mre-mos6502/src/machine.rs

// Copyright (C) 2014 The 6502-rs Developers
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions
// are met:
// 1. Redistributions of source code must retain the above copyright
//    notice, this list of conditions and the following disclaimer.
// 2. Redistributions in binary form must reproduce the above copyright
//    notice, this list of conditions and the following disclaimer in the
//    documentation and/or other materials provided with the distribution.
// 3. Neither the names of the copyright holders nor the names of any
//    contributors may be used to endorse or promote products derived from this
//    software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
// POSSIBILITY OF SUCH DAMAGE.

use std;

use address::{Address, AddressDiff};
use instruction;
use instruction::{DecodedInstr};
use memory::Memory;
use registers::{ Registers, StackPointer, Status, StatusArgs };
use registers::{ PS_NEGATIVE, PS_DECIMAL_MODE, PS_OVERFLOW, PS_ZERO, PS_CARRY,
                 PS_DISABLE_INTERRUPTS };

pub struct Machine {
    pub registers: Registers,
    pub memory:    Memory
}

impl Machine {
    pub fn new() -> Machine {
    	Machine{
    	    registers: Registers::new(),
    	    memory:    Memory::new()
    	}
    }

    pub fn reset(&mut self) {
    	*self = Machine::new();
    }

    pub fn fetch_next_and_decode(&mut self) -> Option<DecodedInstr> {
        let x: u8 = self.memory.get_byte(self.registers.program_counter);

        match instruction::OPCODES[x as uint] {
            Some((instr, am)) => {
                let extra_bytes = am.extra_bytes();
                let num_bytes = AddressDiff(1) + extra_bytes;

                let data_start = self.registers.program_counter
                               + AddressDiff(1);

                let slice = self.memory.get_slice(data_start, extra_bytes);
                let am_out = am.process(self, slice);

                // Increment program counter
                self.registers.program_counter =
                    self.registers.program_counter + num_bytes;

                Some((instr, am_out))
            }
            _ => None
        }
    }

    pub fn execute_instruction(&mut self, decoded_instr: DecodedInstr) {
        match decoded_instr {
            (instruction::ADC, instruction::UseImmediate(val)) => {
                debug!("add with carry immediate: {}", val);
                self.add_with_carry(val as i8);
            }
            (instruction::ADC, instruction::UseAddress(addr)) => {
                let val = self.memory.get_byte(addr) as i8;
                debug!("add with carry. address: {}. value: {}", addr, val);
                self.add_with_carry(val);
            }

            (instruction::AND, instruction::UseImmediate(val)) => {
                self.and(val as i8);
            }
            (instruction::AND, instruction::UseAddress(addr)) => {
                let val = self.memory.get_byte(addr) as i8;
                self.and(val as i8);
            }

            (instruction::ASL, instruction::UseImplied) => {
                // Accumulator mode
                let mut val = self.registers.accumulator as u8;
                Machine::shift_left_with_flags(&mut val,
                                               &mut self.registers.status);
                self.registers.accumulator = val as i8;

            }
            (instruction::ASL, instruction::UseAddress(addr)) => {
                Machine::shift_left_with_flags(
                    self.memory.get_byte_mut_ref(addr),
                    &mut self.registers.status);
            }

            (instruction::BCC, instruction::UseRelative(rel)) => {
                let addr = self.registers.program_counter
                         + AddressDiff(rel as i32);
                self.branch_if_carry_clear(addr);
            }

            (instruction::BCS, instruction::UseRelative(rel)) => {
                let addr = self.registers.program_counter
                         + AddressDiff(rel as i32);
                self.branch_if_carry_set(addr);
            }

            (instruction::BEQ, instruction::UseRelative(rel)) => {
                let addr = self.registers.program_counter
                         + AddressDiff(rel as i32);
                self.branch_if_equal(addr);
            }

            (instruction::BIT, instruction::UseAddress(addr)) => {
                let a: u8 = self.registers.accumulator as u8;
                let m: u8 = self.memory.get_byte(addr);
                let res = a & m;

                // The zero flag is set based on the result of the 'and'.
                let is_zero = 0 == res;

                // The N flag is set to bit 7 of the byte from memory.
                let bit7 = 0 != (0x80 & res);

