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

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

            (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::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 load_register_with_flags(register: &mut i8,
                                status: &mut Status,
                                value: i8) {
        *register = value;

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

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

    fn load_accumulator(&mut self, value: i8) {
        Machine::load_register_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);
        }
    }

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

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 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 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_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));
    }
}