save format.md

Sonic 3 for Sega Genesis save file format

This is a specification for the save file format used by Sonic 3 and Sonic 3 & Knuckles, released in 1994 for the Sega Genesis, and Sonic & Knuckles Collection, released in 1997 for Windows 95.

This document uses the following terms to represent data sizes, consistent with their use in the Motorola 68000 instruction set:

Term	Length	Range
BYTE	1 byte (8 bits)	0–255
WORD	2 bytes (16 bits)	0–65,535
LONG	4 bytes (32 bits)	0–4,294,967,295

All numbers are big endian integers, meaning that the word $00FF has the decimal value of 255 and not 65,280.

Overview

Offset (Console)	Offset (PC)	Size	Description
$008–$05B	$000–$053	84 bytes	Competition, section 1
$05E–$0B1	—	84 bytes	Competition, section 2
$0B4–$0E7	$0C0–$0F3	52 bytes	Sonic 3 single-player, section 1
$0FA–$12D	—	52 bytes	Sonic 3 single-player, section 2
$140–$193	$180–$1D3	84 bytes	Sonic 3 & Knuckles single-player, section 1
$196–$1E9	—	84 bytes	Sonic 3 & Knuckles single-player, section 2

All other bytes are 0.

Console

The save file is 512 bytes long and contains a section for competition times and sections for Sonic 3 or Sonic 3 & Knuckles single-player data (or both, though this is uncommon for emulator saves since emulators give each ROM a separate save file).

Each section contains its own checksum and is duplicated for redundancy. If the checksum for the first section fails, the game tries the second one. If the checksum for the second section fails, the game resets the save data to the default values.

Many emulators, including Gens and Kega Fusion, pad each byte to word-length in big-endian (Motorola) order (so the byte $FF would be written as the word $00FF).

PC

This is the original PC release of the game from 1997. Its save file is named "sonic3k.bin" and located in the program directory. The file is always 1024 bytes in length.

The format is very similar to the console version, but without the duplication (each section only appears once), without the section checksums, and with different starting offsets for each section.

Competition section

The competition section is 84 bytes long. The first section starts at $008 and the second starts at $05E. Both sections should contain identical data unless data corruption has occurred.

Offset 1 (Console)	Offset 2 (Console)	Offset (PC)	Size	Description
$008–$017	$05E–$06D	$000–$00F	16 bytes	Azure Lake
$018–$027	$06E–$07D	$010–$01F	16 bytes	Balloon Park
$028–$037	$07E–$08D	$020–$02F	16 bytes	Desert Palace
$038–$047	$08E–$09D	$030–$03F	16 bytes	Chrome Gadget
$048–$057	$09E–$0AD	$040–$04F	16 bytes	Endless Mine
$058–$059	$0AE–$0AF	$050–$051	WORD	Always $4C44
$05A–$05B	$0B0–$0B1	—	WORD	Checksum

Note that the order of Chrome Gadget and Desert Palace are transposed from their order in the game.

Stage contents

Each stage consists of:

Offset	Size	Description
$0–$3	LONG	1st place time
$4–$7	LONG	2nd place time
$8–$B	LONG	3rd place time
$C–$F	LONG	Characters

See Appendix A for a complete list of competition position offsets.

Time

Each time long is structured as follows:

• First byte is $80 if the position is empty, $00 otherwise.
• Second byte is minutes.
• Third byte is seconds.
• Fourth byte is ticks.

For the PC version, the bytes are in the opposite order:

• First byte is ticks.
• Second byte is seconds.
• Third byte is minutes.
• Fourth byte is $80 if the position is empty, $00 otherwise.

A tick in Sonic 3 is 1/100 of a second.

If the position is empty, all bytes except the first are set to $00. Empty positions are surpassed by any time when the player completes a stage, regardless of the value of the other bytes.

Characters

The characters long is structured as follows:

• First byte is the character for the 1st place position.
• Second byte is the character for the 2nd place position.
• Third byte is the character for the 3rd place position.
• Fourth byte is always 0.

