Rendering

Tile Data

Pixels values are stored in the vram, in 0x8000-0x97FF. A pixel value is defined on 2 bits (so between 0 and 3). A couple of bytes defines 8 pixels values. The bits of the first byte are the least significant bits of the pixels values, and the bits of the second byte are the most significant bits of the pixels values. In both bytes, bit 7 represents the leftmost pixel, and bit 0 the rightmost.

example

0x3366 -> 00110011 01100110

hexa	bin
0x33	00110011
0x66	01100110

Pixels values:

Index	1st bit	2nd bit	Value
0	0	0	0
1	0	1	2
2	1	1	3
3	1	0	1
4	0	0	0
5	0	1	2
6	1	1	3
7	1	0	1

which gives the following line of pixels: 13201320

Tile Map

Tile map values are 1 byte index of the tile.

Map positions

The tile maps are stored in 0x9800-0x9BFF and/or in 0x9C00-0x9FFF.
The Background map adress is defined by the value of the register LCDC bit 3:

• bit is 0: background map is in 0x9800-0x9BFF.
• bit is 0: background map is in 0x9C00-0x9FFF.

The Window map adress is defined by the value of the register LCDC bit 6:

• bit is 0: window map is in 0x9800-0x9BFF.
• bit is 0: window map is in 0x9C00-0x9FFF.

Tile sheet block

Depending on the value of the register LCDC bit 4, the tile index can point to a different block of the tile sheet:

• bit is 0: index from the map start from 0x8000 in the tile sheet. The index is an unsigned byte so the tiles are stored in 0x8000-0x8FFF.
• bit is 1: index from the map start from 0x9000 in the tile sheet. The index is a signed byte so the tiles are stored in 0x8800-0x97FF.

block	obj index	bg/win index if LCDC.4==0	bg/win index if LCDC.4==1
0x8000-0x87FF	0 - 127	0 - 127
0x8800-0x8FFF	128 - 255	128 - 255	-128 - -1
0x9000-0x97FF			0 - 127

OAM

The Object Attribute Table hold the data of 40 objects (sprites) in 0xFE00-0xFE9F. An object can be a 8x8 or a 8x16 sprite. Each object is made of 4 bytes:

• Y position
• X position
• Tile index
• Attributes

At each scanline, the ppu selects up to 10 valid object according to their Y position.

Y position

The Y position of the sprite + 16. A 8xH sprite is fully displayed on the screen with 168 - H > Y >= 16.

X position

The X position of the sprite + 8. A sprite is fully displayed on the screen with 168 > X >= 8.

Tile index

The sprite mode is defined by the bit 2 of LCDC:

• LCDC.2==0: 8x8 sprite mode, since a sprite is made of only one tile the tile index correspond to its index.
• LCDC.2==1: 8x16 sprite mode, a sprite is made of two adjacent tiles and the byte indicate the index of the first one. The first tile is the top of the sprite. Note that the hardware enforce the index to point at a pair value by ignoring the least significant bit of the byte.

Attributes

• Bit 7: BG and Window over OBJ (0=No, 1=BG and Window colors 1-3 over the OBJ)
• Bit 6: Y flip (0=Normal, 1=Vertically mirrored)
• Bit 5: X flip (0=Normal, 1=Horizontally mirrored)
• Bit 4: Palette number Non CGB Mode Only (0=OBP0, 1=OBP1)
• Bit 3: Tile VRAM-Bank CGB Mode Only (0=Bank 0, 1=Bank 1)
• Bit 2-0: Palette number CGB Mode Only (OBP0-7)