decode.rs

use crate::error::Error;
use crate::devices::{Address, Addressable};

use super::state::{M68kType, Exceptions};
use super::instructions::{
    Size,
    Sign,
    Direction,
    ShiftDirection,
    XRegister,
    BaseRegister,
    IndexRegister,
    RegOrImmediate,
    ControlRegister,
    Condition,
    Target,
    Instruction,
    sign_extend_to_long,
};


const OPCG_BIT_OPS: u8 = 0x0;
const OPCG_MOVE_BYTE: u8 = 0x1;
const OPCG_MOVE_LONG: u8 = 0x2;
const OPCG_MOVE_WORD: u8 = 0x3;
const OPCG_MISC: u8 = 0x04;
const OPCG_ADDQ_SUBQ: u8 = 0x5;
const OPCG_BRANCH: u8 = 0x6;
const OPCG_MOVEQ: u8 = 0x7;
const OPCG_DIV_OR: u8 = 0x8;
const OPCG_SUB: u8 = 0x9;
const OPCG_ALINE: u8 = 0xA;
const OPCG_CMP_EOR: u8 = 0xB;
const OPCG_MUL_AND: u8 = 0xC;
const OPCG_ADD: u8 = 0xD;
const OPCG_SHIFT: u8 = 0xE;
const OPCG_FLINE: u8 = 0xF;


#[derive(Clone)]
pub struct M68kDecoder {
    pub cputype: M68kType,
    pub start: u32,
    pub end: u32,
    pub instruction_word: u16,
    pub instruction: Instruction,
}

impl M68kDecoder {
    pub fn new(cputype: M68kType, start: u32) -> M68kDecoder {
        M68kDecoder {
            cputype,
            start: start,
            end: start,
            instruction_word: 0,
            instruction: Instruction::NOP,
        }
    }

    #[inline(always)]
    pub fn init(&mut self, start: u32) {
        self.start = start;
        self.end = start;
    }

    pub fn decode_at(&mut self, memory: &mut dyn Addressable, start: u32) -> Result<(), Error> {
        self.init(start);
        self.instruction = self.decode_one(memory)?;
        Ok(())
    }

    pub fn decode_one(&mut self, memory: &mut dyn Addressable) -> Result<Instruction, Error> {
        let ins = self.read_instruction_word(memory)?;
        self.instruction_word = ins;

        match ((ins & 0xF000) >> 12) as u8 {
            OPCG_BIT_OPS => {
                let optype = (ins & 0x0F00) >> 8;

                if (ins & 0x13F) == 0x03C {
                    match (ins & 0x00C0) >> 6 {
                        0b00 => {
                            let data = self.read_instruction_word(memory)?;
                            match optype {
                                0b0000 => Ok(Instruction::ORtoCCR(data as u8)),
                                0b0010 => Ok(Instruction::ANDtoCCR(data as u8)),
                                0b1010 => Ok(Instruction::EORtoCCR(data as u8)),
                                _ => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
                            }
                        },
                        0b01 => {
                            let data = self.read_instruction_word(memory)?;
                            match optype {
                                0b0000 => Ok(Instruction::ORtoSR(data)),
                                0b0010 => Ok(Instruction::ANDtoSR(data)),
                                0b1010 => Ok(Instruction::EORtoSR(data)),
                                _ => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
                            }
                        },
                        _ => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
                    }
                } else if (ins & 0x138) == 0x108 {
                    let dreg = get_high_reg(ins);
                    let areg = get_low_reg(ins);
                    let dir = if (ins & 0x0080) == 0 { Direction::FromTarget } else { Direction::ToTarget };
                    let size = if (ins & 0x0040) == 0 { Size::Word } else { Size::Long };
                    let offset = self.read_instruction_word(memory)? as i16;
                    Ok(Instruction::MOVEP(dreg, areg, offset, size, dir))
                } else if (ins & 0x0100) == 0x0100 || (ins & 0x0F00) == 0x0800 {
                    let bitnum = if (ins & 0x0100) == 0x0100 {
                        Target::DirectDReg(get_high_reg(ins))
                    } else {
                        Target::Immediate(self.read_instruction_word(memory)? as u32)
                    };

                    let target = self.decode_lower_effective_address(memory, ins, Some(Size::Byte))?;
                    let size = match target {
                        Target::DirectAReg(_) | Target::DirectDReg(_) => Size::Long,
                        _ => Size::Byte,
                    };

