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It has not been revised since its original writing and may be outdated.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


Introduction

I've spent a great deal of time tearing the Sonic savestate files to shreds, and here are my notes. In order to edit these files I recommend a hex utility called Hex Workshop, and I recommend getting the emulators gens and genecyst. Gens is the one I use most of the time, but genecyst has a lot of features that are extremely useful that gens dosen't have.

First of all it's very important that you understand the basics. All data stored on a computer is in the form of 1's and 0's. On a CD for example, a laser hits the surface, and if the laser bounces back and hits the lens it's a 1, and if it dosen't it's a zero. Each 1 or 0 is called a bit, and a bit cannot have any other characters in it other that a 1 or a 0. Now the computer deals with bits in groups of 4. There are 16 possible combinations for a group of four 1's and 0's, so to make it simpler to deal 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 thier 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 fo 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 recogniseable 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 nessicary 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 vaue, you will in fact end up with 80, becuase that value you are entering is actually a hex value, but if you use the base converer 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.

Now onto exactly what a savestate consists of. A savestate is a dump of all the ram that is allocated to the system, but that's not just main system ram, it's sound ram, CPU cache, etc. In a savestate all of this is mashed together in one file. Here's what's where in a genecyst savestate:

0-111 Unknown
112-191 C ram (byteswapped)
192-1E1 VS ram
1E2 and 1E3 A 2 byte break.
1E4-3E3 YM2612 registers
3E4-473 Some kind of sound ram (Z80 internal cache?)(93)
474-2473 Z80 ram (sound ram)
2474-2477 A 4 byte break
2478-12477 System ram
12478-22477 Video ram

And here's what's where in a Kgen savestate:

0-97 File header
98-2097 Z80 ram
2098-12097 System ram
12098-22097 Video ram
22098-22117 C ram
22118-223A5 Unknown (D72)
223A6-225A5 YM2612 registers
225A6-2275B Unknown (1B6)

All the address values listed here are based on a genecyst savestate file, so if you wish to edit one of another type you'll have to convert them yourself. Now any single value that is stored in the system ram can be altered in game by use of a pro action replay code. First of all, here's an example of a Pro action replay code: FFFE10:0800 Now, the first byte in this code is FF, and a value of FF in the first two characters indicates that the code is altering data in the system ram. The next four character after it are the actual memory location in the ram, and the last four are the two byte value to write into that memory address. The pro action replay will always alter two memory addreses at a time by the way, because the Mega Drive (Genesis) is a 16 bit system. It dosen't matter though because you will find that a value will have two bytes assigned to it anyway, or the second byte will be related. In the case of the example, the first byte being altered is the current level, and the second is the current act within that level. The actual address of these values in the savestate are 12288 and 12289. An explination of why lies in the locations of the ram data that is stored in the savestate. If you look at the above table, there are 2478 lines worth of data before the system ram in the savestate, so for any action replay code you want to convert into a file location, you will need to add 2478 to the line number, and for any line number you want to convert into a pro action replay code you will need to minus 2478. Remember that these line numbers are hex values though, so you will need to do this in a hex calculator. (NOTE: Genecyst dosen't do pro action replay codes properly, use gens instead for that function).

Another thing you need to know is that each level in Sonic has a value asigned 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:

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)

And one last thing you need to know is the way that the art is stored in the rom. 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 it's colour from. The palette has 4 lines, each with 16 colurs 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 makes 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 thier x, y, or x and y values reversed when placing them in a 16x16 block. 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:

Internal Groups

System ram
2478-B477 256x256 block mappings
C878-CC77 Level design
D478-E477 16x16 block mappings
11AF8-????? Pattern load cue
11EF8-11F77 Above water palette
11F78-11FF7 Below water palette
Video ram
12478-1E477 Main Pattern info
1E478-1F477 Makeup of buffered foreground
1F478-20477 Patterns for text/computers/rings
20478-21477 Makeup of buffered background
21478-21C77 Patterns for sonic and tails (reloaded from rom each time they change)
21C78-21EF7 Makeup of buffered active objects?
21EF8-22077 Lives counter patterns
22078-223F7 Current state of background tiles
223F8-22477 4 empty blocks

