Nem s1

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 It has not been revised since its original writing and may be outdated.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.

Introduction

Here are my hacking notes on the Sonic 1 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 1s and 0s. 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 1s and 0s, 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 1 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 1:

00	Green Hill zone
01	Labyrinth zone (act 4 is scrap brain zone 3)
02	Marble zone
03	Star Light zone
04	Spring Yard zone
05	Scrap Brain zone (act 3 is Final 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 256x256 block, and these are the things that the actual level info loads. It is made up of 256 16x16 blocks, and each block inside them can use a different palette line. You cannot place anything except a sprite or a 256x256 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 1D000. 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. Also, any patterns that are not used anywhere in the game will be preceeded by this red dot .

0-FF	Vector table
100-200	Header
438-5B8	Platform error messages (ASCII text)
5F0-B0F	Standard numbers/symbols	Uncompressed Further info
1B66-1CA9	Unknown palettes
1D0A-1DAB	Unknown palettes
20C0-20FB	Unknown palette
2168-2207	Palette pointers	Further info
2208	SEGA screen palette	Further info
2288	Title screen palette	Further info
2308	Level select palette	Further info
2388	Primary palette line	Further info
23A8	GHZ level palette	Further info
2408	LZ level palette	Further info
2468	LZ underwater level palette	Further info
24E8	MZ level palette	Further info
2548	SLZ level palette	Further info
25A8	SYZ level palette	Further info
2608	SBZ level palette	Further info
2668	FZ level palette	Further info
26C8	Special stage palette	Further info
2748	SBZ act 3 level palette	Further info
27A8	SBZ act 3 underwater level palette	Further info
2828	SBZ act 3 primary palette line	Further info
2868	Special stage results palette	Further info
28E8	Unknown palette	Further info
2928	Ending sequence level palette	Further info
337A-339F	Level select level order	Further info
379A-37A0	Music playlist for levels	Further info
4100-4117	16x16 collision index pointers	Further info
4E42	Continue screen (sprite)
4F5C	??????? (sprite)
53E2	??????? (sprite)
553C	??????? (sprite)
5616	??????? (sprite)
59E2	??????? (sprite)
5AC2	??????? (sprite)
6112-6181	Character start location array	Further info
6DD0-6DDD	Offset index for dynamic screen resizing	Further info
6DDE	GHZ dynamic screen resizing	Further info
6EF2	LZ dynamic screen resizing	Further info
6F8E	MZ dynamic screen resizing	Further info
70F2	SLZ dynamic screen resizing	Further info
7168	SYZ dynamic screen resizing	Further info
720A	SBZ dynamic screen resizing	Further info
7326	Ending sequence dynamic screen resizing (empty)	Further info
7328	??????? (sprite)
7952	??????? (sprite)
7CA2	??????? (sprite)
7E32	??????? (sprite)
81A8	??????? (sprite)
8210	Collapsable platform (sprite)
8320	Platform in MZ made up of blocks that breaks. (sprite)
87CA	??????? (sprite)
885E	??????? (sprite)
8934	??????? (sprite)
8B4C	Double big red explosion (sprite)
8C1C	Big red explosion (sprite)
8CF2	??????? (sprite)
8D62	Normal explosion (sprite)
8DF6	Large explosion (sprite)
8F22	??????? (sprite)
9412	??????? (sprite)
94F0	Crab enemy from GHZ (sprite)
97DE	Buzz bomber (sprite)
9932	??????? (sprite)
9B26	??????? (sprite)
9CB6	Rings flying out from being hit (sprite)
9E06	Large golden ring (sprite)
9ED6	Special stage entry effect (sprite)
A118	??????? (sprite)
A302	Contents of monitor flying up frm monitor being busted (sprite)
A61C	??????? (sprite)
A69C	??????? (sprite)
AB20	??????? (sprite)
ABDA	??????? (sprite)
ACA4	Drill badnick in LZ (sprite)
AEB8	Platfrom in MZ that moves up and down (sprite)
B1DC	??????? (sprite)
B39E	??????? (sprite)
B67C	Metal block with spikes on bottom from MZ (sprite)
B99A	Sideways spikes that were cut in MZ (sprite)
BD2E	Button in MZ (sprite)
BED2	Moveable block in MZ (sprite)
C306	Title card (sprite)
C4B8	"Game over" sprite (sprite)
C572	End of level results screen (sprite)
C6F4-C723	Main game level order	Further info
C7AE	End of special stage results screen (sprite)
C956	Bubble (sprite)
CE28	Spikes (sprite)
D050	Rock from GHZ (sprite)
D0BE	??????? (sprite)
D11C	Breakable barrier from GHZ (sprite)
D382-D5B1	Object pointer list	Further info
D5B2	??????? (sprite)
DABE	Red spring (sprite)
DD68	Badnick that appears behind wall and fires projectile from GHZ (sprite)
DFFA	??????? (sprite)
E1D4	Wall barrier from GHZ (sprite)
E296	??????? (sprite)
E304	??????? (sprite)
E4E0	??????? (sprite)
E7D0	??????? (sprite)
E90C	??????? (sprite)
E9D0	Bumper (sprite)
EB28	??????? (sprite)
ED84	Lava spurt with lava ripples (sprite)
EE70	Lava spurt (sprite)
F05E	Lava wave from MZ (sprite)
F1B8	Sprite for lava hurting you (sprite)
F61A	Motobug from GHZ (sprite)
F806	??????? (sprite)
F83A	That ground based badnick with spikes ontop from SYZ (sprite)
FC92	Breakable block in MZ (sprite)
FDF8	??????? (sprite)
100A8	Bat from MZ (sprite)
10286	??????? (sprite)
10782	??????? (sprite)
10976	Same as above, but different image (sprite)
10AD2	Raising platforms from SLZ (sprite)
10D5A	Platform moving in circular motion in SLZ (sprite)
10E90	Lowering staircase from SLZ (sprite)
11044	Pylon in foreground frm SLZ (sprite)
110C6	??????? (sprite)
11206	??????? (sprite)
11346	??????? (sprite)
1141A	??????? (sprite)
114FC	Fan from SLZ (sprite)
1163C	See saw without spikeball (sprite)
119F6	Walking bomb (sprite)