                    match (ins & 0x00C0) >> 6 {
                        0b00 => Ok(Instruction::BTST(bitnum, target, size)),
                        0b01 => Ok(Instruction::BCHG(bitnum, target, size)),
                        0b10 => Ok(Instruction::BCLR(bitnum, target, size)),
                        0b11 => Ok(Instruction::BSET(bitnum, target, size)),
                        _ => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
                    }
                } else {
                    let size = get_size(ins);
                    let data = match size {
                        Some(Size::Byte) => (self.read_instruction_word(memory)? as u32 & 0xFF),
                        Some(Size::Word) => self.read_instruction_word(memory)? as u32,
                        Some(Size::Long) => self.read_instruction_long(memory)?,
                        None => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
                    };
                    let target = self.decode_lower_effective_address(memory, ins, size)?;

                    match optype {
                        0b0000 => Ok(Instruction::OR(Target::Immediate(data), target, size.unwrap())),
                        0b0010 => Ok(Instruction::AND(Target::Immediate(data), target, size.unwrap())),
                        0b0100 => Ok(Instruction::SUB(Target::Immediate(data), target, size.unwrap())),
                        0b0110 => Ok(Instruction::ADD(Target::Immediate(data), target, size.unwrap())),
                        0b1010 => Ok(Instruction::EOR(Target::Immediate(data), target, size.unwrap())),
                        0b1100 => Ok(Instruction::CMP(Target::Immediate(data), target, size.unwrap())),
                        _ => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
                    }
                }
            },
            OPCG_MOVE_BYTE => {
                let src = self.decode_lower_effective_address(memory, ins, Some(Size::Byte))?;
                let dest = self.decode_upper_effective_address(memory, ins, Some(Size::Byte))?;
                Ok(Instruction::MOVE(src, dest, Size::Byte))
            },
            OPCG_MOVE_LONG => {
                let src = self.decode_lower_effective_address(memory, ins, Some(Size::Long))?;
                let dest = self.decode_upper_effective_address(memory, ins, Some(Size::Long))?;
                if let Target::DirectAReg(reg) = dest {
                    Ok(Instruction::MOVEA(src, reg, Size::Long))
                } else {
                    Ok(Instruction::MOVE(src, dest, Size::Long))
                }
            },
            OPCG_MOVE_WORD => {
                let src = self.decode_lower_effective_address(memory, ins, Some(Size::Word))?;
                let dest = self.decode_upper_effective_address(memory, ins, Some(Size::Word))?;
                if let Target::DirectAReg(reg) = dest {
                    Ok(Instruction::MOVEA(src, reg, Size::Word))
                } else {
                    Ok(Instruction::MOVE(src, dest, Size::Word))
                }
            },
            OPCG_MISC => {
                let ins_0f00 = ins & 0xF00;
                let ins_00f0 = ins & 0x0F0;

                if (ins & 0x180) == 0x180 {
                    if (ins & 0x040) == 0 {
                        let size = match get_size(ins) {
                            Some(Size::Word) => Size::Word,
                            Some(Size::Long) if self.cputype >= M68kType::MC68020 => Size::Long,
                            // On the 68000, long words in CHK are not supported, but the opcode maps to the word size instruction
                            Some(Size::Long) => Size::Word,
                            _ => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
                        };

