gb-emu/src/processor/mod.rs

use std::mem::transmute;

use crate::{verbose_println, Memory};

mod instructions;
mod opcodes;

#[derive(PartialEq)]
pub(crate) enum FLAGS {
    Zero = 7,
    NSubtract = 6,
    HalfCarry = 5,
    Carry = 4,
}

pub(crate) enum Direction {
    Left,
    Right,
}

pub struct CPU {
    pub memory: Memory,
    pub reg: Registers,
    pub last_instruction: u8,
    pub last_instruction_addr: u16,
}

impl CPU {
    pub fn exec_next(&mut self) {
        self.last_instruction_addr = self.reg.pc;
        let opcode = self.next_opcode();
        self.last_instruction = opcode;
        verbose_println!(
            "exec {:#4X} from pc: {:#X}",
            opcode,
            self.last_instruction_addr
        );
        self.run_opcode(opcode);
    }

    fn next_opcode(&mut self) -> u8 {
        let opcode = self.memory.get(self.reg.pc);
        self.reg.pc = self.reg.pc.wrapping_add(0x1);
        return opcode;
    }
}

pub enum Reg8 {
    A,
    F,
    B,
    C,
    D,
    E,
    H,
    L,
}

#[derive(Clone, Copy)]
pub struct Registers {
    pub af: u16,
    pub bc: u16,
    pub de: u16,
    pub hl: u16,
    pub sp: u16,
    pub pc: u16,
}

impl Default for Registers {
    fn default() -> Self {
        // default post-bootrom values
        Self {
            af: 0x00B0,
            // af: 0x01B0,
            bc: 0x0013,
            de: 0x00D8,
            hl: 0x014D,
            sp: 0xFFFE,
            pc: 0x0100,
        }
    }
}

impl Registers {
    fn get_8(&self, register: Reg8) -> u8 {
        match register {
            Reg8::A => self.af.get_high(),
            Reg8::F => self.af.get_low(),
            Reg8::B => self.bc.get_high(),
            Reg8::C => self.bc.get_low(),
            Reg8::D => self.de.get_high(),
            Reg8::E => self.de.get_low(),
            Reg8::H => self.hl.get_high(),
            Reg8::L => self.hl.get_low(),
        }
    }
    fn set_8(&mut self, register: Reg8, val: u8) {
        match register {
            Reg8::A => self.af.set_high(val),
            Reg8::F => self.af.set_low(val),
            Reg8::B => self.bc.set_high(val),
            Reg8::C => self.bc.set_low(val),
            Reg8::D => self.de.set_high(val),
            Reg8::E => self.de.set_low(val),
            Reg8::H => self.hl.set_high(val),
            Reg8::L => self.hl.set_low(val),
        }
    }
}

trait SplitRegister {
    fn get_low(&self) -> u8;
    fn get_high(&self) -> u8;
    fn set_low(&mut self, val: u8);
    fn set_high(&mut self, val: u8);
}

impl SplitRegister for u16 {
    fn get_low(&self) -> u8 {
        (*self & 0xFF) as u8
    }

    fn get_high(&self) -> u8 {
        ((*self >> 8) & 0xFF) as u8
    }

    fn set_low(&mut self, val: u8) {
        *self = (*self & !0xff) | val as u16;
    }

    fn set_high(&mut self, val: u8) {
        *self = (*self & !0xff00) | (val as u16) << 8;
    }
}

fn as_signed(unsigned: u8) -> i8 {
    unsafe {
        return transmute(unsigned);
    }
}

fn get_bit(byte: u8, flag: u8) -> bool {
    let mask = 1 << flag;
    let got = byte & mask;
    return got > 0x0;
}

fn rotate(byte: u8, direction: &Direction) -> (u8, bool) {
    match direction {
        Direction::Left => {
            let carry = get_bit(byte, 7);
            let r = byte << 1;
            return (r, carry);
        }
        Direction::Right => {
            let carry = get_bit(byte, 0);
            let r = byte >> 1;
            return (r, carry);
        }
    }
}

fn get_rotation_carry(direction: &Direction) -> u8 {
    match direction {
        Direction::Left => 0b1,
        Direction::Right => 0b10000000,
    }
}

fn swap_nibbles(byte: u8) -> u8 {
    (byte & 0x0F) << 4 | (byte & 0xF0) >> 4
}

fn res(byte: u8, bit: u8) -> u8 {
    byte & !(1 << bit)
}

fn set(byte: u8, bit: u8) -> u8 {
    byte | (1 << bit)
}