11CCE	Badnick surrounded by spikeballs that shoots them at you (sprite)
11F04	??????? (sprite)
11FD4	??????? (sprite)
121E8	Stone head that launches projectile in LZ (sprite)
1233C	??????? (sprite)
126A4	Small bubble (sprite)
12A9E	??????? (sprite)
12BD8	Sonic (sprite)
13C98	??????? (sprite)
14158	??????? (sprite)
14254	Old special entry effect (sprite)
142F0	??????? (sprite)
1509A	Spinning wheel in SBZ that shoots you off in different directions (sprite)
154E2	??????? (sprite)
1568E	??????? (sprite)
15730	??????? (sprite)
1597C	??????? (sprite)
15C12	??????? (sprite)
15FF8	??????? (sprite)
16146	??????? (sprite)
1626A	??????? (sprite)
1652A	Big moving rectangle in SBZ (sprite)
16668	??????? (sprite)
16934	Catakiller (sprite)
16E2E	Starpoll (sprite)
1715E	??????? (sprite)
17264	??????? (sprite)
17700	Robotnic in his primary ship (sprite)
17AE4	Large ball from GHZ that was cut (sprite)
17E36	??????? (sprite)
18230	Robotnic flying his default ship (sprite)
186A4	??????? (sprite)
188B8	??????? (sprite)
18CCE	??????? (sprite)
190EE	??????? (sprite)
19692	??????? (sprite)
19832	??????? (sprite)
19B60	??????? (sprite)
19D58	??????? (sprite)
1A44E	??????? (sprite)
1A7DA	??????? (sprite)
1AB22	Egg prison (sprite)
1B984	Sonic in special stage (sprite)
1C022	??????? (sprite)
1C046-1C053	Offset index of animated pattern load cues	Further info
1C054	GHZ animated pattern load cue	Further info
1C112	MZ animated pattern load cue	Further info
1C1AE	Unknown animated pattern load cue	Further info
1C1EC	SBZ animated pattern load cue	Further info
1C2A2	End of game sequence animated pattern load cue	Further info
1C3B6	LZ/SLZ/SYZ animated pattern load cue (empty)	Further info
1C548	??????? (sprite)
1CADA-1CF19	Big and small numbers used on counters in game	Uncompressed Further info
1D16E-1D17B	Offset index of object debug lists	Further info
1D17C	GHZ object debug list	Further info
1D1FE	LZ object debug list	Further info
1D2C8	MZ object debug list	Further info
1D35A	SLZ object debug list	Further info
1D3D4	SYZ object debug list	Further info
1D44E	SBZ object debug list	Further info
1D538	Ending sequence object debug list	Further info
1D5A2	Main level load blocks (patterns/16x16/256x256/palette)	Further info
1D612	Offset index of pattern load cues	Further info
1D652	Pattern load cues	Further info
1DB12	Data block acting as seperator
1DC00	"SEGA" Patterns	Art compression 123 blocks
1E26E	Patterns for foreground in intro	Art compression 254 blocks
1F27A	Patterns for sonic in intro	Art compression 525 blocks
20C48	"TM"	Art compression 00, 2 blocks
20D3A	Hidden Japanese writing on "Sonic team presents" screen	Art compression 00, 166 blocks
211E2-21291	Unknown offset index
21292-217FD	Data that offset index is linking to (something to do with mappings for Sonic)
217FE-218AD	Unknown offset index
218AE-21AFD	Data that offset index is linking to (something to do with mappings for Sonic)
21AFE-2BC1D	Patterns for Sonic	Uncompressed Further info
2BC1E	Smoke/dust trail	Art compression 00, 16 blocks
2BD22	Stars	Art compression 00, 3 blocks
2BD5C	Bubble	Art compression 27 blocks
2BEF2	Stars from invincibility	Art compression 00, 36 blocks
2C03A	Three different sized air bubbles, and a picture of Sonic holding his breath.	Art compression 15 blocks
2C144	Projectile that stone head shoots at you in LZ	Art compression 00, 3 blocks
2C196	Original special stage entrance effect	Art compression 00, 59 blocks
2C5E4	Block in special stage from every angle it rotates to	Art compression 249 blocks
2D050	Background of special stage	Art compression 241 blocks
2DFE0	Bubbles/clouds in background of special stage	Art compression 00, 81 blocks
2E4D4	"Goal" block from special stage	Art compression 18 blocks
2E5C2	"R" block from special stage	Art compression 9 blocks
2E692	Sonic life block from special stage	Art compression 00, 9 blocks
2E788	Stars from getting emerald in special stage	Art compression 00, 9 blocks
2E7E6	Closed one way block from special stage	Art compression 9 blocks
2E878	"Zone 1" block from special stage	Art compression 9 blocks
2E93A	"Zone 2" block from special stage	Art compression 9 blocks
2EA08	"Zone 3" block from special stage	Art compression 9 blocks
2EAD4	"Zone 4" block from special stage	Art compression 9 blocks
2EB9C	"Zone 5" block from special stage	Art compression 9 blocks
2EC66	"Zone 6" block from special stage	Art compression 9 blocks
23D36	"Up/Down" block from special stage	Art compression 27 blocks
2EF2A	4 emeralds present in special stage	Art compression 16 blocks
2F092	One way block from special stage	Art compression 00, 9 blocks
2F142	"W" block from special stage	Art compression 9 blocks
2F21C	Desroyable blocks from special stage	Art compression 9 blocks
2F2A0	Different 4 emeralds present in special stage	Art compression 16 blocks
2F41E	Stalk of flower	Art compression 00, 4 blocks
2F468	Swinging platform	Art compression 14 blocks
2F582	Rope bridge in EHZ	Art compression 00, 10 blocks
2F66E	2x2 block	Art compression 4 blocks
2F6B6	Ball that is swung by robotnic in GHZ	Art compression 38 blocks
2F854	Vertical and horizontal spike	Art compression 8 blocks
2F8D0	Rotating spike log in GHZ	Art compression 44 blocks
2FAE4	Rotating spikes off log in GHZ	Art compression 18 blocks
2FC10	Rock in EHZ	Art compression 24 blocks
2FD3E	Destroyable section of wall in GHZ	Art compression 12 blocks
2FDDC	Normal section of wall in GHZ	Art compression 12 blocks
2FE3C	Surface of water in LZ	Art compression 00, 16 blocks
2FF60	Waterfall and water	Art compression 00, 144 blocks
30754	Swinging spikeball	Art compression 00, 24 blocks