                // The V flag is set to bit 6 of the byte from memory.
                let bit6 = 0 != (0x40 & res);

                self.registers.status.set_with_mask(
                    PS_ZERO | PS_NEGATIVE | PS_OVERFLOW,
                    Status::new(StatusArgs { zero: is_zero,
                                             negative: bit7,
                                             overflow: bit6,
                                             ..StatusArgs::none() } ));
            }

            (instruction::BMI, instruction::UseRelative(rel)) => {
                let addr = self.registers.program_counter
                         + AddressDiff(rel as i32);
                debug!("branch if minus relative. address: {}", addr);
                self.branch_if_minus(addr);
            }

            (instruction::BPL, instruction::UseRelative(rel)) => {
                let addr = self.registers.program_counter
                         + AddressDiff(rel as i32);
                self.branch_if_positive(addr);
            }

            (instruction::BVC, instruction::UseRelative(rel)) => {
                let addr = self.registers.program_counter
                         + AddressDiff(rel as i32);
                self.branch_if_overflow_clear(addr);
            }

            (instruction::BVS, instruction::UseRelative(rel)) => {
                let addr = self.registers.program_counter
                         + AddressDiff(rel as i32);
                self.branch_if_overflow_set(addr);
            }

            (instruction::CLC, instruction::UseImplied) => {
                self.registers.status.and(!PS_CARRY);
            }
            (instruction::CLD, instruction::UseImplied) => {
                self.registers.status.and(!PS_DECIMAL_MODE);
            }
            (instruction::CLI, instruction::UseImplied) => {
                self.registers.status.and(!PS_DISABLE_INTERRUPTS);
            }
            (instruction::CLV, instruction::UseImplied) => {
                self.registers.status.and(!PS_OVERFLOW);
            }

            (instruction::CMP, instruction::UseImmediate(val)) => {
                self.compare(val);
            }
            (instruction::CMP, instruction::UseAddress(addr)) => {
                let val = self.memory.get_byte(addr);
                self.compare(val);
            }

            (instruction::DEC, instruction::UseAddress(addr)) => {
                self.decrement_memory(addr)
            }

            (instruction::DEX, instruction::UseImplied) => {
                self.dec_x();
            }

            (instruction::INC, instruction::UseAddress(addr)) => {
                 let m = self.memory.get_byte(addr);
                 let m = m + 1;
                 self.memory.set_byte(addr, m);
                 let i = m as i8;
                 Machine::set_flags_from_i8(&mut self.registers.status, i);
            }
            (instruction::INX, instruction::UseImplied) => {
                let x = self.registers.index_x + 1;
                self.load_x_register(x);
            }
            (instruction::INY, instruction::UseImplied) => {
                let y = self.registers.index_y + 1;
                self.load_y_register(y);
            }

            (instruction::JMP, instruction::UseAddress(addr)) => {
                self.jump(addr)
            }

            (instruction::LDA, instruction::UseImmediate(val)) => {
                debug!("load A immediate: {}", val);
                self.load_accumulator(val as i8);
            }
            (instruction::LDA, instruction::UseAddress(addr)) => {
                let val = self.memory.get_byte(addr);
                debug!("load A. address: {}. value: {}", addr, val);
                self.load_accumulator(val as i8);
            }

            (instruction::LDX, instruction::UseImmediate(val)) => {
                debug!("load X immediate: {}", val);
                self.load_x_register(val as i8);
            }
            (instruction::LDX, instruction::UseAddress(addr)) => {
                let val = self.memory.get_byte(addr);
                debug!("load X. address: {}. value: {}", addr, val);
                self.load_x_register(val as i8);
            }

            (instruction::LDY, instruction::UseImmediate(val)) => {
                debug!("load Y immediate: {}", val);
                self.load_y_register(val as i8);
            }
            (instruction::LDY, instruction::UseAddress(addr)) => {
                let val = self.memory.get_byte(addr);
                debug!("load Y. address: {}. value: {}", addr, val);
                self.load_y_register(val as i8);
            }

            (instruction::LSR, instruction::UseImplied) => {
                // Accumulator mode
                let mut val = self.registers.accumulator as u8;
                Machine::shift_right_with_flags(&mut val,
                                                &mut self.registers.status);
                self.registers.accumulator = val as i8;
            }
            (instruction::LSR, instruction::UseAddress(addr)) => {
                Machine::shift_right_with_flags(
                    self.memory.get_byte_mut_ref(addr),
                    &mut self.registers.status);
            }