                        let reg = get_high_reg(ins);
                        let target = self.decode_lower_effective_address(memory, ins, Some(size))?;
                        Ok(Instruction::CHK(target, reg, size))
                    } else {
                        let src = self.decode_lower_effective_address(memory, ins, None)?;
                        let dest = get_high_reg(ins);
                        Ok(Instruction::LEA(src, dest))
                    }
                } else if (ins & 0xB80) == 0x880 && (ins & 0x038) != 0 {
                    let size = if (ins & 0x0040) == 0 { Size::Word } else { Size::Long };
                    let data = self.read_instruction_word(memory)?;
                    let target = self.decode_lower_effective_address(memory, ins, None)?;
                    let dir = if (ins & 0x0400) == 0 { Direction::ToTarget } else { Direction::FromTarget };
                    Ok(Instruction::MOVEM(target, size, dir, data))
                } else if (ins & 0xF80) == 0xC00 && self.cputype >= M68kType::MC68020 {
                    let extension = self.read_instruction_word(memory)?;
                    let reg_h = if (extension & 0x0400) != 0 { Some(get_low_reg(ins)) } else { None };
                    let reg_l = ((extension & 0x7000) >> 12) as u8;
                    let target = self.decode_lower_effective_address(memory, ins, Some(Size::Long))?;
                    let sign = if (ins & 0x0800) == 0 { Sign::Unsigned } else { Sign::Signed };
                    match (ins & 0x040) == 0 {
                        true => Ok(Instruction::MULL(target, reg_h, reg_l, sign)),
                        false => Ok(Instruction::DIVL(target, reg_h, reg_l, sign)),
                    }
                } else if (ins & 0x800) == 0 {
                    let target = self.decode_lower_effective_address(memory, ins, Some(Size::Word))?;
                    match (ins & 0x0700) >> 8 {
                        0b000 => {
                            match get_size(ins) {
                                Some(size) => Ok(Instruction::NEGX(target, size)),
                                None => Ok(Instruction::MOVEfromSR(target)),
                            }
                        },
                        0b010 => {
                            match get_size(ins) {
                                Some(size) => Ok(Instruction::CLR(target, size)),
                                None if self.cputype >= M68kType::MC68010 => Ok(Instruction::MOVEfromCCR(target)),
                                None => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
                            }
                        },
                        0b100 => {
                            match get_size(ins) {
                                Some(size) => Ok(Instruction::NEG(target, size)),
                                None => Ok(Instruction::MOVEtoCCR(target)),
                            }
                        },
                        0b110 => {
                            match get_size(ins) {
                                Some(size) => Ok(Instruction::NOT(target, size)),
                                None => Ok(Instruction::MOVEtoSR(target)),
                            }
                        },
                        _ => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
                    }
                } else if ins_0f00 == 0x800 || ins_0f00 == 0x900 {
                    let opmode = (ins & 0x01C0) >> 6;
                    let mode = get_low_mode(ins);
                    match (opmode, mode) {
                        (0b000, 0b001) if self.cputype >= M68kType::MC68020 => {
                            let data = self.read_instruction_long(memory)? as i32;
                            Ok(Instruction::LINK(get_low_reg(ins), data))
                        },
                        (0b000, _) => {
                            let target = self.decode_lower_effective_address(memory, ins, Some(Size::Byte))?;
                            Ok(Instruction::NBCD(target))
                        },
                        (0b001, 0b000) => {
                            Ok(Instruction::SWAP(get_low_reg(ins)))
                        },
                        (0b001, 0b001) => {
                            Ok(Instruction::BKPT(get_low_reg(ins)))
                        },
                        (0b001, _) => {
                            let target = self.decode_lower_effective_address(memory, ins, None)?;
                            Ok(Instruction::PEA(target))
                        },
                        (0b010, 0b000) => {
                            Ok(Instruction::EXT(get_low_reg(ins), Size::Byte, Size::Word))
                        },
                        (0b011, 0b000) => {
                            Ok(Instruction::EXT(get_low_reg(ins), Size::Word, Size::Long))
                        },
                        (0b111, 0b000) => {
                            Ok(Instruction::EXT(get_low_reg(ins), Size::Byte, Size::Long))
                        },
                        _ => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
                    }
                } else if ins_0f00 == 0xA00 {
                    if (ins & 0x0FF) == 0xFC {
                        Ok(Instruction::ILLEGAL)
                    } else {
                        let target = self.decode_lower_effective_address(memory, ins, Some(Size::Word))?;
                        match get_size(ins) {
                            Some(size) => Ok(Instruction::TST(target, size)),
                            None => Ok(Instruction::TAS(target)),
                        }
                    }
                } else if ins_0f00 == 0xE00 {
                    if (ins & 0x80) == 0x80 {
                        let target = self.decode_lower_effective_address(memory, ins, None)?;
                        if (ins & 0b01000000) == 0 {
                            Ok(Instruction::JSR(target))
                        } else {
                            Ok(Instruction::JMP(target))
                        }
                    } else if ins_00f0 == 0x40 {
                        Ok(Instruction::TRAP((ins & 0x000F) as u8))
                    } else if ins_00f0 == 0x50 {
                        let reg = get_low_reg(ins);
                        if (ins & 0b1000) == 0 {
                            let data = (self.read_instruction_word(memory)? as i16) as i32;
                            Ok(Instruction::LINK(reg, data))
                        } else {
                            Ok(Instruction::UNLK(reg))
                        }
                    } else if ins_00f0 == 0x60 {
                        let reg = get_low_reg(ins);
                        let dir = if (ins & 0b1000) == 0 { Direction::FromTarget } else { Direction::ToTarget };
                        Ok(Instruction::MOVEUSP(Target::DirectAReg(reg), dir))
                    } else {
                        match ins & 0x00FF {
                            0x70 => Ok(Instruction::RESET),
                            0x71 => Ok(Instruction::NOP),
                            0x72 => {
                                let data = self.read_instruction_word(memory)?;
                                Ok(Instruction::STOP(data))
                            },
                            0x73 => Ok(Instruction::RTE),
                            0x74 if self.cputype >= M68kType::MC68010 => {
                                let offset = self.read_instruction_word(memory)? as i16;
                                Ok(Instruction::RTD(offset))
                            },
                            0x75 => Ok(Instruction::RTS),
                            0x76 => Ok(Instruction::TRAPV),
                            0x77 => Ok(Instruction::RTR),
                            0x7A | 0x7B if self.cputype >= M68kType::MC68010 => {
                                let dir = if ins & 0x01 == 0 { Direction::ToTarget } else { Direction::FromTarget };
                                let ins2 = self.read_instruction_word(memory)?;
                                let target = match ins2 & 0x8000 {
                                    0 => Target::DirectDReg(((ins2 & 0x7000) >> 12) as u8),
                                    _ => Target::DirectAReg(((ins2 & 0x7000) >> 12) as u8),
                                };
                                let creg = match ins2 & 0xFFF {
                                    0x801 => ControlRegister::VBR,
                                    _ => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
                                };
                                Ok(Instruction::MOVEC(target, creg, dir))
                            },
                            _ => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
                        }
                    }
                } else {
                    return Err(Error::processor(Exceptions::IllegalInstruction as u32));
                }
            },
            OPCG_ADDQ_SUBQ => {
                match get_size(ins) {
                    Some(size) => {
                        let target = self.decode_lower_effective_address(memory, ins, Some(size))?;
                        let mut data = ((ins & 0x0E00) >> 9) as u32;
                        if data == 0 {
                            data = 8;
                        }

