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Using a hex editor it is possible to modify the game, editing it byte-by-byte. By knowing where each function is and modifying it correctly, you and customize the game. You can draw another character in Sonic's place, but it's not too easy, because it needs to be made point-to-point. You can change the levels' name in the Level Select Menu and in the Level Title Cards. But to program the game, it needs to be in the tbinary format. It isn't hard to convert the game to binary form. You can use a conversion program or the Genecyst emulator's DUMP ROM option, which does the conversion.

In spite of Sonic 2 & Knuckles being just Sonic 2 coupled with Sonic & Knuckles, this game can be considered a new version of Sonic 2, because Sonic & Knuckles, after identifying that the cartridge attached is Sonic 2, activates a memory that is at the end of the two games, and this memory tells all the instructions of the game of Sonic 2 & Knuckles just using the Sonic 2 cartridge for the graphs and maps of the levels. In this guide, you will see that the information that's altered in most of the case is above the address 0x300000 and few things are in the addresses 0x200000 to 0x2FFFFF where the game of Sonic 2 is in the memory of Sonic & Knuckles. Because of this, Sonic 2 & Knuckles a new game of Sonic 2 can be considered with Knuckles.

Observe the table below with the levels and their respective ids. These ids are important because the game uses these ids for referencing the certain level.

LEVEL	ID
EMERALD HILL	00
METROPOLIS	04
METROPOLIS	05
WING FORTRESS	06
HILL TOP	07
OIL OCEAN	0A
MYSTIC CAVE	0B
CASINO NIGHT	0C
CHEMICAL PLANT	0D
DEATH EGG	0E
AQUATIC RUIN	0F
SKY CHASE	10

Notice that some numbers are skipped, like 01, 03, 06 and 09. Although these do not have level names and are not in the level select, if you place these lacking IDs, it can access lost levels (try it and see the results). Observe the table below with the standard location of the acts and an explanation will follow.

Level	Act	Address
EMERALD HILL	ACT 1	307906 to 307907
	ACT 2	307908 to 307909
CHEMICAL PLANT	ACT 1	30790A to 3079B
	ACT 2	30790C to 30790D
AQUATIC RUIN	ACT 1	30790E to 30790F
	ACT 2	307910 to 307911
CASINO NIGHT	ACT 1	307912 to 307913
	ACT 2	307914 to 307915
HILL TOP	ACT 1	307916 to 307917
	ACT 2	307918 to 307919
MYSTIC CAVE	ACT 1	30791A to 30791B
	ACT 2	30791C to 30791D
OIL OCEAN	ACT 1	30791E to 30791F
	ACT 2	307920 to 307921
METROPOLIS	ACT 1	307922 to 307923
	ACT 2	307924 to 307925
	ACT 3	307926 to 307927
SKY CHASE		307928 to 307927
WING FORTRESS		30792A to 30792B
DEATH EGG		30792C to 30792D

The table above indicates the location of the configuration bytes in level select screen. The first address of the table indicates the byte of the zone and the 2nd address indicates the byte of the act. For example, in the address 307916 you will see the byte 07 (Hill Top Zone) and in the address 307917 you will see the byte 00 (Act 1).

Levels Music

You can also choose the music to be played in the levels. The format is just a byte for a level and it is very simple. To change what music plays, play the sounds in the sound test, write down the number of the music and place in the byte that represents the chosen level. In the table below, the location of the music is stated.

Note: When modifying the music of the levels, add 80 to the number of the Sound Test song number and place it in the wanted place. The table below has some examples of as the midis they are in Rom and as they appear in Sound Test.

Level	Music in the Rom	Music in the Sound Test	Address
EMERALD HILL	82	02	30397C
ID 01	82	02	30397D
WOOD ZONE	85	05	30397E
ID 03	84	04	30397F
METROPOLIS	85	05	303980
METROPOLIS 3	85	05	303981
WING FORTRESS	8F	0F	303982
HILL TOP	86	06	303983
HIDDEN PALACE	90	10	303984
ID 09	8D	0D	303985
OIL OCEAN	84	04	303986
MYSTIC CAVE	8B	0B	303987
CASINO NIGHT	89	09	303988
CHEMICAL PLANT	8E	0E	303989
DEATH EGG	8A	0A	30398A
AQUATIC RUIN	87	07	30398B
SKY CHASE	8D	0D	30398C

Altering Objects

You can also alter objects in the levels as badnicks, lives, obstacles, etc... The configuration of the objects in the levels uses 6 long bytes, being in the format XX-XX-YY-YY-OO-VV. Explaining to X's better indicates the horizontal position in the screen, Y's indicates the vertical position in the screen, O's indicates the objects they be she placed and V's indicates the possible variations of each object. To obtain the exact location of the place where you want for the objects, make the code of the debug mode and write down the first line of coordinates in the place of SCORE, These will be the coordinates that you should type to place the object in the wanted place. See the location where game it search the objects of the levels. The levels finish the mappings before the bytes FF-FF-00-00-00-00. Starting from these bytes next level begins.