            (instruction::PHA, instruction::UseImplied) => {
                // Push accumulator
                let val = self.registers.accumulator as u8;
                self.push_on_stack(val);
            }
            (instruction::PHP, instruction::UseImplied) => {
                // Push status
                let val = self.registers.status.bits();
                self.push_on_stack(val);
            }
            (instruction::PLA, instruction::UseImplied) => {
                // Pull accumulator
                let val: u8 = self.pull_from_stack();
                self.registers.accumulator = val as i8;
            }
            (instruction::PLP, instruction::UseImplied) => {
                // Pull status
                let val: u8 = self.pull_from_stack();
                // The `truncate` here won't do anything because we have a
                // constant for the single unused flags bit. This probably
                // corresponds to the behavior of the 6502...? FIXME: verify
                self.registers.status = Status::from_bits_truncate(val);
            }

            (instruction::ROL, instruction::UseImplied) => {
                // Accumulator mode
                let mut val = self.registers.accumulator as u8;
                Machine::rotate_left_with_flags(&mut val,
                                                &mut self.registers.status);
                self.registers.accumulator = val as i8;
            }
            (instruction::ROL, instruction::UseAddress(addr)) => {
                Machine::rotate_left_with_flags(
                    self.memory.get_byte_mut_ref(addr),
                    &mut self.registers.status);
            }
            (instruction::ROR, instruction::UseImplied) => {
                // Accumulator mode
                let mut val = self.registers.accumulator as u8;
                Machine::rotate_right_with_flags(&mut val,
                                                 &mut self.registers.status);
                self.registers.accumulator = val as i8;
            }
            (instruction::ROR, instruction::UseAddress(addr)) => {
                Machine::rotate_right_with_flags(
                    self.memory.get_byte_mut_ref(addr),
                    &mut self.registers.status);
            }

            (instruction::SBC, instruction::UseImmediate(val)) => {
                debug!("subtract with carry immediate: {}", val);
                self.subtract_with_carry(val as i8);
            }
            (instruction::SBC, instruction::UseAddress(addr)) => {
                let val = self.memory.get_byte(addr) as i8;
                debug!("subtract with carry. address: {}. value: {}",
                       addr, val);
                self.subtract_with_carry(val);
            }

            (instruction::SEC, instruction::UseImplied) => {
                self.registers.status.or(PS_CARRY);
            }
            (instruction::SED, instruction::UseImplied) => {
                self.registers.status.or(PS_DECIMAL_MODE);
            }
            (instruction::SEI, instruction::UseImplied) => {
                self.registers.status.or(PS_DISABLE_INTERRUPTS);
            }

            (instruction::STA, instruction::UseAddress(addr)) => {
                self.memory.set_byte(addr, self.registers.accumulator as u8);
            }
            (instruction::STX, instruction::UseAddress(addr)) => {
                self.memory.set_byte(addr, self.registers.index_x as u8);
            }
            (instruction::STY, instruction::UseAddress(addr)) => {
                self.memory.set_byte(addr, self.registers.index_y as u8);
            }

            (instruction::TAX, instruction::UseImplied) => {
                let val = self.registers.accumulator;
                self.load_x_register(val);
            }
            (instruction::TAY, instruction::UseImplied) => {
                let val = self.registers.accumulator;
                self.load_y_register(val);
            }
            (instruction::TSX, instruction::UseImplied) => {
                let StackPointer(val) = self.registers.stack_pointer;
                let val = val as i8;
                self.load_x_register(val);
            }
            (instruction::TXA, instruction::UseImplied) => {
                let val = self.registers.index_x;
                self.load_accumulator(val);
            }
            (instruction::TXS, instruction::UseImplied) => {
                // Note that this is the only 'transfer' instruction that does
                // NOT set the zero and negative flags. (Because the target
                // is the stack pointer)
                let val = self.registers.index_x;
                self.registers.stack_pointer = StackPointer(val as u8);
            }
            (instruction::TYA, instruction::UseImplied) => {
                let val = self.registers.index_y;
                self.load_accumulator(val);
            }