308D4	Flat and diagonal blocks from SBZ	Art compression 32 blocks
30A3E	Bubbles and numbers for underwater	Art compression 00, 116 blocks
31094	Block from LZ	Art compression 8 blocks
3114A	Another block from LZ	Art compression 8 blocks
311EC	Horizontal and vertical spear in LZ that extends and retracts	Art compression 18 blocks
31348	Broken bar from LZ	Art compression 8 blocks
313AC	Block from LZ	Art compression 16 blocks
314FE	Wheel from LZ	Art compression 72 blocks
319E0	Head that launches projectile in LZ	Art compression 17 blocks
31B50	Level blocks from LZ	Art compression 104 blocks
31E08	More blocks from LZ	Art compression 24 blocks
31F40	Cork block from LZ	Art compression 16 blocks
3206A	A block from LZ	Art compression 16 blocks
3217A	Metal blocks from MZ	Art compression 69 blocks
32408	Button in LZ and MZ	Art compression 8 blocks
324C6	Ball that swinging platform attaches to in MZ	Art compression 26 blocks
3257E	Unknown. Grass... or something	Art compression 6 blocks
325D2	Fireballs from MZ	Art compression 44 blocks
328B0	Lava stuff from MZ	Art compression 156 blocks
331C6	Moveable block from MZ	Art compression 24 blocks
3332A	Block in background frm MZ	Art compression 00, 16 blocks
33978	Metal block from SLZ	Art compression 00, 8 blocks
333B2	See-saw from SLZ	Art compression 44 blocks
335EE	Spikeballs that flash red and explode that robotnic drops in SLZ	Art compression 00, 18 blocks
33734	Fan from SLZ	Art compression 44 blocks
339DA	Pylon from SLZ	Art compression 16 blocks
33ABC	Spikeball from SLZ	Art compression 32 blocks
33C9E	Misc blocks for SLZ	Art compression 00, 16 blocks
33DAA	Fireball launcher from SLZ	Art compression 8 blocks
33E4E	Bumper from special stage and SYZ	Art compression 22 blocks
33FB8	Small spikeball	Art compression 00, 4 blocks
3401A	Normal button pressed/unpressed, and red button unpressed	Art compression 12 blocks
340FC	Spikeball and chain	Art compression 36 blocks
3438A	Indent on spinning wheel in SBZ	Art compression 4 blocks
343DE	Platforms from SBZ	Art compression 79 blocks
3467A	Pizza slicer thing from SBZ	Art compression 36 blocks
3487E	Stomper from SBZ	Art compression 40 blocks
34A1C	Tilting platform from SBZ	Art compression 00, 48 blocks
34D4C	Some blocks from SBZ	Art compression 49 blocks
34F2A	Metal block from collapsing platform in SBZ	Art compression 00, 4 blocks
34F82	Zapper thing from SPZ	Art compression 00, 20 blocks
35102	Blocks from SBZ	Art compression 28 blocks
35200	Flame pipes from SBZ	Art compression 28 blocks
3538C	One way barrier frm SBZ	Art compression 8 blocks
353DC	Red and metal platforms from SBZ	Art compression 15 blocks
35434	Blocks from SBZ	Art compression 00, 15 blocks
35530	Metal blocks from SBZ	Art compression 00, 12 blocks
35646	Pig badnick	Art compression 47 blocks
35A06	Crab badnick	Art compression 00, 68 blocks
35EF4	Buzz bomber	Art compression 55 blocks
3629A	Fireball	Art compression 36 blocks
36482	Drill noze badnick from LZ	Art compression 00, 90 blocks
36B6C	Fish badnick from GHZ	Art compression 32 blocks
36DD4	Fish badnick from LZ	Art compression 32 blocks
3705E	Enemy that can enter a ball from SYZ	Art compression 80 blocks
37582	Driving enemy from GHZ	Art compression 00, 29 blocks
3780C	Enemy that appears and jets along the ground	Art compression 00, 85 blocks
37E2A	Enemy with spikes on top	Art compression 61 blocks
38212	Bat	Art compression 00, 41 blocks
3850E	Splats badnick	Art compression 29 blocks
38756	Walking bomb	Art compression 41 blocks
389EE	Enemy with spikeballs around it	Art compression 31 blocks
38BCC	Caterkiller	Art compression 16 blocks
38D5A	Level splash screen tiles	Art compression 128 blocks
39368	Rings/Score/Time patterns	Art compression 24 blocks
3945E	Sonic life counter patterns	Art compression 12 blocks
39564	Patterns for rings	Art compression 00, 14 blocks
39658	Monitor/contents of monitor patterns	Art compression 64 blocks
39AB8	Explosion	Art compression 96 blocks
3A11E	Points popping up onscreen from busting badnick	Art compression 9 blocks
3A1CE	"Game over" text	Art compression 34 blocks
3A360	Horizontal Spring	Art compression 00, 16 blocks
3A462	Vertical spring	Art compression 00, 14 blocks
3A53E	End of level signpost	Art compression 58 blocks
3A9BA	Starpoll	Art compression 10 blocks
3AA7A	Flash from entering giant golden ring	Art compression 84 blocks
3ABEE	Hidden points at end of levels	Art compression 00, 36 blocks
3AEF0	Sonic lying down anomation in "Continue?" screen	Art compression 37 blocks
3B1A0	"Continue" and mini sonic for it.	Art compression 30 blocks
3B3DA	Rabbit	Art compression 00, 18 blocks
3B532	Chicken	Art compression 14 blocks
3B68E	Blackbird	Art compression 00, 18 blocks
3B80A	Seal	Art compression 00, 14 blocks
3B926	Pig	Art compression 18 blocks
3BA5C	Bluebird	Art compression 00, 14 blocks
3BB96	Squirrel	Art compression 00, 18 blocks
3BCF2	GHZ 16x16 block mappings	Further info
3C692	GHZ Primary pattern block	Art compression 461 blocks
3DCF2	GHZ secondary pattern block	Art compression 369 blocks
3F09A	GHZ 256x256 block mappings	Further info
411AA	LZ 16x16 block mappings	Further info
414D4	LZ primary pattern block	Art compression 454 blocks
4301C	LZ 256x256 block mappings	Further info
4580C	MZ 16x16 block mappings	Further info
46016	MZ primary pattern block	Art compression 663 blocks
4885A	MZ 256x256 block mappings	Further info
4A01A	SLZ 16x16 block mappings	Further info
4A7FE	SLZ primary pattern block	Art compression 874 blocks
4E352	SLZ 256x256 block mappings	Further info
50782	SYZ 16x16 block mappings	Further info
510F0	SYZ primary pattern block	Art compression 882 blocks
5481C	SYZ 256x256 block mappings	Further info
