cpu.rs

pub const RAM_SIZE: usize = 8192;

pub const INSTRUCTIONS: [(fn(&mut Cpu), u8, &'static str); 11] = [
    // 0x0 opcodes
    (nop, 4, "NOP"),
    (ld_bc_u16, 12, "LD BC, NN"),
    (ld_mbc_a, 8, "LD [BC], A"),
    (inc_bc, 8, "INC BC"),
    (inc_b, 4, "INC B"),
    (dec_b, 4, "DEC B"),
    (ld_b_u8, 8, "LD B, N"),
    (rlca, 4, "RLCA"),
    (ld_mu16_sp, 20, "LD [u16], SP"),
    (add_hl_bc, 8, "ADD HL, BC"),
    // 0x2 opcodes
    (ld_sp_u16, 12, "LD SP, NN"),
];

pub struct Cpu {
    pc: u16,
    sp: u16,
    a: u8,
    b: u8,
    c: u8,
    d: u8,
    e: u8,
    h: u8,
    l: u8,
    zero: bool,
    sub: bool,
    half_carry: bool,
    carry: bool,
    ram: [u8; RAM_SIZE],
}

impl Cpu {
    pub fn new() -> Cpu {
        Cpu {
            pc: 0x0,
            sp: 0x0,
            a: 0x0,
            b: 0x0,
            c: 0x0,
            d: 0x0,
            e: 0x0,
            h: 0x0,
            l: 0x0,
            zero: false,
            sub: false,
            half_carry: false,
            carry: false,
            ram: [0u8; RAM_SIZE],
        }
    }

    pub fn clock(&mut self) {
        // fetches the current instruction and increments
        // the PC (program counter) accordingly
        let _instruction = self.ram[self.pc as usize];
        self.pc += 1;
    }

    #[inline(always)]
    fn af(&self) -> u16 {
        (self.a as u16) << 8 | self.f() as u16
    }

    #[inline(always)]
    fn bc(&self) -> u16 {
        (self.b as u16) << 8 | self.c as u16
    }

    #[inline(always)]
    fn f(&self) -> u8 {
        let mut f = 0x0u8;
        if self.zero {
            f |= 0x80;
        }
        if self.sub {
            f |= 0x40;
        }
        if self.half_carry {
            f |= 0x20;
        }
        if self.carry {
            f |= 0x10;
        }
        f
    }

    #[inline(always)]
    fn set_bc(&mut self, value: u16) {
        self.b = (value >> 8) as u8;
        self.c = value as u8;
    }

    #[inline(always)]
    fn de(&self) -> u16 {
        (self.d as u16) << 8 | self.e as u16
    }

    #[inline(always)]
    fn set_de(&mut self, value: u16) {
        self.d = (value >> 8) as u8;
        self.e = value as u8;
    }

    #[inline(always)]
    fn hl(&self) -> u16 {
        (self.h as u16) << 8 | self.l as u16
    }

    #[inline(always)]
    fn set_hl(&mut self, value: u16) {
        self.h = (value >> 8) as u8;
        self.l = value as u8;
    }

    #[inline(always)]
    fn read_u8(&mut self) -> u8 {
        let byte = self.ram[self.pc as usize];
        self.pc = self.pc.wrapping_add(1);
        byte
    }

    #[inline(always)]
    fn read_u16(&mut self) -> u16 {
        let byte1 = self.read_u8();
        let byte2 = self.read_u8();
        let word = byte1 as u16 | ((byte2 as u16) << 8);
        word
    }

    #[inline(always)]
    fn get_zero(&self) -> bool {
        self.zero
    }

    #[inline(always)]
    fn set_zero(&mut self, value: bool) {
        self.zero = value
    }

    #[inline(always)]
    fn get_sub(&self) -> bool {
        self.sub
    }

    #[inline(always)]
    fn set_sub(&mut self, value: bool) {
        self.sub = value;
    }

    #[inline(always)]
    fn get_half_carry(&self) -> bool {
        self.half_carry
    }

    #[inline(always)]
    fn set_half_carry(&mut self, value: bool) {
        self.half_carry = value
    }

    #[inline(always)]
    fn get_carry(&self) -> bool {
        self.carry
    }

    #[inline(always)]
    fn set_carry(&mut self, value: bool) {
        self.carry = value;
    }
}

fn nop(_cpu: &mut Cpu) {}

fn ld_bc_u16(cpu: &mut Cpu) {
    let word = cpu.read_u16();
    cpu.set_bc(word);
}

fn ld_mbc_a(cpu: &mut Cpu) {
    cpu.ram[cpu.bc() as usize] = cpu.a;
}

fn inc_bc(cpu: &mut Cpu) {
    cpu.set_bc(cpu.bc().wrapping_add(1));
}

fn inc_b(cpu: &mut Cpu) {
    let value = cpu.b.wrapping_add(1);

    cpu.set_sub(false);
    cpu.set_zero(value == 0);
    cpu.set_half_carry((value & 0xf) == 0xf);

    cpu.b = value;
}

fn dec_b(cpu: &mut Cpu) {
    let value = cpu.b.wrapping_sub(1);

    cpu.set_sub(true);
    cpu.set_zero(value == 0);
    cpu.set_half_carry((value & 0xf) == 0xf);

    cpu.b = value;
}

fn ld_b_u8(cpu: &mut Cpu) {
    let byte = cpu.read_u8();
    cpu.b = byte;
}

fn rlca(cpu: &mut Cpu) {
    let carry = cpu.a >> 7;

    cpu.a = cpu.a << 1 | carry;

    cpu.set_sub(false);
    cpu.set_zero(false);
    cpu.set_half_carry(false);
    cpu.set_carry(carry == 1);
}

fn ld_mu16_sp(cpu: &mut Cpu) {
    let word = cpu.read_u16();
    cpu.ram[word as usize] = cpu.sp as u8;
    cpu.ram[word as usize + 1] = (cpu.sp >> 8) as u8;
}

fn add_hl_bc(cpu: &mut Cpu) {
    let value = add_u16_u16(cpu, cpu.hl(), cpu.bc());
    cpu.set_hl(value);
}

fn ld_sp_u16(cpu: &mut Cpu) {
    cpu.sp = cpu.read_u16();
}

fn add_u16_u16(cpu: &mut Cpu, first: u16, second: u16) -> u16 {
    let first = first as u32;
    let second = second as u32;
    let value = first.wrapping_add(second);

    cpu.set_sub(false);
    cpu.set_carry(value & 0x1000 == 0x1000);
    cpu.set_half_carry((first ^ second ^ value) & 0x1000 == 0x1000);

    value as u16
}