            (instruction::NOP, instruction::UseImplied) => {
                debug!("NOP instruction");
            }
            (_, _) => {
                debug!("attempting to execute unimplemented or invalid \
                        instruction");
            }
        };
    }

    pub fn run(&mut self) {
        loop {
            if let Some(decoded_instr) = self.fetch_next_and_decode() {
                self.execute_instruction(decoded_instr);
            } else {
                break
            }
        }
    }

    fn set_flags_from_i8(status: &mut Status, value: i8) {
        let is_zero = value == 0;
        let is_negative = value < 0;

        status.set_with_mask(
            PS_ZERO | PS_NEGATIVE,
            Status::new(StatusArgs { zero: is_zero,
                                     negative: is_negative,
                                     ..StatusArgs::none() } ));
    }

    fn shift_left_with_flags(p_val: &mut u8, status: &mut Status) {
        let mask = 1 << 7;
        let is_bit_7_set = (*p_val & mask) == mask;
        let shifted = (*p_val & !(1 << 7)) << 1;
        *p_val = shifted;
        status.set_with_mask(
            PS_CARRY,
            Status::new(StatusArgs { carry: is_bit_7_set,
                                     ..StatusArgs::none() } ));
        Machine::set_flags_from_i8(status, *p_val as i8);
    }

    fn shift_right_with_flags(p_val: &mut u8, status: &mut Status) {
        let mask = 1;
        let is_bit_0_set = (*p_val & mask) == mask;
        *p_val = *p_val >> 1;
        status.set_with_mask(
            PS_CARRY,
            Status::new(StatusArgs { carry: is_bit_0_set,
                                     ..StatusArgs::none() } ));
        Machine::set_flags_from_i8(status, *p_val as i8);
    }

    fn rotate_left_with_flags(p_val: &mut u8, status: &mut Status) {
        let is_carry_set = status.contains(PS_CARRY);
        let mask = 1 << 7;
        let is_bit_7_set = (*p_val & mask) == mask;
        let shifted = (*p_val & !(1 << 7)) << 1;
        *p_val = shifted + if is_carry_set { 1 } else { 0 };
        status.set_with_mask(
            PS_CARRY,
            Status::new(StatusArgs { carry: is_bit_7_set,
                                     ..StatusArgs::none() } ));
        Machine::set_flags_from_i8(status, *p_val as i8);
    }

    fn rotate_right_with_flags(p_val: &mut u8, status: &mut Status) {
        let is_carry_set = status.contains(PS_CARRY);
        let mask = 1;
        let is_bit_0_set = (*p_val & mask) == mask;
        let shifted = *p_val >> 1;
        *p_val = shifted + if is_carry_set { 1 << 7 } else { 0 };
        status.set_with_mask(
            PS_CARRY,
            Status::new(StatusArgs { carry: is_bit_0_set,
                                     ..StatusArgs::none() } ));
        Machine::set_flags_from_i8(status, *p_val as i8);
    }

    fn set_i8_with_flags(mem: &mut i8, status: &mut Status, value: i8) {
        *mem = value;
        Machine::set_flags_from_i8(status, value);
    }

    fn load_x_register(&mut self, value: i8) {
        Machine::set_i8_with_flags(&mut self.registers.index_x,
                                   &mut self.registers.status,
                                   value);
    }

    fn load_y_register(&mut self, value: i8) {
        Machine::set_i8_with_flags(&mut self.registers.index_y,
                                   &mut self.registers.status,
                                   value);
    }

    fn load_accumulator(&mut self, value: i8) {
        Machine::set_i8_with_flags(&mut self.registers.accumulator,
                                   &mut self.registers.status,
                                   value);
    }

    fn add_with_carry(&mut self, value: i8) {
        if self.registers.status.contains(PS_DECIMAL_MODE) {
            // TODO akeeton: Implement binary-coded decimal.
            debug!("binary-coded decimal not implemented for add_with_carry");
        } else {
            let a_before: i8 = self.registers.accumulator;
            let c_before: i8 = if self.registers.status.contains(PS_CARRY)
                               { 1 } else { 0 };
            let a_after: i8 = a_before + c_before + value;

            debug_assert_eq!(a_after as u8, a_before as u8 + c_before as u8
                                            + value as u8);

            let did_carry = (a_after as u8) < (a_before as u8);

            let did_overflow =
            	   (a_before < 0 && value < 0 && a_after >= 0)
            	|| (a_before > 0 && value > 0 && a_after <= 0);

            let mask = PS_CARRY | PS_OVERFLOW;

            self.registers.status.set_with_mask(mask,
                Status::new(StatusArgs { carry: did_carry,
                                         overflow: did_overflow,
                                         ..StatusArgs::none() } ));

            self.load_accumulator(a_after);

            debug!("accumulator: {}", self.registers.accumulator);
        }
    }

    fn and(&mut self, value: i8) {
        let a_after = self.registers.accumulator & value;
        self.load_accumulator(a_after);
    }