56BCC	SBZ 16x16 block mappings	Further info
57A66	SBZ primary pattern block	Art compression 681 blocks
5A212	SBZ 256x256 block mappings	Further info
5CC62	Robotnic's main ship	Art compression 108 blocks
5D4C6	GHZ boss	Art compression 00, 42 blocks
5D7B0	Egg prison	Art compression 117 blocks
5E034	Robotnic	Art compression 144 blocks
5E860	Stomper boss from final zone	Art compression 156 blocks
5EFC8	Robotnic's ship at end of final zone	Art compression 00, 76 blocks
5F548	Rocket thrust from robotnic's ship	Art compression 00, 17 blocks
5F668	7 chaos emeralds	Art compression 28 blocks
5F866	Patterns for Sonic in ending sequence	Art compression 323 blocks
60A44	Robotnic in Try Again screen	Art compression 139 blocks
61388	Robotnic's ship in flames and blowing up in background	Art compression 00, 47 blocks
61CEE	Grass and flower against both checkered and non checkered background	Art compression 37 blocks
61F76	Alphabet	Art compression 96 blocks
62574	"Sonic the hedgehog" writing from end of game	Art compression 48 blocks
627FC-628FF	Data block acting as seperator
62900-629FF	Curve and resistance mapping	Further info
62A00-649FF	Collision array	Further info
64A00	GHZ 16x16 collision index	Further info
64B9A	LZ 16x16 collision index	Further info
64C62	MZ 16x16 collision index	Further info
64DF2	SLZ 16x16 collision index	Further info
64FE6	SYZ 16x16 collision index	Further info
651DA-65431	SBZ 16x16 collision index	Further info
65432-66AD5	Something to do with special stages
66AD6-66CD5	Waterfall tiles from GHZ	Uncompressed Further info
66CD6-670D5	Flower with pointy petals in GHZ	Uncompressed Further info
670D6-67559	Flower with curved petals in GHZ	Uncompressed Further info
6755A-67835	Lava surface	Uncompressed Further info
67836-67E55	Lava	Uncompressed Further info
67E56-68155	Flames against stone from MZ	Uncompressed Further info
68156-68BD5	Steam in background from SBZ	Uncompressed Further info
68BD6	Offset index of level data	Unique offset Further info
68C7E	GHZ act 1 foreground	Further info
68D74	GHZ act 2 foreground	Further info
68E40	GHZ act 3 foreground	Further info
68F62	GHZ act 1/2/3/ending sequence background	Further info
68F8C	LZ act 1 foreground	Further info
6908E	LZ act 1/2/3/SBZ 3 background	Further info
69194	LZ act 2 foreground	Further info
69232	LZ act 3 foreground	Further info
69350	SBZ act 3 foreground	Further info
6940E	MZ act 1 foreground	Further info
694AC	MZ act 1 background	Further info
69526	MZ act 2 foreground	Further info
695CA	MZ act 2 background	Further info
69660	MZ act 3 foreground	Further info
69742	MZ act 3 background	Further info
697EE	SLZ act 1 foreground	Further info
69900	SLZ act 1/2/3 background	Further info
6997A	SLZ act 2 foreground	Further info
69A6A	SLZ act 3 foreground	Further info
69B88	SYZ act 1 foreground	Further info
69C44	SYZ act 1/2 background	Further info
69C82	SYZ act 2 foreground	Further info
69D8A	SYZ act 3 foreground	Further info
69EEC	SBZ act 1 foreground	Further info
6A00E	SBZ act 1 background	Further info
6A04C	SBZ act 2/FZ foreground	Further info
6A18E	SBZ act 2/FZ background	Further info
6A300	Ending sequence foreground	Further info
6A324-6AF63	Giant spinning golden ring	Uncompressed Further info
6AF64-6AFFF	Data block acting as seperator
6B000-6B095	Offset index of sprite locations	Unique offset
6B096	GHZ act 1/4 sprite locations	Further info
6B5A0	GHZ act 2 sprite locations	Further info
6BB5E	GHZ act 3 sprite locations	Further info
6C218	LZ act 1 sprite locations	Further info
6C686	LZ act 2 sprite locations	Further info
6C9C8	LZ act 3 sprite locations	Further info
6CFAA	SBZ act 3 sprite locations	Further info
6D442	End of game sequence sprite locations	Further info
6D592	MZ act 1/4 sprite locations	Further info
6D8FE	MZ act 2 sprite locations	Further info
6DDA8	MZ act 3 sprite locations	Further info
6E31E	SLZ act 1/4 sprite locations	Further info
6E85E	SLZ act 2 sprite locations	Further info
6ECC6	SLZ act 3 sprite locations	Further info
6F2A8	SYZ act 1/4 sprite locations	Further info
6F734	SYZ act 2 sprite locations	Further info
6FC9E	SYZ act 3 sprite locations	Further info
702A4	SBZ act 1/FZ sprite locations	Further info
709E2	SBZ act 2 sprite locations	Further info
710C0	SBZ act 3 sprite locations	Further info
710D2	Title screen sprite locations	Further info
71204	Level value 06 act 1/2/3/4 sprite locations	Further info
71360	Unknown sprite locations (empty)	Further info
71366-7198F	Data block acting as seperator
71A9C	Midi pointers	Further info
745DC	Track 00
74D44	Track 01
75194	Track 02
755F2	Track 03
75A60	Track 04
75EAC	Track 05
763FC	Track 06
765E2	Track 07
766D0	Track 08
769FC	Track 09
76BA8	Track 0A
76E42	Track 0B
7705E	Track 0C
7727A	Track 0D
773D0	Track 0E
77520	Track 0F
7767E	Track 10
7894C	Track 11
78A76	Track 12
78B44	Sound FX pointers	Further info
78C08	Sound 00
78C1E	Sound 01
78C48	Sound 02
78C68	Sound 03
78C9A	Sound 04
78CD0	Sound 05
78CFC	Sound 06
78D2C	Sound 07
78D5C	Sound 08
78D90	Sound 09
78DA2	Sound 0A
78DE4	Sound 0B
78E04	Sound 0C
78E38	Sound 0D
78E6E	Sound 0E
78EB8	Sound 0F
78EE6	Sound 10
78F18	Sound 11
78F44	Sound 12
78F94	Sound 13
78FDE	Sound 14
7903A	Sound 15
79068	Sound 16
79086	Sound 17
790C2	Sound 18
790E0	Sound 19
79144	Sound 1A
7916C	Sound 1B
79198	Sound 1C
791DA	Sound 1D
7922E	Sound 1E
79268	Sound 1F
792DA	Sound 20
79308	Sound 21
79342	Sound 22
7936C	Sound 23
793E6	Sound 24
7940E	Sound 25
79476	Sound 26
794B8	Sound 27
794E6	Sound 28
794F8	Sound 29
79526	Sound 2A
79554	Sound 2B
795A0	Sound 2C
795DA	Sound 2D
795E8	Sound 2E
79616	Sound 2F
7964E	Sound 30
79687-7FFFF	Sega intro sound	16000Hz mono 8-bit unsigned PCM raw audio Further info