The character is $00 for Sonic, $01 for Tails, or $02 for Knuckles.

The default value is $00010200 if all positions are empty.

Sonic 3 single-player section

The Sonic 3 single-player section is 52 bytes long. The first section starts at $0B4 and the second starts at $0FA. Both sections should contain identical data unless data corruption has occurred.

Offset 1 (Console)	Offset 2 (Console)	Offset (PC)	Size	Description
$0B4–$0BB	$0FA–$101	$0C0–$0C7	8 bytes	Slot 1
$0BC–$0C3	$102–$109	$0C8–$0CF	8 bytes	Slot 2
$0C4–$0CB	$10A–$111	$0D0–$0D7	8 bytes	Slot 3
$0CC–$0D3	$112–$119	$0D8–$0DF	8 bytes	Slot 4
$0D4–$0DB	$11A–$121	$0E0–$0E7	8 bytes	Slot 5
$0DC–$0E3	$122–$129	$0E8–$0EF	8 bytes	Slot 6
$0E4–$0E5	$12A–$12B	$0F0–$0F1	WORD	Always $4244
$0E6–$0E7	$12C–$12D	—	WORD	Checksum

Slot contents

Each slot is structured as follows:

Offset	Size	Description
$0	BYTE	New game
$1	BYTE	Always 0
$2	BYTE	Character
$3	BYTE	Current Zone
$4	BYTE	Next Special Stage
$5	BYTE	Number of emeralds
$6	BYTE	Chaos Emeralds
$7	BYTE	Giant Rings

New game

$80 if new, $00 otherwise.

All other bytes for the slot are $00 if set to new.

Character

Value	Character
$00	Sonic & Tails
$01	Sonic
$02	Tails
$02	Knuckles

Knuckles displays properly on the Data Select screen, but selecting him results in playing as Sonic with some minor changes.

Zone

The current Zone or $07 for clear.

Value	Zone
$00	Angel Island
$01	Hydrocity
$02	Marble Garden
$03	Carnival Night
$04	Flying Battery
$05	Ice Cap
$06	Launch Base
$07	Clear

Flying Battery is selectable but missing most assets. It appears as "Zone 5" on the Data Select screen, the same as Ice Cap.

Next Special Stage

The next Special Stage to be played, between $00 and $06. The game goes through all of the Special Stages in rotation, skipping the ones that have already been successfully completed. This value is set to $00 once the player has collected all of the Chaos Emeralds.

A value of $07 loads an unused Special Stage layout.

If the value corresponds to a Chaos Emerald that has already been collected or is greater than $07, the value is ignored and the game loads the lowest uncompleted Special Stage instead.

Number of emeralds

The number of Chaos Emeralds collected, between $00 and $07. If set to $07, Special Stages can no longer be entered and super forms are enabled.

Chaos Emeralds

Tracks the Chaos Emeralds collected by the player. Each Chaos Emeralds is associated with a particular Special Stage, so they can be collected out of order if the player fails a Special Stage.

This is stored as a bit field of seven bits, each representing one Chaos Emerald:

1 1 1 1 1 1 1 0
| | | | | | |
| | | | | | Green
| | | | | Orange
| | | | Pink
| | | Purple
| | Grey
| Red
Blue

This can also be expressed as a sum of the powers of 2:

Emerald	Value
Green	2
Orange	4
Pink	8
Purple	16
Grey	32
Red	64
Blue	128

A value of $FE means that all of the Chaos Emeralds have been collected.

Giant Rings

Tracks the Giant Rings collected in each Zone. Resets to $00 at the start of a new Zone or after the game has been completed.

This is stored as a bit field of seven bits, each representing one Giant Ring:

1 1 1 1 1 1 1 1
| | | | | | | |
| | | | | | | 1st Giant Ring
| | | | | | 2nd Giant Ring
| | | | | 3rd Giant Ring
| | | | 4th Giant Ring
| | | 5th Giant Ring
| | 6th Giant Ring
| 7th Giant Ring
8th Giant Ring

This can also be expressed as a sum of the powers of 2:

Giant Ring	Value
1st	1
2nd	2
3rd	4
4th	8
5th	16
6th	32
7th	64
8th	128

Sonic 3 & Knuckles single-player section

The Sonic 3 & Knuckles single-player section is 84 bytes long. The first section starts at $140 and the second starts at $196. Both sections should contain identical data unless data corruption has occurred.

