NES Graphics – Part 3

Part 1 and part 2 described CHR data, nametable based backgrounds, sprites, and scrolling. Combined, these cover nearly everything a basic NES cart can do without using additional hardware. To go any further will require a quick tangent to discuss, in detail, how rendering happens.

RENDERING

tv-rendering
Scanline based rendering, with a pause for vblank

Like any older computer, the NES was designed to work with CRT TVs. They draw scanlines to the screen, one at a time, left to right, top to bottom, using an electron gun that physically moves to point at the screen where it draws these lines. Once the bottom corner is reached, a period called “vertical blank” (or vblank) happens, wherein the electron gun moves back to the top left to prepare to draw the next frame. On the NES, the PPU (Picture Processing Unit) does this scanline based rendering automatically, every frame, while code running in the CPU does whatever work the game needs to do. Vblank gives the software an opportunity to change data in the PPU’s memory, as otherwise that memory is being used for rendering. Most of the time, changes to the PPU’s nametable and palettes have to occur during this small window.

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