rom-emulator/bus.rs

use crate::{cartridge::Rom, cpu::Mem};
const RAM: u16 = 0x0000;
const RAM_MIRRORS_END: u16 = 0x1FFF;
const PPU_REGISTERS: u16 = 0x2000;
const PPU_REGISTERS_MIRRORS_END: u16 = 0x3FFF;

pub struct Bus {
    cpu_vram: [u8; 2048],
    rom: Rom,
}

impl Bus {
    pub fn new(rom: Rom) -> Self {
        Bus {
            cpu_vram: [0; 2048],
            rom,
        }
    }
    fn read_prg_rom(&self, mut addr: u16) -> u8 {
        addr -= 0x8000;
        if self.rom.prg_rom.len() == 0x4000 && addr >= 0x4000 {
            addr = addr % 0x4000;
        }
        self.rom.prg_rom[addr as usize]
    }
}

impl Mem for Bus {
    fn mem_read(&self, addr: u16) -> u8 {
        match addr {
            RAM..=RAM_MIRRORS_END => {
                let mirror_down_addr = addr & 0b00000111_11111111;
                self.cpu_vram[mirror_down_addr as usize]
            }
            PPU_REGISTERS..=PPU_REGISTERS_MIRRORS_END => {
                let _mirror_down_addr = addr & 0b00100000_00000111;
                // todo!("PPU is not supported yet")
                0
            }
            0x8000..=0xFFFF => self.read_prg_rom(addr),

            _ => {
                println!("Ignoring mem access at {}", addr);
                0
            }
        }
    }

    fn mem_write(&mut self, addr: u16, data: u8) {
        match addr {
            RAM..=RAM_MIRRORS_END => {
                let mirror_down_addr = addr & 0b11111111111;
                self.cpu_vram[mirror_down_addr as usize] = data;
            }
            PPU_REGISTERS..=PPU_REGISTERS_MIRRORS_END => {
                let _mirror_down_addr = addr & 0b00100000_00000111;
            }
            0x8000..=0xFFFF => {
                panic!("Attempt to write to ROM space")
            }
            _ => {
                println!("Ignoring mem write-access at {}", addr);
            }
        }
    }
}

#[cfg(test)]
mod test {
    use super::*;
    use crate::cartridge::test;

    #[test]
    fn test_mem_read_write_to_ram() {
        let mut bus = Bus::new(test::test_rom(vec![]));
        bus.mem_write(0x01, 0x55);
        assert_eq!(bus.mem_read(0x01), 0x55);
    }
}