    // TODO: Implement binary-coded decimal
    fn subtract_with_carry(&mut self, value: i8) {
        if self.registers.status.contains(PS_DECIMAL_MODE) {
            debug!("binary-coded decimal not implemented for \
                    subtract_with_carry");
        } else {
            // A - M - (1 - C)

            // nc -- 'not carry'
            let nc: i8 = if self.registers.status.contains(PS_CARRY)
                         { 0 } else { 1 };

            let a_before: i8 = self.registers.accumulator;

            let a_after = a_before - value - nc;

            // The carry flag is set on unsigned overflow.
            let did_carry = (a_after as u8) > (a_before as u8);

            // The overflow flag is set on two's-complement overflow.
            //
            // range of A              is  -128 to 127
            // range of - M - (1 - C)  is  -128 to 128
            //                            -(127 + 1) to -(-128 + 0)
            //
            let over = ((nc == 0 && value < 0) || (nc == 1 && value < -1))
                    && a_before >= 0
                    && a_after < 0;

            let under = (a_before < 0) && (-value - nc < 0)
                     && a_after >= 0;

            let did_overflow = over || under;

            let mask = PS_CARRY | PS_OVERFLOW;

            self.registers.status.set_with_mask(mask,
                Status::new(StatusArgs { carry: did_carry,
                                         overflow: did_overflow,
                                         ..StatusArgs::none() } ));

            self.load_accumulator(a_after);
        }
    }

    fn decrement_memory(&mut self, addr: Address) {
        let value_new = self.memory.get_byte(addr) - 1;

        self.memory.set_byte(addr, value_new);

        let is_negative = (value_new as i8) < 0;
        let is_zero     = value_new == 0;

        self.registers.status.set_with_mask(
            PS_NEGATIVE | PS_ZERO,
            Status::new(StatusArgs { negative: is_negative,
                                     zero:     is_zero,
                                     ..StatusArgs::none() } ));
    }

    fn dec_x(&mut self) {
        let val = self.registers.index_x;
        self.load_x_register(val - 1);
    }

    fn jump(&mut self, addr: Address) {
        self.registers.program_counter = addr;
    }

    fn branch_if_carry_clear(&mut self, addr: Address) {
        if !self.registers.status.contains(PS_CARRY) {
            self.registers.program_counter = addr;
        }
    }

    fn branch_if_carry_set(&mut self, addr: Address) {
        if self.registers.status.contains(PS_CARRY) {
            self.registers.program_counter = addr;
        }
    }

    fn branch_if_equal(&mut self, addr: Address) {
        if self.registers.status.contains(PS_ZERO) {
            self.registers.program_counter = addr;
        }
    }

    fn branch_if_minus(&mut self, addr: Address) {
        if self.registers.status.contains(PS_NEGATIVE) {
            self.registers.program_counter = addr;
        }
    }

    fn branch_if_positive(&mut self, addr: Address) {
        if !self.registers.status.contains(PS_NEGATIVE) {
            self.registers.program_counter = addr;
        }
    }

    fn branch_if_overflow_clear(&mut self, addr: Address) {
        if !self.registers.status.contains(PS_OVERFLOW) {
            self.registers.program_counter = addr;
        }
    }

    fn branch_if_overflow_set(&mut self, addr: Address) {
        if self.registers.status.contains(PS_OVERFLOW) {
            self.registers.program_counter = addr;
        }
    }

    // From http://www.6502.org/tutorials/compare_beyond.html:
    //   If the Z flag is 0, then A <> NUM and BNE will branch
    //   If the Z flag is 1, then A = NUM and BEQ will branch
    //   If the C flag is 0, then A (unsigned) < NUM (unsigned) and BCC will branch
    //   If the C flag is 1, then A (unsigned) >= NUM (unsigned) and BCS will branch
    //   ...
    //   The N flag contains most significant bit of the of the subtraction result.
    fn compare(&mut self, val: u8) {
        let a = self.registers.accumulator;

        if a as u8 >= val as u8 {
            self.registers.status.insert(PS_CARRY);
        } else {
            self.registers.status.remove(PS_CARRY);
        }

        if a as i8 == val as i8 {
            self.registers.status.insert(PS_ZERO);
        } else {
            self.registers.status.remove(PS_ZERO);
        }