                        if let Target::DirectAReg(reg) = target {
                            if (ins & 0x0100) == 0 {
                                Ok(Instruction::ADDA(Target::Immediate(data), reg, size))
                            } else {
                                Ok(Instruction::SUBA(Target::Immediate(data), reg, size))
                            }
                        } else {
                            if (ins & 0x0100) == 0 {
                                Ok(Instruction::ADD(Target::Immediate(data), target, size))
                            } else {
                                Ok(Instruction::SUB(Target::Immediate(data), target, size))
                            }
                        }
                    },
                    None => {
                        let mode = get_low_mode(ins);
                        let condition = get_condition(ins);

                        if mode == 0b001 {
                            let reg = get_low_reg(ins);
                            let disp = self.read_instruction_word(memory)? as i16;
                            Ok(Instruction::DBcc(condition, reg, disp))
                        } else {
                            let target = self.decode_lower_effective_address(memory, ins, Some(Size::Byte))?;
                            Ok(Instruction::Scc(condition, target))
                        }
                    },
                }
            },
            OPCG_BRANCH => {
                let mut disp = ((ins & 0xFF) as i8) as i32;
                if disp == 0 {
                    disp = (self.read_instruction_word(memory)? as i16) as i32;
                } else if disp == -1 && self.cputype >= M68kType::MC68020 {
                    disp = self.read_instruction_long(memory)? as i32;
                }
                let condition = get_condition(ins);
                match condition {
                    Condition::True => Ok(Instruction::BRA(disp)),
                    Condition::False => Ok(Instruction::BSR(disp)),
                    _ => Ok(Instruction::Bcc(condition, disp)),
                }
            },
            OPCG_MOVEQ => {
                if (ins & 0x0100) != 0 {
                    return Err(Error::processor(Exceptions::IllegalInstruction as u32));
                }
                let reg = get_high_reg(ins);
                let data = (ins & 0xFF) as u8;
                Ok(Instruction::MOVEQ(data, reg))
            },
            OPCG_DIV_OR => {
                let size = get_size(ins);

                if size.is_none() {
                    let sign = if (ins & 0x0100) == 0 { Sign::Unsigned } else { Sign::Signed };
                    let effective_addr = self.decode_lower_effective_address(memory, ins, Some(Size::Word))?;
                    Ok(Instruction::DIVW(effective_addr, get_high_reg(ins), sign))
                } else if (ins & 0x1F0) == 0x100 {
                    let regx = get_high_reg(ins);
                    let regy = get_low_reg(ins);