Offset 1 (Console)	Offset 2 (Console)	Offset (PC)	Size	Description
$140–$149	$196–$19F	$180–$189	10 bytes	Slot 1
$14A–$153	$1A0–$1A9	$18A–$193	10 bytes	Slot 2
$154–$15D	$1AA–$1B3	$194–$19D	10 bytes	Slot 3
$15E–$167	$1B4–$1BD	$19E–$1A7	10 bytes	Slot 4
$168–$171	$1BE–$1C7	$1A8–$1B1	10 bytes	Slot 5
$172–$17B	$1C8–$1D1	$1B2–$1BB	10 bytes	Slot 6
$17C–$185	$1D2–$1DB	$1BC–$1C5	10 bytes	Slot 7
$186–$18F	$1DC–$1E5	$1C6–$1CF	10 bytes	Slot 8
$190–$191	$1E6–$1E7	$1D0–$1D9	WORD	Always $4244
$192–$193	$1E8–$1E9	—	WORD	Checksum

Slot contents

Each slot is structured as follows:

Offset	Size	Description
$0	BYTE	New or clear value
$1	BYTE	Always 0
$2	BYTE	Character, number of emeralds
$3	BYTE	Current Zone
$4	BYTE	Giant Rings
$5	BYTE	Always 0
$6–$7	WORD	Chaos Emeralds/Super Emeralds
$8	BYTE	Lives
$9	BYTE	Continues

New or clear value

Value	Type
$80	New
$00	Not cleared
$01	Cleared without all Chaos Emeralds
$02	Cleared with all Chaos Emeralds
$03	Cleared with all Super Emeralds

All other bytes for the slot are $00 if set to new.

Character, number of emeralds

The most significant digit is the character:

Value	Character
$00	Sonic & Tails
$10	Sonic
$20	Tails
$30	Knuckles

A value of $40 or higher result in the glitched "Blue Knuckles" character, which uses Knuckles' sprite with Sonic's color palette.

The least significant digit is the number of emeralds collected, between $00 and $07. An emerald is counted as collected if it is a Chaos Emerald, in the Hidden Palace, or a Super Emerald. After all emeralds have been collected, this digit rolls back around to 0.

Current Zone

Value	Zone
$00	Angel Island
$01	Hydrocity
$02	Marble Garden
$03	Carnival Night
$04	Ice Cap
$05	Launch Base
$06	Mushroom Hill
$07	Flying Battery
$08	Sandopolis
$09	Lava Reef
$0A	Hidden Palace
$0B	Sky Sanctuary
$0C	Death Egg
$0D	The Doomsday

Note that the numbering differs from Sonic 3 after Carnival Night since Flying Battery is moved.

If the game is cleared, this value is set to the last Zone ($0B for Knuckles, $0C for Tails and Sonic with fewer than seven emeralds collected, and $0D for Sonic with at least seven emeralds collected).

Giant Rings

Tracks the Giant Rings collected in each Zone. See the Sonic 3 section for a full description.

Chaos Emeralds/Super Emeralds

Tracks the Chaos Emeralds and Super Emeralds collected by the player. Each emerald corresponds to a particular Special Stage.

This is stored in two separate bytes, each consisting of a bit field.

