9. The Picture Processing Unit (PPU)
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A "game" for the NES is made up of three components: graphics displayed on a screen, user input through some kind of controller, and audio for music and sound effects. The game uses the user's input to change the graphics it displays and the audio it plays, until the user turns off the system. In this set of chapters, we will look at each of these three components, beginning with how the NES displays graphics.
As you may remember from Chapter 4, the NES uses a fixed set of 64 colors for all of its graphics.
These colors are used to fill slots in eight four-color palettes. Four palettes are used to draw background objects, and the other four palettes are used to draw sprites, objects in the "foreground". Each thing drawn to the screen uses one of these palettes, limiting a single graphical object to four of the 64 available colors.
What exactly are these "graphical objects"? The NES does not let developers specify what to draw on a pixel-by-pixel basis. At a resolution of 256x240 pixels, each screen of graphics would require the specification of 61,440 pixels of color information, which would be far too much to fit into memory. Instead, the basic unit of NES graphics is the 8x8 pixel "tile". One screen of graphics is 32 tiles wide and 30 tiles tall (960 tiles).
The CHR-ROM in an NES cartridge holds two pattern tables, each of which holds 256
8x8 tiles. One pattern table is used for background graphics, and the other is used for sprite
Another option is to use 8x16 tiles (8px wide and 16px tall), as seen in games like
The Legend of Zelda. While this reduces the number of tiles that can be
stored in the pattern tables, this has the advantage of allowing tiles from either
table to be used as either background or sprite. Depending on your game's graphical
needs, the advantage of being able to re-use tiles in both layers might outweigh the
disadvantage of having fewer tiles available.
These issues largely become moot with the use of mapper chips that allow for bank switching of pattern table graphics, which will be explored much later in this book. Each tile in the table is defined with two "bit planes", specifying which palette color (0-3) is used for each pixel of the tile. One bit plane defines the "low bit" of each pixel in the tile, and the other defines the "high bit". (Two bits, as you may recall, can represent four different values, corresponding to the four colors in a palette.) Each tile takes up 16 bytes of memory, so the CHR-ROM chip's 8KB of storage is just enough to fit the 512 tiles of the two pattern tables. By specifying only a palette index rather than an actual color, the tiles themselves take up less memory and can be re-used with different palettes as needed.
Everything that an NES game draws to the screen is contained in its pattern tables. This is more complicated when CHR-RAM or bank switching are involved, though anything drawn is still, technically, present in the pattern tables at some point. There is no "system font"; if you want to draw text in your NES game, you need to create font tiles in a pattern table. (This is why most NES games, especially early NES games, tend to be all-caps, shouty affairs.) The limited space for tiles also means that efficient re-use of tiles is important. Being able to re-use a tile in multiple ways, or making clever use of palette swaps, can give a game a greater range of visual representations while still fitting within the memory limitations of CHR-ROM.
Sprites represent the "foreground" layer of graphics. Each sprite is a single tile which can be positioned anywhere on the screen, down to the pixel. Sprites can also be flipped vertically or horizontally (but not rotated), and each sprite can specify which of the four sprite palettes it will use. This flexibility comes at a cost, though: memory and processing time constraints mean that the NES can only display 64 sprites at a time, and only eight sprites can be drawn on a scanline (a horizontal row of pixels).
If you've played many NES games, you have likely experienced "flickering", where some sprites appear and disappear rapidly in a way that does not seem intentional.
This flicker is the result of too many sprites being drawn on one scanline. Since the PPU can only draw eight sprites on a scanline, any sprites beyond the first eight will not appear. The flickering effect, however, is a conscious choice on the part of the game developer, to help the player. Flicker results when the developer changes which eight sprites come first each scanline. In doing so, the player can see all of the sprites on the scanline, just not all at the same time. Otherwise, enemy sprites might be entirely invisible, which would be unfair.
When sprites are drawn to the screen, any pixel within the tile that uses the first color of the palette (index zero) is drawn as transparent, allowing the background layer to display at that location. This means that each sprite can only use three colors. By using transparency, it is possible to overlap sprites that use different color palettes over one another, creating graphics that use more than three colors.
Sprites have great flexibility at the expense of only being able to cover a small portion of the screen. Backgrounds have the opposite trade-off. A background can cover the entire screen — 960 8x8 tiles — but background tiles must fit to a grid, and they suffer further limitations on palette use. The background layer can be scrolled in 1-pixel increments, but all the tiles move together. There is no way to scroll different parts of the screen differently without using tricky mid-frame updates (e.g. "Sprite Zero Hit", scanline IRQ).
Backgrounds are defined via nametables, which live in PPU memory. Each nametable is 960 bytes, and each of those bytes stores the tile number of one of the 256 tiles in the background pattern table. The PPU memory map has space for four nametables arranged in a square pattern, meaning that, in theory, you could set up four TV screens worth of background at once.
I say in theory because the Famicom was designed to be cheap, as we discussed in Chapter 1, and at the time, memory was very expensive. As a compromise, the Famicom/NES has enough physical memory for two nametables. These are "real" nametables that behave as expected. The memory ranges for the other two nametables act as "mirrors" of the real nametables, so that asking for a byte of memory from a mirror returns a byte from the corresponding real nametable. The developer can configure which two nametables are "real" and which two are "mirrored". On a hardware NES cartridge, this is done with a pad of solder over one of two contacts on the cartridge board. For emulators, a game's mirroring setting is part of its iNES header.
Mirroring can be vertical or horizontal. In vertical mirroring, nametables 1 and 2 are "real", and 3 and 4 are mirrors. This gives the developer two screens in a left-to-right layout, perfect for horizontally-scrolling games. Horizontal mirroring, in contrast, makes nametables 1 and 3 the "real" nametables, and nametables 2 and 4 the mirrors. Horizontal mirroring results in two screens in a top-to-bottom layout, which is designed for vertically-scrolling games. While mirroring is hard-soldered in older NES games, later cartridges that add mapper chips give the developer the ability to change mirroring at any time. The MMC1 chip, for example, allows Metroid to switch between vertical and horizontal mirroring when the player moves through a doorway, allowing for a mix of horizontal and vertical scrolling sections.
A nametable is just a list of tile numbers. In order to color each tile with a palette, we need a second type of table. At the end of each nametable is a 64-byte region called an attribute table, which specifies which palette to use for each tile of background. 960 + 64 = 1024 bytes, so each nametable / attribute table pair takes one kilobyte of memory.
Since the attribute table is only 64 bytes, there isn't enough space to specify a palette for each individual background tile. Instead, each byte of the attribute table specifies the palette colors for sixteen background tiles, in a four-by-four square. Bits 0-1 of each byte specify the palette used for the top left two-by-two portion of the four-by-four area, bits 2-3 the top right, bits 4-5 cover the bottom left and bits 6-7 select a palette for the bottom right. This means that in addition to background tiles being fixed to a grid, color information is tied to its own grid as well.
As a consequence of attribute table limitations, background objects are generally drawn in 2x2 tile units. We call these larger objects metatiles.
The attribute table grid is also the reason why few NES games use an "isometric", or angled, display. Trying to draw backgrounds at an angle can cause color glitches when a large background section crosses attribute table boundaries.