        let diff: i8 = (a as i8) - (val as i8);
        if diff < 0 {
            self.registers.status.insert(PS_NEGATIVE);
        } else {
            self.registers.status.remove(PS_NEGATIVE);
        }
    }

    fn push_on_stack(&mut self, val: u8) {
        let addr = self.registers.stack_pointer.to_address();
        self.memory.set_byte(addr, val);
        self.registers.stack_pointer.decrement();
    }

    fn pull_from_stack(&mut self) -> u8 {
        let addr = self.registers.stack_pointer.to_address();
        let out = self.memory.get_byte(addr);
        self.registers.stack_pointer.increment();
        out
    }
}

impl std::fmt::Show for Machine {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        write!(f, "Machine Dump:\n\nAccumulator: {}",
               self.registers.accumulator)
    }
}

#[test]
fn add_with_carry_test() {
    let mut machine = Machine::new();

    machine.add_with_carry(1);
    assert_eq!(machine.registers.accumulator, 1);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    false);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), false);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), false);

    machine.add_with_carry(-1);
    assert_eq!(machine.registers.accumulator, 0);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    true);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     true);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), false);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), false);

    machine.add_with_carry(1);
    assert_eq!(machine.registers.accumulator, 2);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    false);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), false);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), false);

    let mut machine = Machine::new();

    machine.add_with_carry(127);
    assert_eq!(machine.registers.accumulator, 127);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    false);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), false);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), false);

    machine.add_with_carry(-127);
    assert_eq!(machine.registers.accumulator, 0);
    assert_eq!(machine.registers.status.contains(PS_CARRY),     true);
    assert_eq!(machine.registers.status.contains(PS_ZERO),      true);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), false);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), false);

    machine.registers.status.remove(PS_CARRY);
    machine.add_with_carry(-128);
    assert_eq!(machine.registers.accumulator, -128);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    false);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE),  true);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), false);

    machine.add_with_carry(127);
    assert_eq!(machine.registers.accumulator, -1);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    false);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE),  true);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), false);

    let mut machine = Machine::new();

    machine.add_with_carry(127);
    assert_eq!(machine.registers.accumulator, 127);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    false);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), false);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), false);

    machine.add_with_carry(1);
    assert_eq!(machine.registers.accumulator, -128);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    false);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE),  true);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW),  true);
}

#[test]
fn and_test() {
    let mut machine = Machine::new();

    machine.registers.accumulator = 0;
    machine.and(-1);
    assert_eq!(machine.registers.accumulator, 0);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     true);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), false);

    machine.registers.accumulator = -1;
    machine.and(0);
    assert_eq!(machine.registers.accumulator, 0);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     true);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), false);

    machine.registers.accumulator = -1;
    machine.and(0x0f);
    assert_eq!(machine.registers.accumulator, 0x0f);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), false);

    machine.registers.accumulator = -1;
    machine.and(-128);
    assert_eq!(machine.registers.accumulator, -128);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), true);
}

#[test]
fn subtract_with_carry_test() {
    let mut machine = Machine::new();

    machine.execute_instruction((instruction::SEC, instruction::UseImplied));
    machine.registers.accumulator = 0;

    machine.subtract_with_carry(1);
    assert_eq!(machine.registers.accumulator, -1);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    true);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), true);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), false);

    machine.execute_instruction((instruction::SEC, instruction::UseImplied));
    machine.registers.accumulator = -128;
    machine.subtract_with_carry(1);
    assert_eq!(machine.registers.accumulator, 127);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    false);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), false);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), true);

    machine.execute_instruction((instruction::SEC, instruction::UseImplied));
    machine.registers.accumulator = 127;
    machine.subtract_with_carry(-1);
    assert_eq!(machine.registers.accumulator, -128);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    true);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), true);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), true);

    machine.execute_instruction((instruction::CLC, instruction::UseImplied));
    machine.registers.accumulator = -64;
    machine.subtract_with_carry(64);
    assert_eq!(machine.registers.accumulator, 127);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    false);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), false);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), true);

    machine.execute_instruction((instruction::SEC, instruction::UseImplied));
    machine.registers.accumulator = 0;
    machine.subtract_with_carry(-128);
    assert_eq!(machine.registers.accumulator, -128);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    true);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), true);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), true);

    machine.execute_instruction((instruction::CLC, instruction::UseImplied));
    machine.registers.accumulator = 0;
    machine.subtract_with_carry(127);
    assert_eq!(machine.registers.accumulator, -128);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    true);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), true);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), false);
}

