gb-emu/src/processor/memory/rom/mbcs.rs

use crate::processor::memory::Address;

pub(super) trait Mbc {
    fn get(&self, address: Address) -> u8;
    fn get_ram(&self, address: Address) -> u8;
    fn set(&mut self, address: Address, data: u8);
    fn set_ram(&mut self, address: Address, data: u8);
    fn mbc_type(&self) -> &str;
}

pub(super) struct None {
    pub(super) data: Vec<u8>,
}

impl None {
    pub(super) fn init(data: Vec<u8>) -> Self {
        Self { data }
    }
}

impl Mbc for None {
    fn get(&self, address: Address) -> u8 {
        self.data[address as usize]
    }

    fn get_ram(&self, _address: Address) -> u8 {
        0xFF
    }

    fn set_ram(&mut self, _address: Address, _data: u8) {}

    fn set(&mut self, _address: Address, _data: u8) {}

    fn mbc_type(&self) -> &str {
        "None"
    }
}

#[derive(Clone, Copy)]
enum BankingMode {
    Simple,
    Advanced,
}

pub(super) struct Mbc1 {
    pub(super) data: Vec<u8>,
    rom_len: usize,
    rom_bank: u8,
    ram_enabled: bool,
    ram: Option<Vec<u8>>,
    ram_bank: u8,
    upper_banks: u8,
    bank_mode: BankingMode,
}

const KB: usize = 1024;
const ROM_BANK_SIZE: usize = 16 * KB;
const RAM_BANK_SIZE: usize = 8 * KB;

impl Mbc1 {
    pub(super) fn init(
        data: Vec<u8>,
        rom_size: u8,
        ram_size: u8,
        _save_file: Option<Vec<u8>>,
    ) -> Self {
        let rom_len = match rom_size {
            0x00 => 2,
            0x01 => 4,
            0x02 => 8,
            0x03 => 16,
            0x04 => 32,
            0x05 => 64,
            0x06 => 128,
            0x07 => 256,
            0x08 => 512,
            0x52 => 72,
            0x53 => 80,
            0x54 => 96,
            _ => panic!("unacceptable rom size"),
        } * ROM_BANK_SIZE;
        // in kb
        let ram = match ram_size {
            0x00 => None,
            0x01 => Some(vec![0; 2 * KB]),
            0x02 => Some(vec![0; 8 * KB]),
            0x03 => Some(vec![0; 32 * KB]),
            0x04 => Some(vec![0; 128 * KB]),
            0x05 => Some(vec![0; 64 * KB]),
            _ => panic!("unacceptable ram size"),
        };
        Self {
            data,
            rom_len,
            rom_bank: 0x1,
            ram_enabled: false,
            ram,
            ram_bank: 0,
            upper_banks: 0,
            bank_mode: BankingMode::Simple,
        }
    }
}

impl Mbc for Mbc1 {
    fn get(&self, address: Address) -> u8 {
        self.data[self.get_rom_addr(address)]
    }

    fn get_ram(&self, address: Address) -> u8 {
        if self.ram_enabled && let Some(ram) = &self.ram {
            let addr = self.get_ram_addr(address)%ram.len();
            return ram[addr];
        }
        0xFF
    }

    fn set_ram(&mut self, address: Address, data: u8) {
        let mut addr = self.get_ram_addr(address);
        if self.ram_enabled && let Some(ram) = &mut self.ram {
            addr %= ram.len();
            ram[addr] = data;
        }
    }

    fn set(&mut self, address: Address, data: u8) {
        match address {
            0x0..0x2000 => {
                // enable/disable ram
                self.ram_enabled = (data & 0x0F) == 0xA;
            }
            0x2000..0x4000 => {
                // rom bank number - lower 5 bits
                let mut set_data = data & 0b00011111;
                if set_data == 0 {
                    set_data = 1;
                }
                self.rom_bank = set_data;
            }
            0x4000..0x6000 => {
                // ram bank OR upper bits 5 & 6 of rom bank
                self.upper_banks = data & 0b11;
            }
            0x6000..0x8000 => {
                // mode select
                self.bank_mode = if data == 0x1 {
                    BankingMode::Advanced
                } else if data == 0x0 {
                    BankingMode::Simple
                } else {
                    self.bank_mode
                };
            }
            _ => {}
        }
    }

    fn mbc_type(&self) -> &str {
        "MBC1"
    }
}

impl Mbc1 {
    fn get_rom_addr(&self, address: Address) -> usize {
        (match address {
            0x0..0x4000 => match self.bank_mode {
                BankingMode::Simple => address as usize,
                BankingMode::Advanced => {
                    (address as usize) + (self.upper_banks as usize * 512 * KB)
                }
            },
            0x4000..0x8000 => {
                (address - 0x4000) as usize
                    + (ROM_BANK_SIZE * self.rom_bank as usize)
                    + (self.upper_banks as usize * 512 * KB)
            }

            0xA000..0xC000 => panic!("passed ram address to rom address function"),
            _ => panic!("address {address} incompatible with MBC1"),
        } % self.rom_len)
    }

    fn get_ram_addr(&self, address: Address) -> usize {
        match address {
            0x0..0x8000 => panic!("passed rom address to ram address function"),
            0xA000..0xC000 => match self.bank_mode {
                BankingMode::Simple => {
                    (address - 0xA000) as usize + (RAM_BANK_SIZE * self.ram_bank as usize)
                }
                BankingMode::Advanced => {
                    (address - 0xA000) as usize
                        + (RAM_BANK_SIZE * self.ram_bank as usize)
                        + (self.upper_banks as usize * 16 * KB)
                }
            },
            _ => panic!("address {address} incompatible with MBC1"),
        }
    }
}