nesemu/src/native_core/cpu.rs

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum CpuError {
    UnsupportedOpcode { opcode: u8, pc: u16 },
}

pub trait CpuBus {
    fn read(&mut self, addr: u16) -> u8;
    fn write(&mut self, addr: u16, value: u8);
    fn poll_nmi(&mut self) -> bool {
        false
    }
    fn poll_irq(&mut self) -> bool {
        false
    }
}

const FLAG_CARRY: u8 = 0b0000_0001;
const FLAG_ZERO: u8 = 0b0000_0010;
const FLAG_IRQ_DISABLE: u8 = 0b0000_0100;
const FLAG_DECIMAL: u8 = 0b0000_1000;
const FLAG_BREAK: u8 = 0b0001_0000;
const FLAG_UNUSED: u8 = 0b0010_0000;
const FLAG_OVERFLOW: u8 = 0b0100_0000;
const FLAG_NEGATIVE: u8 = 0b1000_0000;

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Cpu6502 {
    pub a: u8,
    pub x: u8,
    pub y: u8,
    pub sp: u8,
    pub pc: u16,
    pub p: u8,
    pub halted: bool,
    pub irq_delay: bool,
    pub pending_nmi: bool,
    pub pending_irq: bool,
}

impl Default for Cpu6502 {
    fn default() -> Self {
        Self {
            a: 0,
            x: 0,
            y: 0,
            sp: 0xFD,
            pc: 0,
            p: FLAG_IRQ_DISABLE | FLAG_UNUSED,
            halted: false,
            irq_delay: false,
            pending_nmi: false,
            pending_irq: false,
        }
    }
}

impl Cpu6502 {
    pub fn reset<B: CpuBus>(&mut self, bus: &mut B) {
        let lo = bus.read(0xFFFC) as u16;
        let hi = bus.read(0xFFFD) as u16;
        self.pc = (hi << 8) | lo;
        self.sp = self.sp.wrapping_sub(3);
        self.p = (self.p | FLAG_IRQ_DISABLE | FLAG_UNUSED) & !FLAG_BREAK;
        self.halted = false;
        self.irq_delay = false;
        self.pending_nmi = false;
        self.pending_irq = false;
    }

    pub fn step<B: CpuBus>(&mut self, bus: &mut B) -> Result<u8, CpuError> {
        if self.halted {
            return Ok(2);
        }
        if self.pending_nmi {
            self.pending_nmi = false;
            self.pending_irq = false;
            self.service_interrupt(bus, 0xFFFA, false);
            self.p |= FLAG_UNUSED;
            return Ok(7);
        }
        let irq_delayed = self.irq_delay;
        self.irq_delay = false;
        if self.pending_irq && !irq_delayed && (self.p & FLAG_IRQ_DISABLE) == 0 {
            self.pending_irq = false;
            self.service_interrupt(bus, 0xFFFE, false);
            self.p |= FLAG_UNUSED;
            return Ok(7);
        }

        let pc_before = self.pc;
        let opcode = self.fetch(bus);
        let cycles = self.execute_opcode(bus, opcode, pc_before)?;

        // Real 6502 polls IRQ/NMI near the end of the current instruction and
        // services it before the next opcode fetch.
        if bus.poll_nmi() {
            self.pending_nmi = true;
            self.pending_irq = false;
        } else if !irq_delayed && (self.p & FLAG_IRQ_DISABLE) == 0 && bus.poll_irq() {
            self.pending_irq = true;
        }
        Ok(cycles)
    }
}

// Addressing modes, ALU helpers, stack and interrupt internals.
mod helpers;
// Opcode decoder/executor table split out from cpu.rs for readability.
mod opcodes;

#[cfg(test)]
mod tests;