#[test]
fn decrement_memory_test() {
    let mut machine = Machine::new();
    let addr        = Address(0xA1B2);

    machine.memory.set_byte(addr, 5);

    machine.decrement_memory(addr);
    assert_eq!(machine.memory.get_byte(addr), 4);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), false);

    machine.decrement_memory(addr);
    assert_eq!(machine.memory.get_byte(addr), 3);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), false);

    machine.decrement_memory(addr);
    machine.decrement_memory(addr);
    machine.decrement_memory(addr);
    assert_eq!(machine.memory.get_byte(addr), 0);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     true);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), false);

    machine.decrement_memory(addr);
    assert_eq!(machine.memory.get_byte(addr) as i8, -1);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), true);
}

#[test]
fn logical_shift_right_test() {
    // Testing UseImplied version (which targets the accumulator) only, for now

    let mut machine = Machine::new();
    machine.execute_instruction((instruction::LDA,
                                 instruction::UseImmediate(0)));
    machine.execute_instruction((instruction::LSR,
                                 instruction::UseImplied));
    assert_eq!(machine.registers.accumulator, 0);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    false);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     true);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), false);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), false);

    machine.execute_instruction((instruction::LDA,
                                 instruction::UseImmediate(1)));
    machine.execute_instruction((instruction::LSR,
                                 instruction::UseImplied));
    assert_eq!(machine.registers.accumulator, 0);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    true);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     true);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), false);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), false);

    machine.execute_instruction((instruction::LDA,
                                 instruction::UseImmediate(255)));
    machine.execute_instruction((instruction::LSR,
                                 instruction::UseImplied));
    assert_eq!(machine.registers.accumulator, 0x7F);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    true);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), false);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), false);

    machine.execute_instruction((instruction::LDA,
                                 instruction::UseImmediate(254)));
    machine.execute_instruction((instruction::LSR,
                                 instruction::UseImplied));
    assert_eq!(machine.registers.accumulator, 0x7F);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    false);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), false);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), false);
}

#[test]
fn dec_x_test() {
    let mut machine = Machine::new();

    machine.dec_x();
    assert_eq!(machine.registers.index_x, -1);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    false);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), true);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), false);

    machine.dec_x();
    assert_eq!(machine.registers.index_x, -2);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    false);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), true);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), false);

    machine.load_x_register(5);
    machine.dec_x();
    assert_eq!(machine.registers.index_x, 4);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    false);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), false);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), false);

    machine.dec_x();
    machine.dec_x();
    machine.dec_x();
    machine.dec_x();

    assert_eq!(machine.registers.index_x, 0);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    false);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     true);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), false);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), false);

    machine.dec_x();
    assert_eq!(machine.registers.index_x, -1);
    assert_eq!(machine.registers.status.contains(PS_CARRY),    false);
    assert_eq!(machine.registers.status.contains(PS_ZERO),     false);
    assert_eq!(machine.registers.status.contains(PS_NEGATIVE), true);
    assert_eq!(machine.registers.status.contains(PS_OVERFLOW), false);
}

#[test]
fn jump_test() {
    let mut machine = Machine::new();
    let     addr    = Address(0xA1B1);

    machine.jump(addr);
    assert_eq!(machine.registers.program_counter, addr);
}

#[test]
fn branch_if_carry_clear_test() {
    let mut machine = Machine::new();

    machine.execute_instruction((instruction::SEC, instruction::UseImplied));
    machine.branch_if_carry_clear(Address(0xABCD));
    assert_eq!(machine.registers.program_counter, Address(0));

    machine.execute_instruction((instruction::CLC, instruction::UseImplied));
    machine.branch_if_carry_clear(Address(0xABCD));
    assert_eq!(machine.registers.program_counter, Address(0xABCD));
}

#[test]
fn branch_if_carry_set_test() {
    let mut machine = Machine::new();

    machine.execute_instruction((instruction::CLC, instruction::UseImplied));
    machine.branch_if_carry_set(Address(0xABCD));
    assert_eq!(machine.registers.program_counter, Address(0));

    machine.execute_instruction((instruction::SEC, instruction::UseImplied));
    machine.branch_if_carry_set(Address(0xABCD));
    assert_eq!(machine.registers.program_counter, Address(0xABCD));
}