The first byte:

1 1 1 1 1 1 1 1
| | | | | | | |
| | | | | | | Purple Chaos Emerald
| | | | | | Purple Emerald in Hidden Palace
| | | | | Pink Chaos Emerald
| | | | Pink Emerald in Hidden Palace
| | | Orange Chaos Emerald
| | Orange Emerald in Hidden Palace
| Green Chaos Emerald
Green Emerald in Hidden Palace

The second byte:

1 1 1 1 1 1 0 0
| | | | | |
| | | | | |
| | | | | |
| | | | | Blue Chaos Emerald
| | | | Blue Emerald in Hidden Palace
| | | Red Chaos Emerald
| | Red Emerald in Hidden Palace
| Grey Chaos Emerald
Grey Emerald in Hidden Palace

Each emerald has two bits associated with it, representing one of four possible states:

Bits	State
%00	Not collected
%01	Chaos Emerald collected
%10	In Hidden Palace
%11	Super Emerald collected

A value of $FFFC means that all of the Super Emeralds have been collected.

For the PC version, these bytes are in the opposite order (such that $FCFF signifies that all Super Emeralds have been collected).

Lives

The number of lives, between 0 and 99 ($63).

Continues

The number of continues, between 0 and 99 ($63).

Checksum algorithm

The original algorithm for calculating and verifying the checksum is implemented in Motorola 68000 assembly:

Create_SRAMChecksum:
    moveq   #0,d7

loc_C364:
    move.w  (a6)+,d5
    eor.w   d5,d7
    lsr.w   #1,d7
    bcc.s   loc_C370
    eori.w  #$8810,d7

loc_C370:
    dbf     d6,loc_C364
    rts

The address register a6 points to the save data. The data register d7 holds the checksum and d6 is the counter.

We initialize the checksum to $00 and the counter to the length of the section in words, minus the checksum ($28 for the competition and Sonic 3 & Knuckles single-player sections, $18 for the Sonic 3 single-player section).

Then we iterate through the save data one word at a time and xor each word with the checksum. We then do a logical shift right, which saves the least significant bit before the shift as the carry bit. If the carry bit is 0, we xor the checksum with the value $8810.

The algorithm implemented in JavaScript:

function calculateChecksum(bytes) {
	const BIT_MASK = 0x8810;
	let checksum = 0;

	for (let i = 0; i < bytes.length - 2; i += 2) {
		checksum ^= (bytes[i] << 8) | bytes[i + 1];

		const carry = checksum & 1; // gets least significant bit before shift
		checksum >>>= 1;

		if (carry != 0) {
			checksum ^= BIT_MASK;
		}
	}

	return checksum;
}

The checksum is only used by the console version.

Appendices

A. Competition offsets

Offset (Console)	Offset (PC)	Size	Description
$008–$00B	$000–$003	LONG	Azure Lake, 1st place time
$00C–$00F	$004–$007	LONG	Azure Lake, 2nd place time
$010–$013	$008–$00B	LONG	Azure Lake, 3rd place time
$014–$017	$00C–$00F	LONG	Azure Lake, characters
$018–$01B	$010–$013	LONG	Balloon Park, 1st place time
$01C–$01F	$014–$017	LONG	Balloon Park, 2nd place time
$020–$023	$018–$01B	LONG	Balloon Park, 3rd place time
$024–$027	$01C–$01F	LONG	Balloon Park, characters
$028–$02B	$020–$023	LONG	Desert Palace, 1st place time
$02C–$02F	$024–$027	LONG	Desert Palace, 2nd place time
$030–$033	$028–$02B	LONG	Desert Palace, 3rd place time
$034–$037	$02C–$02F	LONG	Desert Palace, characters
$038–$03B	$030–$033	LONG	Chrome Gadget, 1st place time
$03C–$03F	$034–$037	LONG	Chrome Gadget, 2nd place time
$040–$043	$038–$03B	LONG	Chrome Gadget, 3rd place time
$044–$047	$03C–$03F	LONG	Chrome Gadget, characters
$048–$04B	$040–$043	LONG	Endless Mine, 1st place time
$04C–$04F	$044–$047	LONG	Endless Mine, 2nd place time
$050–$053	$048–$04B	LONG	Endless Mine, 3rd place time
$054–$057	$04C–$04F	LONG	Endless Mine, characters

Authors

• J.C. Fields jcfields@jcfields.dev

Checksum algorithm taken from Sonic Retro's Sonic 3 & Knuckles disassembly by Stealth and contributors.