gb-emu/src/main.rs

#![feature(exclusive_range_pattern)]

use clap::Parser;
use std::{fs, io};

/// Simple program to greet a person
#[derive(Parser, Debug)]
#[command(author, version, about, long_about = None)]
struct Args {
    /// ROM path
    #[arg(short, long)]
    rom: String,

    /// BootROM path
    #[arg(short, long)]
    bootrom: String,
}

type Address = u16;
type ROM = Vec<u8>;

#[derive(Clone, Copy)]
struct Inner {
    left: u8,
    right: u8,
}

union Register {
    as_u8s: Inner,
    as_u16: u16,
}

struct Memory {
    rom: ROM,
    vram: [u8; 8192],
    ram: [u8; 8192],
}

impl Memory {
    fn init(rom: ROM) -> Self {
        Self {
            rom,
            vram: [0x0; 8192],
            ram: [0x0; 8192],
        }
    }
    fn get(&self, address: Address) -> u8 {
        match address {
            0x0..0x8000 => {
                // rom access
                // todo - switchable rom banks
                return self.rom[address as usize];
            }
            0x8000..0xA000 => {
                return self.vram[(address - 0x8000) as usize];
            }
            0xA000..0xC000 => {
                panic!("switchable ram bank");
            }
            0xC000..0xE000 => {
                return self.ram[(address - 0xC000) as usize];
            }
            0xE000..0xFE00 => {
                return self.ram[(address - 0xE000) as usize];
            }
            0xFE00..0xFEA0 => {
                panic!("sprite attrib memory");
            }
            0xFEA0..0xFF00 => {
                panic!("empty")
            }
            0xFF00..0xFF4C => {
                panic!("I/O");
            }
            0xFF4C..0xFF80 => {
                panic!("empty");
            }
            0xFF80..0xFFFF => {
                panic!("internal ram");
            }
            0xFFFF => {
                panic!("interrupt enable register");
            }
        }
    }
}

struct State {
    af: Register,
    bc: Register,
    de: Register,
    hl: Register,
    sp: Register,
    pc: Register,
}

impl Default for State {
    fn default() -> Self {
        // default post-bootrom values
        Self {
            af: Register { as_u16: 0x01B0 },
            bc: Register { as_u16: 0x0013 },
            de: Register { as_u16: 0x00D8 },
            hl: Register { as_u16: 0x014D },
            sp: Register { as_u16: 0xFFFE },
            pc: Register { as_u16: 0x0100 },
        }
    }
}

struct CPU {
    memory: Memory,
    state: State,
}

impl CPU {
    fn exec_next(&mut self) -> u8 {
        let opcode = self.next_opcode();
        println!("opcode: {:#X}", opcode);

        match opcode {
            0x0 => {
                // noop
            }
            0x01 => {
                self.state.bc.as_u8s.left = self.next_opcode();
                self.state.bc.as_u8s.right = self.next_opcode();
            }
            0x11 => {
                self.state.de.as_u8s.left = self.next_opcode();
                self.state.de.as_u8s.right = self.next_opcode();
            }
            0x21 => {
                self.state.de.as_u8s.left = self.next_opcode();
                self.state.de.as_u8s.right = self.next_opcode();
            }
            0x2C => {
                unsafe {
                    self.state.hl.as_u8s.right += 1;
                };
            }
            0x66 => {
                unsafe {
                    self.state.hl.as_u8s.left = self.memory.get(self.state.hl.as_u16);
                };
            }
            0xC3 => {
                let (a, b) = (self.next_opcode(), self.next_opcode());
                self.state.pc.as_u8s.left = a;
                self.state.pc.as_u8s.right = b;
            }
            _ => {
                panic!("unimplemented opcode: {:#X}", opcode);
            }
        };
        return opcode;
    }
    fn next_opcode(&mut self) -> u8 {
        unsafe {
            let opcode = self.memory.get(self.state.pc.as_u16);
            self.state.pc.as_u16 += 0x1;
            return opcode;
        };
    }
}

fn main() {
    let args = Args::parse();

    let rom: ROM = fs::read(args.rom).expect("Could not load ROM");
    let _bootrom: ROM = fs::read(args.bootrom).expect("Could not load BootROM");
    let mut cpu = CPU {
        memory: Memory::init(rom),
        state: State::default(),
    };
    loop {
        cpu.exec_next();
        pause();
    }
}

fn pause() {
    io::stdin().read_line(&mut String::new()).unwrap();
}