrap) \| Assembly Hacking Guide \| 68000 Instruction Set \| 68000 ASM-to-Hex Code Reference \| SMPS Music Hacking Guide \| Mega Drive technical information

SCHG