EMERALD HILL	ACT 1	DF3FE
	ACT 2	DF7B2
METROPOLIS ZONE	ACT 1	DFBD2
	ACT 2	E005E
	ACT 3	E05A4
WING FORTRESS		E0C10
HILL TOP	ACT 1	E1018
	ACT 2	E1396
HIDDEN PALACE		2E8C80
OIL OCEAN	ACT 1	E1A4A
	ACT 2	E1F18
MYSTIC CAVE	ACT 1	E23DA
	ACT 2	E26F2
CASINO NIGHT	ACT 1	33F06E
	ACT 2	33F74C
CHEMICAL PLANT	ACT 1	E2A88
	ACT 2	E2EFC
DEATH EGG		2EB230
AQUATIC RUIN	ACT 1	E34D8
	ACT 2	E398E
SKY CHASE		2EBBDE

I Begin of Knuckles in the Phases

In this part you can define in that point of the screen Knuckles begins. To select a place different from the original. Use the debug mode. Go to the place from where you want to begin and write down the coordinates (first line in the place of Score), Tends in hands the coordinates it places them in the place of the level chosen accordingly below in the address of the table.

Level	Location	Coordinates
Emerald Hill 1	30A488	0060028F
Emerald Hill 2	30A48C	006002AF
01-00	30A490	0060028F
01-01	30A494	006002AF
Wood 1	30A498	006001AC
Wood 2	30A49C	006001AC
03-00	30A4A0	0060028F
03-01	30A4A4	006002AF
Metropolis 1	30A4A8	0060028C
Metropolis 2	30A4AC	006005EC
Metropolis 3	30A4B0	0060020C
05-01	30A4B4	006002AF
Wing Fortress 1	30A4B8	006004CC
Wing Fortress 2	30A4BC	01E004CC
Hill Top 1	30A4C0	006003EF
Hill Top 2	30A4C4	006006AF
Hidden Palace 1	30A4C8	023001AC
Hidden Palace 2	30A4CC	023001AC
09-00	30A4D0	0060028F
09-01	30A4D4	006002AF
Oil Ocean 1	30A4D8	006006AC
Oil Ocean 2	30A4DC	0060056C
Mystic Cave 1	30A4E0	006006AC
Mystic Cave 2	30A4E4	006005AC
Casino Night 1	30A4E8	006002AC
Casino Night 2	30A4EC	0060058C
Chemical Plant 1	30A4F0	006001EC
Chemical Plant 2	30A4F4	0060012C
Death Egg 1	30A4F8	0060012D
Death Egg 2	30A4FC	0060012D
Aquatic Ruin 1	30A500	0060037E
Aquatic Ruin 2	30A504	0060037E
Sky Chase 1	30A508	01200070
Sky Chase 2	30A50C	01400070

Events

In the games of Sonic, the Sega Logo, The screen of I title, the levels, the Special Stages, etc.. they are different events and they are called with different codes and you it can you work them placing its values in the appropriate place, the numbers that you will see below in the table they simply belong the code of the game and theses numbers don't polish by chance of 4 in 4, If you go until the address 0x3000A0 you will see several numbers 6000xxxxx it notices that the neighbor 6000xxxx has one it distances of 4 bytes of the address 0x3000A0, the neighbor 6000xxxx has 8 bytes of the address 0x3000A0 and so on. Before you wonder: what does this have of important? - I answer: being known the exact initialization place of the events you can reprogram them without affecting the operation of the game. See below in the table the events and its respective initialization addresses.

Event	ID	Start
Sega Logo	00	0x303310
Screen Titles	04	0x3034E8
Demo Mode	08	0x3039A0
Normal Gameplay	0C	0x3039A0
Special Stage	10	0x304A0A
Continue	14	0x3072A6
Level Select Menu	18-1C	0x3076C2
End Sequence	20	0x308008
Level Select Menu	24	0x3076C2

Codes of the Game

Level Select Menu

In this part you will know the codes of the game and as to alter them, the access code to the Level Select Menu is: up, up, up, down, down, down, left, right, left, right. In the Rom this code this stored in the format of the keys of the game and you it can it simply alters placing the value of the corresponding key. The code this stored in the address 0x3038E2 and you will see the following bytes: -01-01-01-02-02-02-04-08-04-08-00-FF - where: 01-is up, 02 is down, 04 is left, 08 is right and 00-FF indicate end of the code, you can modify the access code to level select menu even so the game only accepts code for the directional keys, you should place the bytes 00-FF in the end to indicate that the code finished

TABLE SUMMARY

Code	Location
Level Select Menu	0x3038E2 á 0x3038ED
Keys	Value
UP	01
DOWN	02
LEFT	04
RIGHT	08

Debug Mode and Super Knuckles

Them code to proceed is made after having been acessed the level select menu and they are inserted in Sound Test playing the respective music. These are easier to alter because are stored in Rom in the same way that are typed in Sound Test. The only difference in relation to the access code to the level select menu is that you have to type FF instead of 00-FF to indicate it I finish of the code. See below in the table the codes and its respective locations.

Event	Code	Location
Debug Mode	01-09-09-04-01-00-01-08	0x307C7A to 0x307C81
All Emeralds (Super Knuckles)	01-06-07-07-07-02-01-06	0x307C83 to 0x307C8A

Curiousities

You will see the codes of Sonic 2 alone, but without none usefulness in this game, now the onlooker is that if these codes were used the game it would not join in the music Oil Ocean when being made the code (01-01-02-04), because if you observe in the address 0x307C4C you will see the bytes 10-3C-00-BF and not 10-3C-00-3F as it happens in the game of Sonic 2 and that it causes the lock in the music Oil Ocean, I am commenting this because it Sega it didn't correct this bug nor in Sonic Jam in spite of not joining in the music OIL OCEAN SONIC JAM doesn't play the Sound of the it continues that should be played when doing the code 01-01-02-04. Many other bugs that happen in the cartridge of Sonic 2 doesn't happen in this version, a simple example is the bug of the debug that when you die and it tries to place an object in the screen the game it doesn't place the object in the screen while you doesn't leave in the edition way and to come back, being avoided the game as soon as freezes.