SEGA Genesis format

This document describes the format of data in Prince of Persia 1 for the SEGA Genesis, also known as the SEGA Mega Drive.

The game has two versions: One for the USA (Genesis), and one for Europe (Mega Drive). They have data at different offsets, and I will always note which offsets are for which versions.

General

Byte order: Big-endian (Motorola, network) byte order is used, unless noted otherwise. This is the native endianness of the Motorola 68k CPU in the SEGA Genesis.

Numbering of bits: The least significant bit of a byte is bit 0, the most significant bit of a byte is bit 7.

Addresses: Since the CPU is 32-bit, there are no segments, banks, paging, or anything like that. You can just use each address as it is. (Only the bottom 24 bits are used by the hardware. The top 8 bits are usually 0, though.)

Compression: The European version of the game uses RNC compression for some graphics. This is a popular library by Rob Northen Computing.

Hardware: You can find some documentation about the SEGA Genesis at Zophar's Domain.

Levels

The level table

Starting address

Game version	Address of level table	Address of starting place table	Number of levels
USA	0x6A5C	0x20140	13 levels
Europe	0x358C2	0x504B2	17 levels

Contents of the level table

The table stores 20 bytes about each level:

Size	Contents
2 bytes	Level's height, in pixels. Must be a multiple of 192, the height of a room. Divide by 64 to get the height in tiles.
2 bytes	Level's width, in pixels. Must be a multiple of 320, the width of a room. Divide by 32 to get the width in tiles.
2 bytes	Size of each level map, in tiles (bytes). (width * height)
4 bytes	Start address of level's #Graphics map.
4 bytes	Start address of level's #Objects map.
2 bytes	Y coordinate of the top of the starting room, in pixels. Subject to the same restrictions as level height.
2 bytes	X coordinate of the left of the starting room, in pixels. Subject to the same restrictions as level width.
2 bytes	Level type: 0=dungeon, 1=palace.

Contents of the starting place table

The table stores 6 bytes about each level:

• 2 bytes: starting Y coordinate, in pixels (bottom)
• 2 bytes: starting X coordinate, in pixels (left)
• 2 bytes: starting direction: 0=left, 0x800=right

Maps

Both maps store tiles in column-major order. That is, you start from the top-left corner of the level, and go down. After you finished a column, you continue at the top of the next column.

Note that maps are not split into rooms (only when displayed), and therefore there are no room links or room numbers.

Graphics map

Each byte tells which #Level graphics block is to be displayed here. The same index is used for both layers.

Objects map

Each byte tells what object is in the corresponding tile.

The possible values are:

• 0x00: empty
• 0x01: wall
• 0x02: floor (and all objects below include a floor)
• 0x03: opener button (*)
• 0x04: closer button (*)
• 0x05: gate (*)
• 0x06: loose floor (*)
• 0x07: spikes (*)
• 0x08: chomper (*)
• 0x09: potion (*)
• 0x0A: level door (*)
• 0x0B: sword

Further info about objects marked with (*) (and also guards) can be found in the #Object tables.

Object tables

type of object	USA address	Europe address	bytes per object
opener buttons	0x1E592	0x4E8F6	14
closer buttons	0x1EBFC	0x4F138	14
spikes	0x25F94	0x56612	8
potions	0x27BAE	0x581B0	10
chompers	0x228DE	0x52D86	10
loose floors	0x1CD3E	0x4CF3C	10
gates	0x5B96	0x61CE	12
level doors	0x41DE	0x46C2	10
guards	0x1FAC	0x2B02	24

Each table has a header. The header contains 6 bytes per level:

• 2 bytes for the number of objects on that level,
• 4 bytes for the start address of the sub-table of the objects on this level. (It's 0 if the count is 0.)

(The two bytes before the header always contain 0x4E75. This is the RTS instruction that ends the preceding subroutine.)

The contents of each table depend on the type of the object.