                    match (ins & 0x08) != 0 {
                        false => Ok(Instruction::SBCD(Target::DirectDReg(regy), Target::DirectDReg(regx))),
                        true => Ok(Instruction::SBCD(Target::IndirectARegDec(regy), Target::IndirectARegDec(regx))),
                    }
                } else {
                    let data_reg = Target::DirectDReg(get_high_reg(ins));
                    let effective_addr = self.decode_lower_effective_address(memory, ins, size)?;
                    let (from, to) = if (ins & 0x0100) == 0 { (effective_addr, data_reg) } else { (data_reg, effective_addr) };
                    Ok(Instruction::OR(from, to, size.unwrap()))
                }
            },
            OPCG_SUB => {
                let reg = get_high_reg(ins);
                let dir = (ins & 0x0100) >> 8;
                let size = get_size(ins);
                match size {
                    Some(size) => {
                        if (ins & 0b100110000) == 0b100000000 {
                            let src = get_low_reg(ins);
                            let dest = get_high_reg(ins);
                            match (ins & 0x08) == 0 {
                                true => Ok(Instruction::SUBX(Target::DirectDReg(src), Target::DirectDReg(dest), size)),
                                false => Ok(Instruction::SUBX(Target::IndirectARegDec(src), Target::IndirectARegDec(dest), size)),
                            }
                        } else {
                            let target = self.decode_lower_effective_address(memory, ins, Some(size))?;
                            if dir == 0 {
                                Ok(Instruction::SUB(target, Target::DirectDReg(reg), size))
                            } else {
                                Ok(Instruction::SUB(Target::DirectDReg(reg), target, size))
                            }
                        }
                    },
                    None => {
                        let size = if dir == 0 { Size::Word } else { Size::Long };
                        let target = self.decode_lower_effective_address(memory, ins, Some(size))?;
                        Ok(Instruction::SUBA(target, reg, size))
                    },
                }
            },
            OPCG_CMP_EOR => {
                let reg = get_high_reg(ins);
                let optype = (ins & 0x0100) >> 8;
                let size = get_size(ins);
                match (optype, size) {
                    (0b1, Some(size)) => {
                        if get_low_mode(ins) == 0b001 {
                            Ok(Instruction::CMP(Target::IndirectARegInc(get_low_reg(ins)), Target::IndirectARegInc(reg), size))
                        } else {
                            let target = self.decode_lower_effective_address(memory, ins, Some(size))?;
                            Ok(Instruction::EOR(Target::DirectDReg(reg), target, size))
                        }
                    },
                    (0b0, Some(size)) => {
                        let target = self.decode_lower_effective_address(memory, ins, Some(size))?;
                        Ok(Instruction::CMP(target, Target::DirectDReg(reg), size))
                    },
                    (_, None) => {
                        let size = if optype == 0 { Size::Word } else { Size::Long };
                        let target = self.decode_lower_effective_address(memory, ins, Some(size))?;
                        Ok(Instruction::CMPA(target, reg, size))
                    },
                    _ => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
                }
            },
            OPCG_MUL_AND => {
                let size = get_size(ins);

                if (ins & 0b0001_1111_0000) == 0b0001_0000_0000 {
                    let regx = get_high_reg(ins);
                    let regy = get_low_reg(ins);

                    match (ins & 0x08) != 0 {
                        false => Ok(Instruction::ABCD(Target::DirectDReg(regy), Target::DirectDReg(regx))),
                        true => Ok(Instruction::ABCD(Target::IndirectARegDec(regy), Target::IndirectARegDec(regx))),
                    }
                } else if (ins & 0b0001_0011_0000) == 0b0001_0000_0000 && !size.is_none() {
                    let regx = get_high_reg(ins);
                    let regy = get_low_reg(ins);
                    match (ins & 0x00F8) >> 3 {
                        0b01000 => Ok(Instruction::EXG(Target::DirectDReg(regx), Target::DirectDReg(regy))),
                        0b01001 => Ok(Instruction::EXG(Target::DirectAReg(regx), Target::DirectAReg(regy))),
                        0b10001 => Ok(Instruction::EXG(Target::DirectDReg(regx), Target::DirectAReg(regy))),
                        _ => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
                    }
                } else if size.is_none() {
                    let sign = if (ins & 0x0100) == 0 { Sign::Unsigned } else { Sign::Signed };
                    let effective_addr = self.decode_lower_effective_address(memory, ins, Some(Size::Word))?;
                    Ok(Instruction::MULW(effective_addr, get_high_reg(ins), sign))
                } else {
                    let data_reg = Target::DirectDReg(get_high_reg(ins));
                    let effective_addr = self.decode_lower_effective_address(memory, ins, size)?;
                    let (from, to) = if (ins & 0x0100) == 0 { (effective_addr, data_reg) } else { (data_reg, effective_addr) };
                    Ok(Instruction::AND(from, to, size.unwrap()))
                }
            },
            OPCG_ADD => {
                let reg = get_high_reg(ins);
                let dir = (ins & 0x0100) >> 8;
                let size = get_size(ins);
                match size {
                    Some(size) => {
                        if (ins & 0b100110000) == 0b100000000 {
                            let src = get_low_reg(ins);
                            let dest = get_high_reg(ins);
                            match (ins & 0x08) == 0 {
                                true => Ok(Instruction::ADDX(Target::DirectDReg(src), Target::DirectDReg(dest), size)),
                                false => Ok(Instruction::ADDX(Target::IndirectARegDec(src), Target::IndirectARegDec(dest), size)),
                            }
                        } else {
                            let target = self.decode_lower_effective_address(memory, ins, Some(size))?;
                            if dir == 0 {
                                Ok(Instruction::ADD(target, Target::DirectDReg(reg), size))
                            } else {
                                Ok(Instruction::ADD(Target::DirectDReg(reg), target, size))
                            }
                        }
                    },
                    None => {
                        let size = if dir == 0 { Size::Word } else { Size::Long };
                        let target = self.decode_lower_effective_address(memory, ins, Some(size))?;
                        Ok(Instruction::ADDA(target, reg, size))
                    },
                }
            },
            OPCG_SHIFT => {
                let dir = if (ins & 0x0100) == 0 { ShiftDirection::Right } else { ShiftDirection::Left };
                match get_size(ins) {
                    Some(size) => {
                        let reg = get_low_reg(ins);
                        let rotation = get_high_reg(ins);
                        let count = if (ins & 0x0020) == 0 {
                            Target::Immediate(if rotation != 0 { rotation as u32 } else { 8 })
                        } else {
                            Target::DirectDReg(rotation)
                        };

