Tile data is stored in VRAM in the memory area at $8000-$97FF; with each tile taking 16 bytes, this area defines data for 384 tiles. In CGB Mode, this is doubled (768 tiles) because of the two VRAM banks.
Each tile has 8x8 pixels and has a color depth of 4 colors/gray shades. Tiles can be displayed as part of the Background/Window maps, and/or as OBJ tiles (foreground sprites). Note that OBJs don’t use color 0 - it’s transparent instead.
There are three “blocks” of 128 tiles each:
|Block||VRAM Address||Corresponding Tile IDs|
|OBJs||BG/Win if LCDC.4=1||BG/Win if LCDC.4=0|
Tiles are always indexed using a 8-bit integer, but the addressing method may differ. The “$8000 method” uses $8000 as its base pointer and uses an unsigned addressing, meaning that tiles 0-127 are in block 0, and tiles 128-255 are in block 1. The “$8800 method” uses $9000 as its base pointer and uses a signed addressing, meaning that tiles 0-127 are in block 2, and tiles -128 to -1 are in block 1, or to put it differently, “$8800 addressing” takes tiles 0-127 from block 2 and tiles 128-255 from block 1. (You can notice that block 1 is shared by both addressing methods)
Sprites always use “$8000 addressing”, but the BG and Window can use either mode, controlled by LCDC bit 4.
Each tile occupies 16 bytes, where each line is represented by 2 bytes:
Byte 0-1 Topmost Line (Top 8 pixels) Byte 2-3 Second Line etc.
For each line, the first byte specifies the least significant bit of the color ID of each pixel, and the second byte specifies the most significant bit. In both bytes, bit 7 represents the leftmost pixel, and bit 0 the rightmost. For example: let’s say you have $57 $36 (in this order in memory), which in binary are 01010111 and 00110110. To obtain the color ID for the leftmost pixel, you take bit 7 of both bytes: 0, and 0. Thus the index is binary 00 = 0. For the second pixel, repeat with bit 6: 1, and 0. Thus the index is binary 01 = 1 (remember to flip the order of the bits!). If you repeat the operation you’ll find that the IDs for the eight pixels are 0 1 2 3 0 3 3 1.
A more visual explanation can be found here.
So, each pixel has a color ID of 0 to 3. The color numbers are translated into real colors (or gray shades) depending on the current palettes, except that when the tile is used in a OBJ the color ID 0 means transparent. The palettes are defined through registers BGP, OBP0 and OBP1, and BCPS/BGPI, BCPD/BGPD, OCPS/OBPI and OCPD/OBPD (CGB Mode).