All coordinates are in pixels.

opener and closer buttons

• 2 bytes: 1
• 2 bytes: Y coordinate (bottom)
• 2 bytes: X coordinate (left)
• 2 bytes: 0
• 2 bytes: first trigger
• 2 bytes: second trigger
• 2 bytes: third trigger

The triggers can have these values:

• 0-based index into gates table
• 0xFFFF = exit door
• 0xFFFE = used for the button that opens the level 4 exit (=mirror?)
• 0xFFFD = none

spikes

• 2 bytes: 1
• 2 bytes: Y coordinate (bottom)
• 2 bytes: X coordinate (left)
• 2 bytes: 0xFFFF

potions

• 2 bytes: bubble colors: 1 (red) or 3 (green)
  • Even values (like 0 or 2) will make the potion gone.
• 2 bytes: Y coordinate (bottom)
• 2 bytes: X coordinate (left)
• 2 bytes: 0
• 2 bytes: effect:
  • 0 = heal (+1 HP)
  • 1 = life (extra HP)
  • 2 = hurt (-1 HP)
  • 3 = slow-fall
  • 4 = shake screen and drop all loose floors in the whole level
  • 5 = stop time temporarily
  • 6..9 = extra time (+5 minutes)

Potion effects 4..9 are available only in the European version.

chompers

• 2 bytes: 1
• 2 bytes: Y coordinate (top)
• 2 bytes: X coordinate (left)
• 2 bytes: 0
• 2 bytes: 0

loose floors

• 2 bytes: 1 (other values hang the game)
• 2 bytes: Y coordinate (bottom)
• 2 bytes: X coordinate (left)
• 2 bytes: 0 (animation frame)
• 2 bytes: 0

gates

• 2 bytes: state: (animation)
  • 0=closed
  • 1=open on start (not used)
  • 3=open
  • 7=close on start
• 2 bytes: Y coordinate (top)
• 2 bytes: X coordinate (left edge of the gate's tile, but the gate is on the right edge of this tile)
• 2 bytes: state: (amount of openness)
  • 0=closed
  • 1..0x0B=partially open (not used)
  • 0x0C=open / close on start
• 2 bytes: state: (time before gate starts closing)
  • 0=closed / close on start
  • 0xFFFF=open
• 2 bytes: unknown, 0 or 1

level doors

• 2 bytes: type: 0=start, 2=exit door
• 2 bytes: Y coordinate (top)
• 2 bytes: X coordinate (left)
• 2 bytes: 0
• 2 bytes: 0

guards

• 2 bytes: 0
• 2 bytes: Y coordinate (bottom)
• 2 bytes: X coordinate (left)
• 2 bytes: direction: 0=left, 0x800=right
• 2 bytes: 0
• 4 bytes: guard sprite (values below are for USA / Europe) (unused?)
  • 0x23838 / 0x53C8C = regular guard
  • 0x23954 / 0x53DA2 = skeleton
  • 0x23DD8 / 0x54226 = fat
  • 0x24000 / 0x54448 = shadow
  • 0x241B2 / 0x545FA = Jaffar
• 2 bytes: guard type
  • 0 = regular guard
  • 1 = skeleton
  • 2 = fat
  • 3 = shadow
  • 4 = Jaffar
• 2 bytes: skill
• 2 bytes: hit points
• 2 bytes: 0
• 2 bytes: 0

Level graphics

USA:

Level type	Palette addr	Tiles addr	Blocks addr	Number of tiles
0 (dungeon)	0x47B20	0x40AC0	0xDCEE	898
1 (palace)	0x4E680	0x47B40	0x154EE	858

Europe:

Level type	Palette addr	Tiles addr	Blocks addr	Number of tiles
0 (dungeon)	0x7976C	0x732D6	0x3DEEC	898
1 (palace)	0x7FA38	0x797AC	0x456EC	858

Palettes

A palette contains 16 colors. Each color is stored on 16 bits (2 bytes), in X:R:G:B = 4:4:4:4 bits format. (This means that a palette takes up 32 bytes.)

But each level graphics has two palettes, so there are 32 colors (64 bytes) at the pointed address!

Tiles

Each tile has 8*8 pixels. Each pixel is stored on 4 adjacent bits. (This means that each tile takes up 32 bytes.)

Within a byte, bits 7..4 store the left pixel, and bits 3..0 store the right pixel. (There is no interleaving or separated planes, as on some other hardware. (SNES, etc.))

Color 0 is transparent!

Blocks

There are 2*256 blocks. The first 256 are for the back layer, the next 256 for the front layer. Block with the same index from the two layers are always used together.

Each block is 4 tiles (32 pixels) wide and 8 tiles (64 pixels) high.

Each tile-position is stored on 2 bytes (16 bits):

bits	mask	meaning
15	0x8000	Priority. 0 = behind sprites, 1 = in front of sprites.
14..13	0x6000	Palette index (0..3). For level graphics, only 0 and 1 are used, corresponding to the two palettes mentioned in #Palettes.
12	0x1000	Vertical flip.
11	0x0800	Horizontal flip.
10..0	0x07FF	Tile number.

TODO

• Texts
• Sprites
• Full-screen images
• Sound samples?
• (Music???)