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 vaues, 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 in the S2 rom. 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.

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

These are the blocks that tie all the pieces for a level together. Pretty much everything major to do with a level is linked back to this block at some point.

One thing that's very imporant to understand is that the game engine used in S1 was not completed. It looks like some things that Yuji Naka planned to implement were not completed due to lack of time. Some of these things show up in the main level load block. First of all, the pointer to the main level patterns in this block is not used by the game at all. Instead the main level patterns are included on the pattern load cue for that level. In S2 another compression format was made, and the main level patterns were stored in this format, which was less dence to allow the main level patterns to be loaded into the VRAM faster, and for that this pointer was used. Several other things that were only to use one byte were also allowed for in this block, such as the music track to play for that level. Some of these were later moved to seperate sections of the code though, so only the pattern load cue and palette index numbers are used.

Here's the main level load block for MZ:

0804		6016		0904		580C		0004		885A		0083		0606
1	2	3	4	5	6	7	8	9	A	B	C	D	E	F	10

And here's what each byte does:

1	Index number of first pattern load cue	Further info
2-4	Main level patterns	Unused
5	Index number of second pattern load cue	Further info
6-8	16x16 block mappings	Further info
9	Blank	Unused
A-C	256x256 block mappings	Further info
D	Blank	Unused
E	Music track to play for level	Unused
F	Unknown	Unused
10	Index number of palette to use for level	Further info

Now that you know that, here's a breakdown of the main level load blocks in the S1 rom:

1D5A2: Main level load blocks (patterns/16x16/256x256/palette)

1D5A2-1D5B1: Green hill zone (00)

1D5A2: GHZ main level patterns (3DCF2) [04]

1D5A6: GHZ 16x16 block mappings (3BCF2) [05]

1D5AA: GHZ 256x256 bock mappings (3F09A) [00]

1D5AE: GHZ main level palette (04) [00 81 04]

1D5B2-1D5C1: Labyrinth zone (01)

1D5B2: LZ main level patterns (414D4) [06]

1D5B6: LZ 16x16 block mappings (411AA) [07]

1D5BA: LZ 256x256 block mappings (4301C) [00]

1D5BE: LZ main level palette (05) [00 82 05]

1D5C2-1D5D1: Marble zone (02)

1D5C2: MZ main level patterns (46016) [08]

1D5C6: MZ 16x16 block mappings (4580C) [09]

1D5CA: MZ 256x256 block mappings (4885A) [00]