#[test]
fn branch_if_equal_test() {
    let mut machine = Machine::new();

    machine.branch_if_equal(Address(0xABCD));
    assert_eq!(machine.registers.program_counter, Address(0));

    machine.registers.status.or(PS_ZERO);
    machine.branch_if_equal(Address(0xABCD));
    assert_eq!(machine.registers.program_counter, Address(0xABCD));
}

#[test]
fn branch_if_minus_test() {
    {
        let mut machine      = Machine::new();
        let registers_before = machine.registers;

        machine.branch_if_minus(Address(0xABCD));
        assert_eq!(machine.registers, registers_before);
        assert_eq!(machine.registers.program_counter, Address(0));
    }

    {
        let mut machine = Machine::new();

        machine.registers.status.or(PS_NEGATIVE);
        let registers_before = machine.registers;

        machine.branch_if_minus(Address(0xABCD));
        assert_eq!(machine.registers.status, registers_before.status);
        assert_eq!(machine.registers.program_counter, Address(0xABCD));
    }
}

#[test]
fn branch_if_positive_test() {
    let mut machine = Machine::new();

    machine.registers.status.insert(PS_NEGATIVE);
    machine.branch_if_positive(Address(0xABCD));
    assert_eq!(machine.registers.program_counter, Address(0));

    machine.registers.status.remove(PS_NEGATIVE);
    machine.branch_if_positive(Address(0xABCD));
    assert_eq!(machine.registers.program_counter, Address(0xABCD));
}

#[test]
fn branch_if_overflow_clear_test() {
    let mut machine = Machine::new();

    machine.registers.status.insert(PS_OVERFLOW);
    machine.branch_if_overflow_clear(Address(0xABCD));
    assert_eq!(machine.registers.program_counter, Address(0));

    machine.registers.status.remove(PS_OVERFLOW);
    machine.branch_if_overflow_clear(Address(0xABCD));
    assert_eq!(machine.registers.program_counter, Address(0xABCD));
}

#[test]
fn branch_if_overflow_set_test() {
    let mut machine = Machine::new();

    machine.branch_if_overflow_set(Address(0xABCD));
    assert_eq!(machine.registers.program_counter, Address(0));

    machine.registers.status.insert(PS_OVERFLOW);
    machine.branch_if_overflow_set(Address(0xABCD));
    assert_eq!(machine.registers.program_counter, Address(0xABCD));
}

#[test]
fn compare_test() {
    let mut machine = Machine::new();

    machine.execute_instruction(
        (instruction::LDA, instruction::UseImmediate(127))
    );

    machine.compare(127);
    assert!( machine.registers.status.contains(PS_ZERO    ));
    assert!( machine.registers.status.contains(PS_CARRY   ));
    assert!(!machine.registers.status.contains(PS_NEGATIVE));


    machine.execute_instruction(
        (instruction::LDA, instruction::UseImmediate(127))
    );

    machine.compare(1);
    assert!(!machine.registers.status.contains(PS_ZERO    ));
    assert!( machine.registers.status.contains(PS_CARRY   ));
    assert!(!machine.registers.status.contains(PS_NEGATIVE));


    machine.execute_instruction(
        (instruction::LDA, instruction::UseImmediate(1))
    );

    machine.compare(2);
    assert!(!machine.registers.status.contains(PS_ZERO    ));
    assert!(!machine.registers.status.contains(PS_CARRY   ));
    assert!( machine.registers.status.contains(PS_NEGATIVE));


    machine.execute_instruction(
        (instruction::LDA, instruction::UseImmediate(20))
    );

    machine.compare(-50);
    assert!(!machine.registers.status.contains(PS_ZERO    ));
    assert!(!machine.registers.status.contains(PS_CARRY   ));
    assert!(!machine.registers.status.contains(PS_NEGATIVE));


    machine.execute_instruction(
        (instruction::LDA, instruction::UseImmediate(1))
    );

    machine.compare(-1);
    assert!(!machine.registers.status.contains(PS_ZERO    ));
    assert!(!machine.registers.status.contains(PS_CARRY   ));
    assert!(!machine.registers.status.contains(PS_NEGATIVE));


    machine.execute_instruction(
        (instruction::LDA, instruction::UseImmediate(127))
    );

    machine.compare(-128);
    assert!(!machine.registers.status.contains(PS_ZERO    ));
    assert!(!machine.registers.status.contains(PS_CARRY   ));
    assert!( machine.registers.status.contains(PS_NEGATIVE));
}