diff options
| -rw-r--r-- | assets/ppu-level-1.svg | 3 | ||||
| -rw-r--r-- | docs/ppu.drawio | 1 | ||||
| -rw-r--r-- | docs/research.md | 144 |
3 files changed, 138 insertions, 10 deletions
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\ No newline at end of file diff --git a/docs/research.md b/docs/research.md index 7eca454..1c13739 100644 --- a/docs/research.md +++ b/docs/research.md @@ -20,36 +20,160 @@ this chip's features and limitations: - composite output at 256x224 @ 60Hz (NTSC) or 256x240 @ 50Hz (PAL) - 256 tiles per tilemap (with 8x8 tiles) -- 2 tilemaps +- 2 tilemaps (pattern tables) - 4 palettes per sprite with 64 possible colors -- 512x480 background palette with scrolling +- 512x480 background canvas with scrolling - background scrolling splits -- 64 total sprites on screen (8 per scanline) +- 64 total sprites on screen (max 8 per scanline) - sprites can be drawn before or after the background layer - PPU control using DMA - 8K character memory (tilemaps) -- 2K nametable memory +- 2K nametable memory (OAM for background tiles) - 256B object attribute memory (OAM) - tiles can be flipped using OAM - no frame buffer +### Usage + +The NES PPU has a lot of capabilities, so here's a quick run-down of how the +PPU is used by games to produce pictures. + +On boot, the NES copies the contents of the so-called CHR-ROM (in game +cartridge) into the PPU's pattern tables. The NES has two 256-tile tilemaps +called the pattern tables which store all sprites in the game. By modifying the +nametable memory directly, the game can control which tile, and which palette +will be used for any given tile on the background layer. Because the background +layer only displays tiles in a fixed grid, the nametable memory area is not +very big. + +The same process happens for the foreground sprites, though these can each be +positioned using screen coordinates, and don't have to conform to any grid. The +NES also has some limitations on how many sprites can be drawn and how many +palettes can be used. An important note is that the first color of any palette +used on any foreground sprite, is treated as the transparency color. Our PPU +also copies this behavior. + +The following is a small section of pseudocode, depicting a program that will +display a triangle moving in a circle: + +```c +unsigned frame = 0; + +void setup() { + // copy character rom to PPU + memcpy(CHR, PPU->CHR, 0x8000); + // character rom contains a sprite of a triangle at index 0 + + // set palette index 0 to have 4 random colors + PAL[0] = (palette) { + PALETTE_COLOR_25, + PALETTE_COLOR_F3, + PALETTE_COLOR_00, + PALETTE_COLOR_3D, + }; + // copy palette data to PPU + memcpy(PAL, PPU->PAL, 0x40); +} + +void loop() { + frame++; // increment frame counter + + OAM[0] = (sprite) { + .x = sin(frame) * 20 + 10, // calculate circle position using frame counter + .y = cos(frame) * 20 + 10, // calculate circle position using frame counter + .pattern_index = 0, // triangle sprite + .palette_index = 0, // palette 0 (see setup) + .attributes = PPU_FX_FLIP_H | PPU_FX_FLIP_V, // flip horizontally and vertically + }; + + memcpy(OAM, PPU->OAM, 0x100); // update PPU with local copy of OAM +} + +int main() { + setup(); + while(1) loop(); +} +``` + ## Custom PPU Here's a list of features our PPU should have: <!-- TODO: expand list with PPU spreadsheet --> -- 256x240 @ 60Hz VGA output -- single tilemap with room for 2048 tiles of 8x8 pixels +- 256x224 @ 60Hz VGA output +- single tilemap with room for 1024 tiles of 16x16 pixels - 8 colors per palette, with 4096 possible colors (12-bit color depth) -- 512x480 background palette with scrolling -- **NO** background scrolling splits -- 128 total sprites on screen (**NO** scanline sprite limit) +- 512x448 background canvas with scrolling +- NO background scrolling splits +- 128 total sprites on screen (NO scanline sprite limit) - sprites are always drawn on top of the background layer - PPU control using DMA (dual-port asynchronous RAM) -- tiles can be flipped using OAM +- tiles can be flipped using FAM or BAM - no frame buffer - vertical and horizontal sync output +Notable differences: + +- NES nametable equivalent is called BAM (background attribute register) +- NES OAM equivalent is called FAM (foreground attribute register) +- No scanline sprite limit + + Unless not imposing any sprite limit makes the hardware implementation + impossible, or much more difficult, this is a restriction that will likely + lead to frustrating debugging sessions, so will not be replicated in our + custom PPU. +- Sprites are 16x16 + + Most NES games already tile multiple 8x8 tiles together into "metatiles" to + create the illusion of larger sprites. This was likely done to save on memory + costs as RAM was expensive in the '80s, but since we're running on an FPGA + cost is irrelevant. +- Single 1024 sprite tilemap shared between foreground and background sprites + + The NES OAM registers contain a bit to select which tilemap to use (of two), + which effectively expands each tile's index address by one byte. Instead of + creating the illusion of two separate memory areas for tiles, having one + large tilemap seems like a more sensible solution to indexed tiles. +- 8 total palettes, with 8 colors each + + More colors is better. Increasing the total palette count is a very memory + intensive operation, while increaing the palette color count is likely slower + when looking up color values for each pixel on real hardware. +- Sprites can be positioned paritally off-screen on all screen edges using only + the offset bits in the FAM register + + The NES has a separate PPUMASK register to control special color effects, and + to shift sprites off the left and top screen edges, as the sprite offsets + count from 0. Our PPU's FAM sprite offset bits count from -15, so the sprite + can shift past the top and left screen edges, as well as the standard bottom + and right edges. +- No status line register, only V-sync and H-sync outputs are supplied back to + CPU + + The NES status line register contains some handy lines, such as a buggy + status line for reaching the max sprite count per scanline, and a status line + for detecting collisions between background and foreground sprites. Our PPU + doesn't have a scanline limit, and all hitbox detection is done in software. + Software hacks involving swapping tiles during a screen draw cycle can still + be achieved by counting the V-sync and H-sync pulses using interrupts. +- No background scrolling splits + + This feature allows only part of the background canvas to be scrolled, while + another portion stays still. This was used to draw HUD elements on the + background layer for displaying things like health bars or score counters. + Since we are working with a higher foreground sprite limit, we'll use regular + foreground sprites to display HUD elements. +- Sprites are always drawn on top of the background layer + + Our game doesn't need this capability for any visual effects. Leaving this + feature out will lead to a simpler hardware design + +### Hardware design schematics + + + + + [nesppuspecs]: https://www.copetti.org/writings/consoles/nes/ [nesppudocs]: https://www.nesdev.org/wiki/PPU_programmer_reference [nesppupinout]: https://www.nesdev.org/wiki/PPU_pinout |