                        match (ins & 0x0018) >> 3 {
                            0b00 => Ok(Instruction::ASd(count, Target::DirectDReg(reg), size, dir)),
                            0b01 => Ok(Instruction::LSd(count, Target::DirectDReg(reg), size, dir)),
                            0b10 => Ok(Instruction::ROXd(count, Target::DirectDReg(reg), size, dir)),
                            0b11 => Ok(Instruction::ROd(count, Target::DirectDReg(reg), size, dir)),
                            _ => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
                        }
                    },
                    None => {
                        if (ins & 0x800) == 0 {
                            let target = self.decode_lower_effective_address(memory, ins, Some(Size::Word))?;
                            let count = Target::Immediate(1);
                            match (ins & 0x0600) >> 9 {
                                0b00 => Ok(Instruction::ASd(count, target, Size::Word, dir)),
                                0b01 => Ok(Instruction::LSd(count, target, Size::Word, dir)),
                                0b10 => Ok(Instruction::ROXd(count, target, Size::Word, dir)),
                                0b11 => Ok(Instruction::ROd(count, target, Size::Word, dir)),
                                _ => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
                            }
                        } else if self.cputype > M68kType::MC68020 {
                            // Bitfield instructions (MC68020+)
                            let ext = self.read_instruction_word(memory)?;
                            let reg = ((ext & 0x7000) >> 12) as u8;

                            let offset = match (ext & 0x0800) == 0 {
                                true => RegOrImmediate::Immediate(((ext & 0x07C0) >> 6) as u8),
                                false => RegOrImmediate::DReg(((ext & 0x01C0) >> 6) as u8),
                            };

                            let width = match (ext & 0x0020) == 0 {
                                true => RegOrImmediate::Immediate((ext & 0x001F) as u8),
                                false => RegOrImmediate::DReg((ext & 0x0007) as u8),
                            };

                            let target = self.decode_lower_effective_address(memory, ins, Some(Size::Word))?;
                            match (ins & 0x0700) >> 8 {
                                0b010 => Ok(Instruction::BFCHG(target, offset, width)),
                                0b100 => Ok(Instruction::BFCLR(target, offset, width)),
                                0b011 => Ok(Instruction::BFEXTS(target, offset, width, reg)),
                                0b001 => Ok(Instruction::BFEXTU(target, offset, width, reg)),
                                0b101 => Ok(Instruction::BFFFO(target, offset, width, reg)),
                                0b111 => Ok(Instruction::BFINS(reg, target, offset, width)),
                                0b110 => Ok(Instruction::BFSET(target, offset, width)),
                                0b000 => Ok(Instruction::BFTST(target, offset, width)),
                                _ => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
                            }
                        } else {
                            return Err(Error::processor(Exceptions::IllegalInstruction as u32));
                        }
                    },
                }
            },
            OPCG_ALINE => {
                Ok(Instruction::UnimplementedA(ins))
            },
            OPCG_FLINE => {
                Ok(Instruction::UnimplementedF(ins))
            },
            _ => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
        }
    }