Single Variables

F47A and F47B Patterns for Sonic. First value specifies palette line to use and mirroring/flipping of sprite. refer to table in 16x16 mappings section. Next three values specify the number of the pattern that Sonic's 8x8 blocks start at.
F480 and F481 Sonic's x position
F484 and F485 Sonic's y position
F48C and F48D Sonic's horizontal speed (8000 is max forward, 7FFF is max back and working back to either 0000 or FFFF accordingly)
F493 Sonic's current animation frame
F4A8 and F4A9 Sonic flashing (Enter how many frames to flash for. There are 60 frames per second on an NTSC system.)
F500 and F501 x position of level name on screen
F502 and F503 y position of level name on screen
F512 Image for level name
F516 and F517 Time until level name moves off screen
F51C "Press start button" text enabled/disabled (set to 00 to enable)
F540 and F541 x position of "zone" on screen
F542 and F543 y position of "zone" on screen
F552 Image for "zone"
F556 and F557 Time until "zone" moves off screen
F580 and F581 x position of act number on screen
F582 and F583 y position of act number on screen
F592 Image for act number
F596 and D597 Time until act number moves off screen
F5C0 and F5C1 x position of sonic thingy on screen
F5C2 and F5C3 y position of sonic thingy on screen
F5D2 Image for sonic thingy
F5D6 and D5D7 Time until sonic thingy moves off screen
11A78 Master level trigger (This trigger tells the game what you're in, be it a level, special stage, or a menu. View breakdown for table of values.)
11AC0 and 11AC1 Current Water level (specify a y axis value. Note that the y axis values work as 0 being the top of the level and increase as the character moves further down the level. If this value differs to what it should be, the water level will rise/lower as is neccisary to end up where it should be.)
11AC4 The speed at which the water moves (If this is set to 00 the water will not move if the set water level changes)
11BD8 and 11BD9 Sonic's Top speed
11BDA and 11BDB Sonic's Acceleration
11BDC and 11BDD Sonic's Deceleration
11BE8-11BEF Sprites (includes map triggers. NOTE: this is not the actual data on the sprite locations. Read breakdown for details).
1227E Sprite currently selected in sprite placement mode
12281 Sprite placement mode (00=off 01 up=on)
12288 Level (hex vaue assigned to level, not level position on list. NOTE: Changing this value in level will also switch info for background movement, animated sprites, and animated palette sections.)
12289 Act
1228A Lives (convert from decimal to hex for desired number)
12290 Continues (convert from decimal to hex for desired number)
12298 and 12299 Rings (convert from decimal to hex for desired number)
1229B Minutes on clock (beyond 09 it loads images beyond it's range, hence

screwup)

1229C Seconds
1229D Milliseconds
1229E-122A1 Score (convert from decimal to hex for desired score)
122A8 Number of star polls that have been hit (based thier positions on the x axis)
122CF Number of emeralds you have
12388 and 12389 Camera x position
1238C and 1238D Camera y position
123FA and 123FB Item in level select menu that is selected
123FD Sound that is selected in options menu (minus (80) of any value you enter, becuase for some weird reason they have the counter starting at 80)
12404 Box that is selected in options menu
12429 Number of emeralds you have
12468 and 12469 Demo mode (00=off 01 up=on)
12472 Debug mode (00=off 01 up=on)

Pattern breakdown

8x8 blocks (12478-1C477)
All 8x8 patterns are stored sequentially, each one taking up 32(20) bytes. One pixel takes up one hex value, not one byte. The value of this hex value being 0-16 represents the colour that pixel is on the palette, 0 being the start, and F being the end (NOTE: specifying a value of 0 will not load up the first colour on the palette line, but instead make that pixel transparent).

16x16 block mappings: (D478-????)
Each 16x16 block is made up of four 8x8 blocks. Delt with in blocks of 8 bytes, 2 bytes per pattern to be used in block. First value determins which palette line to use, and which orientation the pattern is at. Refer to table for explination:

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

The next three values represent the pattern number to use. Specify the number that the pattern is in the list (EFF is the last of the useable patterns). The block mappings use the standard method of layout, start at x0 y0 and move across x values until end of line is reached, then repeating on next y value. If a space is blank in the 16x16 mappings, it will have the values 4000 in every box.

256x256 block mappings: (2478-B477)
Delt with in blocks of 512 (200) bytes. Each 256x256 block is made up of 256 16x16 blocks, hence two bytes per 16x16 block are used. The first value is to do with the flipping of the block. An odd number will flip it vertically, while an even number will load it normally. The next 3 values are the number of the 16x16 block to use.

