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NES Library |
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2022-09-22 |
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Last edit: 2022-09-22 |
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--------------------- |
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I wanted to learn more about how the NES works, so I consulted the resources on |
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Nesdev Wiki (https://www.nesdev.org) |
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and I discovered that from an NES program (`.nes` file) you could do cool stuff like extracting some tilesets from the CHR ROM (Character Read-Only Memory) or a PRNG (Pseudorandom Number Generator) with the instructions interpreted by the NES. |
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## CLI |
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I made an example binary that can run all the main features of the `nes-utils` library to make it more convenient to test. |
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https://github.com/theobori/nes-utils-cli (https://github.com) |
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## Tilesets dumping |
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A NES program include a 16 bytes header, we can represent it like this: |
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```rust |
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const NES_HEADER_FIELDS_ORDER: [(&str, usize, usize); 9] = { |
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[ |
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("magic", 0, 4), |
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("len_prg_rom", 4, 1), |
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("len_chr_rom", 5, 1), |
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("f6", 6, 1), |
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("f7", 7, 1), |
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("len_prg_ram", 8, 1), |
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("f9", 9, 1), |
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("f10", 10, 1), |
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("reserved", 11, 5) |
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] |
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}; |
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``` |
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In this example, the field magic is at position 0 and has a lenght of 4 bytes. |
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The CHR ROM is linked to the PPU (Picture Process Unit), it means if the CHR ROM length is superior to zero, it contains some graphics. |
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There are not all the tilesets of the game in the CHR, you can find `0x2000 * len_chr_rom` bytes in it, with two banks of `0x1000` bytes, which makes two images of 8 kilobytes, there is one bank for one image. |
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So, let's take the example of `Kirby's Adventure`, below are the graphical data of the CHR rom: |
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There are only four colors because the rest is calculated at runtime. Below are the main parts of code that generate this 2 images: |
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### Colors |
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```rust |
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type Rgb = (u8, u8, u8); |
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const BLACK_PIXEL: Rgb = (0, 0, 0); |
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const COLOR_SCHEME: [Rgb; 4] = [ |
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(0, 0, 0), |
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(126, 126, 126), |
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(189, 189, 189), |
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(255, 255, 255) |
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]; |
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fn bits_to_rgb(left: u8, right: u8) -> Rgb { |
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let color = right << 1 | left; |
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COLOR_SCHEME[color as usize] |
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} |
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``` |
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### Bytes to RGB |
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```rust |
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pub fn fill_with_bank(&mut self, bank: &[u8]) { |
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let mut mem_x = 0; |
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let mut mem_y = 0; |
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for byte in (0..bank.len()).step_by(16) { |
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for y in 0..8 { |
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if mem_x >= NesImage::W { |
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mem_y += NesImage::TILE_H; |
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mem_x = 0; |
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} |
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let lower = bank[byte + y]; |
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let upper = bank[byte + y + 8]; |
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for bit in 0..8 { |
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let pixel = bits_to_rgb( |
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lower >> (7 - bit) & 1, |
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upper >> (7 - bit) & 1 |
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); |
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self.put_pixel(bit + mem_x, y + mem_y, pixel); |
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} |
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} |
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mem_x += NesImage::TILE_W; |
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} |
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} |
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``` |
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## Disassembling |
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The bytecodes are in the PGR ROM (Program Read-Only Memory) of size `0x4000 * len_chr_rom` (value in the header) bytes. |
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So, for `Kirby's Adventure`, the assembler code should looks like: |
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```asm6502 |
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; Mapped registers |
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SQ1_VOL equ $4000 |
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SQ1_SWEEP equ $4001 |
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PPUMASK equ $2001 |
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SQ1_LO equ $4002 |
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SQ1_HI equ $4003 |
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SQ2_SWEEP equ $4005 |
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... |
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; Header |
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hex 4e 45 53 1a |
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hex 20 |
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hex 20 |
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hex 43 |
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hex 00 |
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hex 00 |
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hex 00 |
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hex 00 |
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hex 00 00 00 00 00 |
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; PRG ROM |
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and ($0f, x) ; 21 0f |
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slo $280f ; 0f 0f 28 |
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slo $2121 ; 0f 21 21 |
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jsr $2020 ; 20 20 20 |
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jsr $0221 ; 20 21 02 |
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slo $200f ; 0f 0f 20 |
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and ($0f, x) ; 21 0f |
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... |
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``` |
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## Game Genie code |
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- *The Game Genie is a enhancement cart for the NES designed by Camerica and distributed by Galoob and Cameric* |
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- *it provides a simple interface to enter up to three cheat codes* |
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*Source: |
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NesDev Wiki (https://www.nesdev.org) |
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* |
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Obviously I didn't buy the enhancement cart, so I can't use the available codes. But on some emulators it is possible to access the memory and inject values. |
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So I made a feature that decodes the Game Genie codes. |
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For example for `Kirby's Adventure`, it is possible to have infinite energy with the following code: |
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```text |
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$ nes-utils-cli --code SZEPSVSE |
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Address 0x1e05 |
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Value 0xad |
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Compare value 0x8d |
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``` |
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## Links |
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https://github.com/theobori/nes-utils (https://github.com) |
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