    fn read_instruction_word(&mut self, memory: &mut dyn Addressable) -> Result<u16, Error> {
        let word = memory.read_beu16(self.end as Address)?;
        self.end += 2;
        Ok(word)
    }

    fn read_instruction_long(&mut self, memory: &mut dyn Addressable) -> Result<u32, Error> {
        let word = memory.read_beu32(self.end as Address)?;
        self.end += 4;
        Ok(word)
    }

    fn decode_lower_effective_address(&mut self, memory: &mut dyn Addressable, ins: u16, size: Option<Size>) -> Result<Target, Error> {
        let reg = get_low_reg(ins);
        let mode = get_low_mode(ins);
        self.get_mode_as_target(memory, mode, reg, size)
    }

    fn decode_upper_effective_address(&mut self, memory: &mut dyn Addressable, ins: u16, size: Option<Size>) -> Result<Target, Error> {
        let reg = get_high_reg(ins);
        let mode = get_high_mode(ins);
        self.get_mode_as_target(memory, mode, reg, size)
    }

    fn get_extension_displacement(&mut self, memory: &mut dyn Addressable, select: u16) -> Result<i32, Error> {
        let result = match select {
            0b00 | 0b01 => 0,
            0b10 => sign_extend_to_long(self.read_instruction_word(memory)? as u32, Size::Word),
            0b11 => self.read_instruction_long(memory)? as i32,
            _ => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
        };
        Ok(result)
    }

    fn decode_extension_word(&mut self, memory: &mut dyn Addressable, areg: Option<u8>) -> Result<Target, Error> {
        let brief_extension = self.read_instruction_word(memory)?;

        let use_brief = (brief_extension & 0x0100) == 0;

        // Decode Index Register
        let xreg_num = ((brief_extension & 0x7000) >> 12) as u8;
        let xreg = if (brief_extension & 0x8000) == 0 { XRegister::DReg(xreg_num) } else { XRegister::AReg(xreg_num) };
        let size = if (brief_extension & 0x0800) == 0 { Size::Word } else { Size::Long };

        if self.cputype <= M68kType::MC68010 {
            let index_reg = IndexRegister { xreg, scale: 0, size };
            let displacement = sign_extend_to_long((brief_extension & 0x00FF) as u32, Size::Byte);

            match areg {
                Some(areg) => Ok(Target::IndirectRegOffset(BaseRegister::AReg(areg), Some(index_reg), displacement)),
                None => Ok(Target::IndirectRegOffset(BaseRegister::PC, Some(index_reg), displacement)),
            }
        } else if use_brief {
            let scale = ((brief_extension & 0x0600) >> 9) as u8;
            let index_reg = IndexRegister { xreg, scale, size };
            let displacement = sign_extend_to_long((brief_extension & 0x00FF) as u32, Size::Byte);

            match areg {
                Some(areg) => Ok(Target::IndirectRegOffset(BaseRegister::AReg(areg), Some(index_reg), displacement)),
                None => Ok(Target::IndirectRegOffset(BaseRegister::PC, Some(index_reg), displacement)),
            }
        } else {
            let scale = ((brief_extension & 0x0600) >> 9) as u8;
            let index_reg = IndexRegister { xreg, scale, size };

            let use_base_reg = (brief_extension & 0x0080) == 0;
            let use_index = (brief_extension & 0x0040) == 0;
            let use_sub_indirect = (brief_extension & 0x0007) != 0;
            let pre_not_post = (brief_extension & 0x0004) == 0;

            let opt_base_reg = match (use_base_reg, areg) {
                (false, _) => BaseRegister::None,
                (true, None) => BaseRegister::PC,
                (true, Some(areg)) => BaseRegister::AReg(areg),
            };
            let opt_index_reg = if use_index { Some(index_reg) } else { None };
            let base_disp = self.get_extension_displacement(memory, (brief_extension & 0x0030) >> 4)?;
            let outer_disp = self.get_extension_displacement(memory, brief_extension & 0x0003)?;

            match (use_sub_indirect, pre_not_post) {
                (false, _) => Ok(Target::IndirectRegOffset(opt_base_reg, opt_index_reg, base_disp)),
                (true, true) => Ok(Target::IndirectMemoryPreindexed(opt_base_reg, opt_index_reg, base_disp, outer_disp)),
                (true, false) => Ok(Target::IndirectMemoryPostindexed(opt_base_reg, opt_index_reg, base_disp, outer_disp)),
            }
        }
    }

