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SCHG

Nem s1

From Sonic Retro

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It has not been revised since its original writing and may be outdated.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

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
Reddot.gif2C03A 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
Reddot.gif2C196 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
Reddot.gif2E692 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
Reddot.gif2E878 "Zone 1" block from special stage Art compression
9 blocks
Reddot.gif2E93A "Zone 2" block from special stage Art compression
9 blocks
Reddot.gif2EA08 "Zone 3" block from special stage Art compression
9 blocks
Reddot.gif2EAD4 "Zone 4" block from special stage Art compression
9 blocks
Reddot.gif2EB9C "Zone 5" block from special stage Art compression
9 blocks
Reddot.gif2EC66 "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
Reddot.gif2F142 "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
Reddot.gif2F66E 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
Reddot.gif3257E 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
Reddot.gif3850E 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
Reddot.gif61388 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]


Sonic Community Hacking Guide
General
Sonic Music Hacking Guide | SonED2 Manual
Game-Specific
Sonic the Hedgehog (16-bit) | Sonic the Hedgehog (8-bit) | Sonic Eraser | Sonic CD (prototype 510) | Sonic CD | Sonic CD (PC) | Sonic 2 (Nick Arcade prototype) | Sonic 2 (Simon Wai prototype) | Sonic 2 (16-bit) | Sonic 2 (8-bit) | Sonic 2 & Knuckles | Dr. Robotnik's Mean Bean Machine | Sonic Triple Trouble | Sonic 3 | Sonic & Knuckles | Sonic 3 & Knuckles | Sonic & Knuckles Collection | Sonic Crackers | Sonic 3D: Flickies' Island | Chaotix | Sonic R PC | Sonic Jam 6 | Sonic Advance | Sonic Advance 2 | Sonic Advance 3 | Sonic Battle | 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 | Shadow the Hedgehog | Sonic Riders | Sonic the Hedgehog (2006) | Sonic & Sega All-Stars Racing | Sonic Unleashed (Xbox 360/PS3) | Sonic Generations
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 | Subroutine Equivalency List | 68000 Instruction Set | 68000 ASM-to-Hex Code Reference | VDP Documentation | Official Sega 32X Documentation | Official Mega-CD Documentation | Official Sega Mega Drive Documentation