1D5CE: MZ main level palette (06) [00 83 06]

1D5D2-1D5E1: Star light zone (03)

1D5D2: SLZ main level patterns (4A7FE) [0A]

1D5D6: SLZ 16x16 block mappings (50782) [0B]

1D5DA: SLZ 256x256 block mappins (5481C) [00]

1D5DE: SLZ main level palette (07) [00 84 07]

1D5E2-1D5F1: Spring yard zone (04)

1D5E2: SYZ main level patterns (510F0) [0C]

1D5E6: SYZ 16x16 block mappings (50782) [0D]

1D5EA: SYZ 256x256 block mappins (5481C) [00]

1D5EE: SYZ main level palette (08) [00 85 08]

1D5F2-1D601: Scrap brain zone (05)

1D5F2: SBZ main level patterns (57A66) [0E]

1D5F6: SBZ 16x16 block mappings (56BCC) [0F]

1D5FA: SBZ 256x256 block mappins (5A212) [00]

1D5FE: SBZ main level palette (09) [00 86 09]

1D602-1D611: End of game (06)

1D602: GHZ main level patterns (3DCF2) [00]

1D606: GHZ 16x16 block mappings (3BCF2) [00]

1D60A: GHZ 256256 block mappins (3F09A) [00]

1D60E: GHZ main level palette (13) [00 86 13]

I think that pretty much summs it up. Just change the vaue of the thing you want to modify to the appropriate value. If you wanted to give GHZ the MZ palette, just change the main level palette index number from 04 to 06 and it's done.

Pattern load cues

The pattern load cues are what loads all the peices 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 S1 rom:

00	1D652	Standard block 1 (starpoll/rings\score\time/life/ring/points)	(4)
01	1D672	Standard block 2 (monitor/bubble/stars)	(2)
02	1D686	Explosion (explosion)	(0)
03	1D68E	Game/Time over (game\time over)	(0)
04	1D696	GHZ (GHZ primary/GHZ secondary/stalk/rock/crabmeat/buzz bomber/fish/jet badnick/driving badnick/spikes/H spring/V spring)	(B)
05	1D6E0	GHZ (platform/bridge/wooden spikes/ball/breakable wall/wall block)	(5)
06	1D706	LZ (LZ patterns/block/blocks/waterfall/water surface/spikeball/blocks/bubbles/block/block/spear/drill badnick)	(B)
07	1D750	LZ (broken bar/block/wheel/stone head/sonic holding breath/blocks/enemy with spikeballs/fish/button/cork/spikes/H spring/V spring)	(C)
08	1D7A0	MZ (MZ patterns/metal/fireball/platform/ball/lava/buzz bomber/spikey badnick/bat/caterkiller)	(9)
09	1D7DE	MZ (button/spikes/H spring/V spring/moveable block)	(4)
0A	1D7FE	SLZ (SLZ patterns/bomb/enemy with spikeballs/fireballs/blocks/block/spikes/H spring/V spring)	(8)
0B	1D836	SLZ (see-saw/fan/pylon/spikeball/fireball launcher/spikeball)	(5)
0C	1D85C	SYZ (SYZ patterns/crabmeat/buzz bomber/spikey badnick/metal sonic)	(4)
0D	1D87C	SYZ (bumper/spikeball/spikeball/caterkiller/button/spikes/H spring/V spring)	(7)
0E	1D8AE	SBZ (SBZ patterns/stomper/barrier/blocks/pig/indent/platform/spikeball/spikewheel/flamepipe/zapper/blocks)	(B)
0F	1D8F8	SBZ (caterkiller/bomb/badnick with spikeballs/platforms/zapper/blocks/collapsing platform/tilting platform/button/spikes/H spring/V spring)	(C)
10	1D948	Title card (title card)	(0)
11	1D950	End of boss level (robotnic's ship/GHZ boss/egg prizon/bomb/spikeball/rocket thrust)	(5)
12	1D976	End of signpost level (signpost/hidden bonuses/flash)	(2)
13	1D98A	Lost warp effect (original bonus stage entry effect)	(0)
14	1D992	Special stage (bubbles\clouds/background/block/bumper/"goal"/up\down/"R"/life block/stars/closed one way block/one way block/"W"/destroyable blocks/emeralds/"zone 1"/"zone 2"/"zone 3"/"zone 4"/"zone 5"/"zone 6")	(10) [13]
15	1DA0C	GHZ animals (rabbit/bluebird)	(1)
16	1DA1A	LZ animals (blackbird/seal)	(1)
17	1DA28	MZ animals (squirrel/seal)	(1)
18	1DA36	SLZ animals (pig/bluebird)	(1)
19	1DA44	SYZ animals (pig/chicken)	(1)
1A	1DA52	SBZ animals (rabbit/chicken)	(1)
1B	1DA60	End of special stage (emeralds/continue)	(1)
1C	1DA6E	End of game sequence (GHZ primary/GHZ secondary/stalk/weird flowers/emeralds/sonic in ending sequence/robotnic dying/rabbit/chicken/blackbird/seal/pig/bluebird/squirrel/"sonic the hedgehog")	(E)
1D	1DACA	End/try again screen (emeralds/robotnic/alphabet)	(2)
1E	1DADE	End of SBZ 2 (blocks/robotnic/button)	(2)
1F	1DAF2	FZ (robotnic's ship at end/stomper boss/robotnic's ship/robotnic/rocket thrust)	(4)

This pointer list has an offset index to locate the correct cue. Go here for an explanation of how this works. Note that the number in parenthesis is the recorded number of requests in the cue. If a number follows it in brackets, it means the actual number that exists differs from the recorded one. This means any load requests past the recorded number were not being loaded, but still exist in the rom. 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 cues 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 4ADC-4B63. 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 layout in S1 is broken up into two peices, one for the foreground of the level, and one for the background. When being read into the ram, this data is interleaved to give the entire level layout. The offset index that proceeds the actual level data is a unique offset index. There are three offset values for each act, and four acts for each level value. The first value is the offset to the foreground to use for that act. The value is the offset to the background to use for that act. I have no idea what the third offset was intended to be used for, but it appears that it is not used in the game, so just ignore it.