Sprite loading addresses (11BE8-11BEF)

The sprite information is discarded when screen moves too far away, and information is then reloaded from rom when character reenters area. These values specify a location in the rom to load the information from. The first four bytes are the address to load the sprites from when approaching from the left. The four bytes after that is the address to load the sprite info when approaching from the right. There is also another block of 8 bytes after this one that appears to be the same, but it dosen't seem to do anything. And no, it's not where to load the sprites from when approaching from the top and bottom.

NOTE: Altering these values to those of another level will transfer the information for the location of sprites, and the key files (definitions about movement/reactions) for the sprites, but unless the correct patterns for them are loaded into the ram, it will look like a jumbled peice of crap.

Title card breakdown (11BE8-11BEF)

The title card is quite a simple thing. It's really just 4 sprites on different levels that are bound to the screen. There's 64 (40) bytes of data that defines each one of these sprites. Look in the single variables section for these values. WIth the Image variable, these values will load up these images:

Value Image
00 Green Hill
01 Labyrinth
02 Marble
03 Star Light
04 Spring Yard
05 Scrap Brain
06 Zone
07 Act 1
08 Act 2
09 Act 3
0A Blue circle thingy
0B Final

We can really see from this just how much of a last minute thing adding a final zone rather than having it as part of Scrap Brain zone 3 was.

Buffered sprite breakdown

Final: (D878-F477)
Stored in blocks of 64 bytes (40) per sprite. The sprites are listed in the order that they appear starting from the top left and scrolling across x values, then beginning on next line. The following example uses address locations for the first sprite on the list as a referance. You will need to increase the line values on this list by 40(hex) for every sprite that comes before the one you want to modify.

FB78 Key file to use for defining the actions/movements of sprite. 94 is lava snake, 26 is monitor, A4 is exploding star, just to name a few. I have compiled an early list of values for it in the final version of Sonic 2 here
FBFA and FBFB Patterns to use for sprite. Specify number of 8x8 block to start from,

not address value.

FC00 and FC01 x position of sprite
FC04 and FC05 y position of sprite
FC18 Collision response (this value determines what happens when your character's sprite hits this one. A long list of different values so one will have to be compiled).
FC19 Special flag for collision (Setting this value to 01 or above will set a special thing about the sprite for the collision group it belongs to. If it's set as an enemy this trigger will make you bounce off the sprite like the enemy in Casino night. If it's a computer it will turn off collision responces all together. The other two do nothing.)

Palette breakdown

(Above water: 11EF8-11F77, Below water: 11F78-11FF7)
A simple 12 bit RGB value. One colour on palette is represented by two bytes, and working back from the last value forward an RGB value is specified like this: 0BGR. So if you wanted that colour on the palette to be completely red, you would enter a value of 000F. The first colour on the palette is the default colour of the stage. If you had a value of 0 in one of the background tiles, this colour will be substituted in instead of making it transparent.

Pattern load cue (11AF8-?????)

Requests are processed in the order they are listed. If you try to load multiple patterns to the same area, the last one will overwrite the first. A six byte value per request. First four bytes are the address to load the patterns from the rom (eg, 0008 30D2). The last two bytes are the address to load the patterns into in the video ram. The pattern view window in genecyst is the easiest way to determine this. Note than whenever clearing out the pattern cue you must also set 11B71 to 00, or else some crap will be written over the first few patterns in the video ram. I have yet to narrow down the exact relationship this variable has with the load cue.

Master level trigger (11A78)

This trigger is the main trigger that specifies the current "mode" if you will that the game is in. Changing this will make the game switch to the mode you specify once it finishes it's current process.

00 SEGA logo
04 Title screen
08 Demo (will make the screen fade out and return to title when time reaches 27 seconds)
0C Normal level
10 Special stage
14 Continue screen
18 End of game sequence
1C Credits

Values above 1C have not been used, so you get some strange results. If you change this value in level the screen will fade out and the data for the thing you have selected will be loaded instead. If you specify a value betwen the values I have listed, you will get some strange results most of the time.

Level design breakdown (C878-????)

Level is delt with as a grid, starting with first tile in top left and working across to the right doing one x value at a time. One byte represents one tile in one grid location. A value of 00 will not load up the first large tile, but instead will simply display no block. A value of 01 will load up the first block, 02 the second, etc. The maximum possible number of large tiles on the x-axis is 127 (80). One x line is stored in the form of 128 (40) lines for foreground tiles, then another 128 (40) lines for background tiles per y value. That totals an increase of 0100 per line, so if level data started at 0300 (not possible), 0300-03FF would be all the data for the top row of 128x128 tiles, 0400-04FF would be all the data for the second row of 256x256 tiles, etc.


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