    pub fn get_mode_as_target(&mut self, memory: &mut dyn Addressable, mode: u8, reg: u8, size: Option<Size>) -> Result<Target, Error> {
        let value = match mode {
            0b000 => Target::DirectDReg(reg),
            0b001 => Target::DirectAReg(reg),
            0b010 => Target::IndirectAReg(reg),
            0b011 => Target::IndirectARegInc(reg),
            0b100 => Target::IndirectARegDec(reg),
            0b101 => {
                let displacement = sign_extend_to_long(self.read_instruction_word(memory)? as u32, Size::Word);
                Target::IndirectRegOffset(BaseRegister::AReg(reg), None, displacement)
            },
            0b110 => {
                self.decode_extension_word(memory, Some(reg))?
            },
            0b111 => {
                match reg {
                    0b000 => {
                        let value = sign_extend_to_long(self.read_instruction_word(memory)? as u32, Size::Word) as u32;
                        Target::IndirectMemory(value, Size::Word)
                    },
                    0b001 => {
                        let value = self.read_instruction_long(memory)?;
                        Target::IndirectMemory(value, Size::Long)
                    },
                    0b010 => {
                        let displacement = sign_extend_to_long(self.read_instruction_word(memory)? as u32, Size::Word);
                        Target::IndirectRegOffset(BaseRegister::PC, None, displacement)
                    },
                    0b011 => {
                        self.decode_extension_word(memory, None)?
                    },
                    0b100 => {
                        let data = match size {
                            Some(Size::Byte) | Some(Size::Word) => self.read_instruction_word(memory)? as u32,
                            Some(Size::Long) => self.read_instruction_long(memory)?,
                            None => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
                        };
                        Target::Immediate(data)
                    },
                    _ => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
                }
            },
            _ => return Err(Error::processor(Exceptions::IllegalInstruction as u32)),
        };
        Ok(value)
    }

    pub fn dump_disassembly(&mut self, memory: &mut dyn Addressable, start: u32, length: u32) {
        let mut next = start;
        while next < (start + length) {
            match self.decode_at(memory, next) {
                Ok(()) => {
                    self.dump_decoded(memory);
                    next = self.end;
                },
                Err(err) => {
                    println!("{:?}", err);
                    match err {
                        Error { native, .. } if native == Exceptions::IllegalInstruction as u32 => {
                            println!("    at {:08x}: {:04x}", self.start, memory.read_beu16(self.start as Address).unwrap());
                        },
                        _ => { },
                    }
                    return;
                },
            }
        }
    }

    pub fn dump_decoded(&mut self, memory: &mut dyn Addressable) {
        let ins_data: Result<String, Error> =
            (0..((self.end - self.start) / 2)).map(|offset|
                Ok(format!("{:04x} ", memory.read_beu16((self.start + (offset * 2)) as Address).unwrap()))
            ).collect();
        println!("{:#010x}: {}\n\t{}\n", self.start, ins_data.unwrap(), self.instruction);
    }
}

#[inline(always)]
fn get_high_reg(ins: u16) -> u8 {
    ((ins & 0x0E00) >> 9) as u8
}

#[inline(always)]
fn get_low_reg(ins: u16) -> u8 {
    (ins & 0x0007) as u8
}

#[inline(always)]
fn get_high_mode(ins: u16) -> u8 {
    ((ins & 0x01C0) >> 6) as u8
}

#[inline(always)]
fn get_low_mode(ins: u16) -> u8 {
    ((ins & 0x0038) >> 3) as u8
}

#[inline(always)]
fn get_size(ins: u16) -> Option<Size> {
    match (ins & 0x00C0) >> 6 {
        0b00 => Some(Size::Byte),
        0b01 => Some(Size::Word),
        0b10 => Some(Size::Long),
        _ => None,
    }
}

#[inline(always)]
fn get_condition(ins: u16) -> Condition {
    match (ins & 0x0F00) >> 8 {
        0b0000 => Condition::True,
        0b0001 => Condition::False,
        0b0010 => Condition::High,
        0b0011 => Condition::LowOrSame,
        0b0100 => Condition::CarryClear,
        0b0101 => Condition::CarrySet,
        0b0110 => Condition::NotEqual,
        0b0111 => Condition::Equal,
        0b1000 => Condition::OverflowClear,
        0b1001 => Condition::OverflowSet,
        0b1010 => Condition::Plus,
        0b1011 => Condition::Minus,
        0b1100 => Condition::GreaterThanOrEqual,
        0b1101 => Condition::LessThan,
        0b1110 => Condition::GreaterThan,
        0b1111 => Condition::LessThanOrEqual,
        _ => Condition::True,
    }
}