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 S1 rom.

Main level block mappings

In S1 both the 16x16 and 256x256 block mappings are compressed. This means that in order to edit the block mappings, you'd need to be able to decompress, alter, then recompress them. Right now there is no way to do that, so you can only use the block mapings that are in the rom unless you want to do a savestate hack. Note also that becuase S1 uses 256x256 block mappings, you can't just port the block mappings from later sonic games.

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 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 1 savestate breakdown.

Mappings for Sonic

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 sprite for Sonic, 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 an 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.

Animated pattern load cues

While the function of these cues is identical to the ones in S2 and S2B, these ones are structured entirely differently, and as of yet I haven't figured the entire system of this one out. You can switch the animated pattern load cues between levels easy enough by changing the offset index preceeding them though.

Palettes

All the main palettes used in the game are linked in a pointer table at 2168. 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 S1 rom:

00	2168	SEGA screen palette (2208)
01	2170	Title screen palette (2288)
02	2178	Level select palette (2308)
03	2180	Primary palette line (2388)
04	2188	GHZ level palette (23A8)
05	2190	LZ level palette (2408)
06	2198	MZ level palette (24E8)
07	21A0	SLZ level palette (2548)
08	21A8	SYZ level palette (25A8)
09	21B0	SBZ level palette (2608)
0A	21B8	Special stage palette (26C8)
0B	21C0	LZ underwater level palette (2468)
0C	21C8	SBZ act 3 level palette (2748)
0D	21D0	SBZ act 3 underwater level palette (27A8)
0E	21D8	FZ level palette (2668)
0F	21E0	LZ underwater primary palette line (2828)
10	21E8	SBZ act 3 underwater primary palette line (2848)
11	21F0	Special stage results palette (2868)
12	21F8	Unknown palette (28E8)
13	2200	Ending sequence level palette (2928)

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.

Character start location array

The start location for the characters and the camera for each level are stored in this array at address 6112. 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

The music playlist is very simple to modify. There is one byte per level value, and that byte stores the value of the music track you want to play for that level. The values for the music tracks can be obtained from the sound test in the level select screen.

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 LZ act 1 to load up when you finish SBZ act 2, you would take the location of the table, which in this case is C6F4, and you would add 42 bytes to that location, and then you would replace the next two bytes from that point with 0100. That's becuase you add 8 bytes to the address for every level that comes before it, and in this case there are 5, 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.

Dynamic screen resizing

In S1, the programmers allowed for some very fancy effects to do with the height and width of the sceen. They allowed for the X/Y start, and X/Y ending locations to be changed at any point during the level, based on any event they cared to use. The method they used made the programmers job a little harder though and ate away some performance. They completely redesigned the way this worked for the S2 platform. All the programming for the screen resizing is compiled, so it's very difficult to edit directly.

Music/Sound FX pointers

The music and sound effects in S1 are very easy to change. Note that the midi's in the game start at a sound value of 80, and the sound effects start at a value of A0. If you want to determine the value of the sound or midi you want to work with, just read it staright off the sound test in the level select menu. The pointers to the midi's are at address 71A9C, and the pointers for the sound ffects are at 78B44. Both are simply 4 byte pointers to the address in the rom that the data is located at, so to find the pointer to the sound or midi you want to edit, add 4 bytes to the starting address of the pointer list for every sound or midi that comes before it after it's starting value. To switch a track, just switch thier pointer values.

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 D382. It’s pretty simple, just one pointer per address value. Here's the pointer list in Sonic 1:

