SNES Images

Overview

The display is 256×224 pixels in the normal NTSC setup. The picture is not a single bitmap; it is assembled every scanline from up to four scrolling background layers plus the object layer. Each background is a grid of 8×8 tiles, arranged by a tilemap, coloured from a shared 256-colour palette. The chosen background mode decides how many layers exist and how deep their colour is.

Background modes

The PPU offers eight background modes, numbered 0 to 7. A mode fixes how many background layers are active and how many colours each can use. More layers means fewer colours per layer, because deeper tiles cost more VRAM and more per-pixel fetches. The table gives every mode with its layers and their colour depth.

Modes 0 to 6 differ only in the layer and depth mix; the tile and tilemap machinery is the same. Mode 1 is the workhorse for most games, giving two richly coloured layers and a third for text or a status bar.

Mode 7

Mode 7 replaces the tiled layers with one large background the PPU can transform. It is a single 8bpp layer, 128×128 tiles at most, wrapped or clamped, that the hardware multiplies through a 2×2 matrix each scanline. That matrix can rotate, scale and shear the layer, producing the tilting floors, spinning maps and racing roads the machine is known for.

Changing the matrix values between scanlines, usually with a raster interrupt, bends the transform down the screen and gives the perspective effect of a ground plane receding to the horizon.

Tile character format

A tile, also called a character, is an 8×8 block of pixels. Its colour depth is set by the mode and layer: 2bpp for 4 colours, 4bpp for 16, or 8bpp for 256. The pixels are stored as bitplanes. A 2bpp tile is two planes, 16 bytes; each pair of bytes holds one row, the two planes giving the two bits of each of the eight pixels. Deeper tiles simply add more planes.

In a 4bpp tile the first two planes are stored interleaved by row, then the next two planes follow as a second block. Colour index 0 is transparent, so it shows the backdrop or a lower layer. The final on-screen colour is that index plus the palette offset chosen by the tilemap entry.

Palette and CGRAM

All colour on the SNES comes from CGRAM, a 512-byte palette RAM holding 256 colours, two bytes each. A colour is a 15-bit value with five bits per channel in blue-green-red order, so the top bit of the 16-bit word is unused.

The 256 colours are shared out among the layers. A 4bpp layer draws from one of sixteen 16-colour sub-palettes, chosen per tile by the tilemap; an 8bpp layer or Mode 7 uses the whole 256 at once. Objects take the upper 128 colours as eight object palettes.

Video memory

The PPU owns three separate memories. VRAM holds the tiles and tilemaps, CGRAM holds the palette, and OAM holds the sprites. The CPU cannot see VRAM directly; it writes through the PPU data ports, usually driven by DMA during the vertical blank.

Because VRAM is only 64 KB, deep tiles and large tilemaps compete for space, so a game balances how many 8bpp characters it keeps against the size of its scrolling maps.