gb-emu/src/processor.rs

use crate::{Inner, Memory, Register, State};

#[allow(dead_code)]
enum FLAGS {
    Z = 7,
    N = 6,
    H = 5,
    C = 4,
}

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

impl CPU {
    pub fn exec_next(&mut self) -> u8 {
        let opcode = self.next_opcode();
        match opcode {
            0x0 => {
                // noop
            }
            0x01 => self.state.bc = self.ld_immediate_word(),
            0x02 => unsafe {
                let address = self.state.bc.as_u16;
                self.memory.set(address, self.state.af.as_u8s.left);
            },
            0x03 => unsafe { self.state.bc.as_u16 += 1 },
            0x04 => unsafe { self.state.bc.as_u8s.left += 1 },
            0x05 => unsafe { self.state.bc.as_u8s.left -= 1 },
            0x06 => self.state.bc.as_u8s.left = self.ld_immediate_byte(),
            0x07 => panic!("RCLA instruction: 0x07"),
            0x08 => unsafe {
                let address = self.ld_immediate_word().as_u16;
                let word = self.state.sp;
                self.store_word(address, word);
            },
            0x09 => unsafe { self.state.hl.as_u16 += self.state.bc.as_u16 },
            0x0A => unsafe { self.state.af.as_u8s.left = self.memory.get(self.state.bc.as_u16) },
            0x0B => unsafe { self.state.bc.as_u16 -= 0x1 },
            0x0C => unsafe { self.state.bc.as_u8s.right += 0x1 },
            0x0D => unsafe { self.state.bc.as_u8s.right -= 0x1 },
            0x0E => self.state.bc.as_u8s.right = self.ld_immediate_byte(),
            0x0F => panic!("RRCA instruction: 0x0F"),
            0x10 => panic!("STOP instruction"),
            0x11 => self.state.de = self.ld_immediate_word(),
            0x12 => unsafe {
                let address = self.state.de.as_u16;
                let data = self.state.af.as_u8s.left;
                self.memory.set(address, data);
            },
            0x13 => unsafe { self.state.de.as_u16 -= 0x1 },
            0x14 => unsafe { self.state.de.as_u8s.left += 0x1 },
            0x15 => unsafe { self.state.de.as_u8s.left -= 0x1 },
            0x16 => self.state.de.as_u8s.left = self.ld_immediate_byte(),
            0x17 => panic!("RLA instruction: 0x17"),
            0x18 => unsafe { self.state.pc.as_u16 += self.ld_immediate_byte() as u16 },
            0x19 => unsafe { self.state.hl.as_u16 += self.state.de.as_u16 },
            0x1A => unsafe { self.state.af.as_u8s.left = self.memory.get(self.state.de.as_u16) },
            0x1B => unsafe { self.state.de.as_u16 -= 1 },
            0x1C => unsafe { self.state.de.as_u8s.right += 1 },
            0x1D => unsafe { self.state.de.as_u8s.right -= 1 },
            0x1E => self.state.de.as_u8s.right = self.ld_immediate_byte(),
            0x1F => panic!("RRA instruction: 0x1F"),
            0x20 => {
                let jump_size = self.ld_immediate_byte();
                if self.get_flag(FLAGS::Z) == 0 {
                    unsafe { self.state.pc.as_u16 += jump_size as u16 }
                }
            }
            0x21 => self.state.hl = self.ld_immediate_word(),
            0x22 => unsafe {
                self.memory
                    .set(self.state.hl.as_u16, self.state.af.as_u8s.left);
                self.state.hl.as_u16 += 1;
            },
            0x23 => unsafe { self.state.hl.as_u16 += 1 },
            0x2C => {
                unsafe {
                    self.state.hl.as_u8s.right += 1;
                };
            }
            0x3E => {
                self.state.af.as_u8s.left = self.ld_immediate_byte();
            }
            0x4A => {
                unsafe {
                    self.state.bc.as_u8s.right = self.state.de.as_u8s.left;
                };
            }
            0x4B => {
                unsafe {
                    self.state.bc.as_u8s.right = self.state.de.as_u8s.right;
                };
            }
            0x53 => {
                unsafe {
                    self.state.de.as_u8s.left = self.state.de.as_u8s.right;
                };
            }
            0x66 => {
                unsafe {
                    self.state.hl.as_u8s.left = self.memory.get(self.state.hl.as_u16);
                };
            }
            0xC3 => {
                self.state.pc = self.ld_immediate_word();
            }
            0xEA => {
                unsafe {
                    let address = self.ld_immediate_word().as_u16;
                    self.memory.set(address, self.state.af.as_u8s.left);
                };
            }
            _ => {
                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 store_word(&mut self, address: u16, word: Register) {
        unsafe {
            self.memory.set(address, word.as_u8s.left);
            self.memory.set(address + 1, word.as_u8s.right);
        };
    }

    fn ld_immediate_word(&mut self) -> Register {
        Register {
            as_u8s: Inner {
                left: self.next_opcode(),
                right: self.next_opcode(),
            },
        }
    }

    fn ld_immediate_byte(&mut self) -> u8 {
        self.next_opcode()
    }

    fn get_flag(&mut self, flag: FLAGS) -> u8 {
        unsafe {
            return self.state.af.as_u8s.right & (1 << flag as u8);
        }
    }
}