01	D382	Sonic [12BD8]
02	D386	??????? [D5B2]
03	D38A	??????? [D5B2]
04	D38E	??????? [D5B2]
05	D392	??????? [D5B2]
06	D396	??????? [D5B2]
07	D39A	??????? [D5B2]
08	D39E	??????? [142F0]
09	D3A2	Sonic in special stage [1B984]
0A	D3A6	??????? [13C98]
0B	D3AA	Bar you hold onto in LZ [11206]
0C	D3AE	Opening blocks that open to pasage sonic gets sucked down in LZ [11346]
0D	D3B2	Unknown. Warps you to somewhere. [EB28]
0E	D3B6	??????? [A61C]
0F	D3BA	??????? [A69C]
10	D3BE	??????? [1C022]
11	D3C2	??????? [7328]
12	D3C6	Spinning light behind glass from SYZ [E90C]
13	D3CA	Fireball from MZ [E296]
14	D3CE	Unknown. Dives through the floor. [E304]
15	D3D2	Swinging spikeball on a chain from SBZ [7952]
16	D3D6	Spear from LZ [11F04]
17	D3DA	Unknown. Flashes between hitable and not hitable, but you can't hurt it, only the other way around. Frame changes in accordance with this. [7CA2]
18	D3DE	Misc standard platforms [7E32]
19	D3F2	??????? [81A8]
1A	D3F6	Collapsable platform [8210]
1B	D3FA	??????? [110C6]
1C	D3FE	Fireball launcher from SLZ [87CA]
1D	D402	??????? [885E]
1E	D406	Double big red explosion [8B4C]
1F	D40A	Crab enemy from GHZ and SYZ [94F0]
20	D40E	Big red explosion [8C1C]
21	D412	Unknown. It just sits there. [1C548]
22	D416	Buzz bomber [97DE]
23	D41A	Unknown. Sprite sits there and flashes. [9932]
24	D41E	Unknown. Sprite changes frame and disappears. [8CF2]
25	D422	Ring [9B26]
26	D426	Monitor [A118]
27	D42A	Normal explosion [8D62]
28	D42E	Unknown. Edges along grund at a terminal rate. As soon (as in immediately) as it leaves the screen it disappears. [8F22]
29	D432	??????? [9412]
2A	D436	One way barrier from SBZ [8934]
2B	D43A	Unknown. Sprite moves off bottom of screen. [AB20]
2C	D43E	Unknown. Sprite changes a few frames, then sits there moving quickly back and forth in a small area. [ABDA]
2D	D442	Drill badnick in LZ [ACA4]
2E	D446	Contents of monitor flying up frm monitor being busted [A302]
2F	D44A	Platfrom in MZ that moves up and down [AEB8]
30	D44E	Unknown. Big block. [B39E]
31	D452	Metal block with spikes on bottom from MZ [B67C]
32	D456	Button [BD2E]
33	D45A	Moveable block in MZ and LZ [BED2]
34	D45E	Title card [C306]
35	D462	??????? [B1DC]
36	D466	Spikes [CE28]
37	D46A	Rings flying out from being hit [9CB6]
38	D46E	??????? [14158]
39	D472	"Game over" sprite [C4B8]
3A	D476	End of level results screen [C572]
3B	D47A	Rock from GHZ [D050]
3C	D47E	Breakable barrier from GHZ [D11C]
3D	D482	Robotnic in his primary ship [17700]
3E	D486	Egg prison [1AB22]
3F	D48A	Large explosion [8DF6]
40	D48E	Motobug from GHZ [F61A]
41	D492	Red spring [DABE]
42	D496	Badnick that appears behind wall and fires projectile from GHZ [DD68]
43	D49A	Metal dude that goes in a ball like sonic from SYZ [DFFA]
44	D49E	Wall barrier from GHZ [E1D4]
45	D4A2	Sideways spikes that were cut in MZ [B99A]
46	D4A6	Block from MZ [E7D0]
47	D4AA	Bumper [E9D0]
48	D4AE	Large ball from GHZ that was cut [17AE4]
49	D4B2	??????? [D0BE]
4A	D4B6	Unknown. Some sort of teleport effect. Most likely belongs to those lost patterns. [14254]
4B	D4BA	Large golden ring [9E06]
4C	D4BE	Lava spurt with lava ripples [ED84]
4D	D4C2	Lava spurt [EE70]
4E	D4C6	Lava wave from MZ [F05E]
4F	D4CA	??????? [F806]
50	D4CE	That ground based badnick with spikes ontop from SYZ [F83A]
51	D4D2	Breakable block in MZ [FC92]
52	D4D6	Misc platforms [FDF8]
53	D4DA	Platform made up of small blocks that collapses [8320]
54	D4DE	Sprite for lava hurting you [F1B8]
55	D4E2	Bat from MZ [100A8]
56	D4E6	Block from SYZ [10286]
57	D4EA	Unknown. A small sprite with a small collision box that hurts you, but you can't bust it. [10782]
58	D4EE	Same as above, but different image [10976]
59	D502	Raising platforms from SLZ [10AD2]
5A	D506	Platform moving in circular motion in SLZ [10D5A]
5B	D50A	Lowering staircase from SLZ [10E90]
5C	D50E	Pylon in foreground frm SLZ [11044]
5D	D512	Fan from SLZ [114FC]
5E	D516	See saw [1163C]
5F	D51A	Walking bomb [119F6]
60	D51E	Badnick surrounded by spikeballs that shoots them at you [11CCE]
61	D522	Misc blocks from LZ [11FD4]
62	D526	Stone head that launches projectile in LZ [121E8]
63	D52A	Rotating wheel in LZ [1233C]
64	D52E	Small bubble [126A4]
65	D532	moving water sprites from LZ [12A9E]
66	D536	Spinning wheel in SBZ that shoots you off in different directions [1509A]
67	D53A	Spinning circle on wheel from SBZ [154E2]
68	D53E	??????? [1568E]
69	D542	Various platforms from SBZ [15730]
6A	D546	Giant pizza slicer thing from SBZ [1597C]
6B	D54A	Misc blocks in SBZ [15C12]
6C	D54E	Spikewheel from SBZ that runs through floor [15FF8]
6D	D552	Unknown. Looks like flamepipe frm SBZ [E4E0]
6E	D556	Broken pipe that lets out fire from SBZ [16146]
6F	D55A	??????? [1626A]
70	D55E	Big moving rectangle in SBZ [1652A]
71	D562	Unknown. A collsion box but no image. [1141A]
72	D566	Unknown. A spin tube? [16668]
73	D56A	Robotnic flying his default ship. [18230]
74	D56E	Unknown. Changes frames back and forth for awhile, and it can hurt you but you can't hurt it, then it diappears. [186A4]
75	D572	??????? [190EE]
76	D576	??????? [19692]
77	D57A	??????? [17E36]
78	D57E	Catakiller [16934]
79	D582	Starpoll [16E2E]
7A	D586	??????? [188B8]
7B	D58A	Unknown. Makes flashing stuff appear at top of screen [18CCE]
7C	D58E	Special stage entry effect [9ED6]
7D	D592	??????? [1715E]
7E	D596	End of special stage results screen [C7AE]
7F	D59A	Bubble [C956]
80	D59E	Continue screen [4E42]
81	D5A2	??????? [4F5C]
82	D5A6	??????? [19832]
83	D5AA	??????? [19B60]
84	D5AE	??????? [1A44E]
85	D5B2	??????? [19D58]
86	D5B6	??????? [1A7DA]
87	D5BA	??????? [53E2]
88	D5BE	??????? [553C]
89	D5C2	??????? [5616]
8A	D5C6	??????? [17264]
8B	D5CA	??????? [59E2]
8C	D5CE	??????? [5AC2]