first working CPU implementation with 3 op codes

2026-08-08 13:21:31 -05:00
parent 09d0319ae3
commit faef20555b
5 changed files with 241 additions and 3 deletions
@@ -10,15 +10,21 @@ pub struct NesBus {
    cart: Cartridge,
 }

+impl NesBus {
+    pub fn new(cart: Cartridge) -> Self {
+        Self { ram: [0; 0x800], cart }
+    }
+}
+
 impl Bus for NesBus {
    fn read(&mut self, address: u16) -> u8 {
        match address {
            0x0000..=0x1FFF => self.ram[(address & 0x07FF) as usize], // Zero-Page my beloved
            0x2000..=0x3FFF => 0, //PPU registers stub, temporal
            0x4000..=0x401F => 0, //APU controller stub, temporal
-            0x4020..=0x5FFF => 0, //Expansion ROM stub
+            0x4020..=0x5FFF => 0, //Expansion ROM stub. temporal
            0x6000..=0x7FFF => 0, //SRAM stub, no reading saves for you :3
-            0x8000..=0xFFFF => self.cart.read_prg(address),
+            0x8000..=0xFFFF => self.cart.read_prg(address), // this is te space PRG Rom gets "loaded" to in real hardware
        }
    }

@@ -0,0 +1,213 @@
+use crate::bus::Bus;
+
+// Flag bit constants
+const FLAG_CARRY: u8 = 0b0000_0001;
+const FLAG_ZERO: u8 = 0b0000_0010;
+const FLAG_INTERRUPT: 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;
+
+pub struct Cpu {
+    a: u8, // Accumulator Register
+    x: u8, // X Register
+    y: u8, // Y Register
+    sp: u8, // Stack Pointer
+    pc: u16, // Program Counter
+    p: u8, // Status Register
+    cycles: u64,
+    irq_pending: bool,
+    nmi_pending: bool,
+}
+
+impl Cpu {
+    // Testing related
+    pub fn set_test_start(&mut self, pc: u16, cycles: u64) {
+        self.pc = pc;
+        self.cycles = cycles;
+    }
+
+    // Helpers for changing flags on the CPU
+    fn set_flag(&mut self, bit: u8) { self.p |= bit; }
+    fn clear_flag(&mut self, bit: u8) { self.p &= !bit; }
+    fn flag(&self, bit: u8) -> bool { self.p & bit != 0 }
+
+    fn set_zp_flags(&mut self, value: u8) {
+        // Clear before setting
+        self.clear_flag(FLAG_ZERO);
+        self.clear_flag(FLAG_NEGATIVE);
+
+        if value == 0 { self.set_flag(FLAG_ZERO); } // Set Zero if Zero 
+        if value & 0x80 != 0 { self.set_flag(FLAG_NEGATIVE) } // Set if Negative
+    }
+
+    // New and reset methods 
+    pub fn new() -> Self {
+        Self { a: 0, x: 0, y: 0, sp: 0xFD, pc: 0, p: FLAG_UNUSED | FLAG_INTERRUPT, cycles: 0, irq_pending: false, nmi_pending: false } // kick stars a NES CPU
+    }
+
+    pub fn reset(&mut self, bus: &mut impl Bus) {
+        self.sp = 0xFD;
+        self.p = FLAG_UNUSED | FLAG_INTERRUPT;
+        self.pc = self.read16(bus, 0xFFFC);
+    } 
+
+    // Memory helpers
+    fn fetch(&mut self, bus: &mut impl Bus) -> u8 {
+        let value: u8 = bus.read(self.pc);
+        self.pc = self.pc.wrapping_add(1);
+        value
+    }
+
+    fn fetch16(&mut self, bus: &mut impl Bus) -> u16 {
+        let low = self.fetch(bus) as u16;
+        let high = self.fetch(bus) as u16;
+        high << 8 | low
+    }
+
+    fn read(&self, bus: &mut impl Bus, address: u16) -> u8 {
+        bus.read(address)
+    }
+
+    fn write(&mut self, bus: &mut impl Bus, address: u16, value: u8) {
+        bus.write(address, value);
+    }
+
+    fn read16(&self, bus: &mut impl Bus, address: u16) -> u16 {
+        let low: u16 = bus.read(address) as u16;
+        let high: u16 = bus.read(address.wrapping_add(1)) as u16;
+        high << 8 | low // returt in little-endian format
+    }
+
+    // Stack related Memory helpers
+    fn push(&mut self, bus: &mut impl Bus, value: u8) {
+        self.write(bus, 0x0100 + self.sp as u16, value);
+        self.sp = self.sp.wrapping_sub(1);
+    }
+
+    fn pop(&mut self, bus: &mut impl Bus) -> u8 {
+        self.sp = self.sp.wrapping_add(1);
+        self.read(bus, 0x0100 + self.sp as u16)
+    }
+
+    // Step shell
+    pub fn step(&mut self, bus: &mut impl Bus) -> u8 {
+        let opcode: u8 = self.fetch(bus);
+        let cycles = match opcode {
+            0x4C => self.jmp_abs(bus),
+            0xA2 => self.ldx_imm(bus),
+            0x86 => self.stx_zp(bus),
+            _ => panic!("illegal/unkown opcode {opcode:#04X} at {:#06X}", self.pc.wrapping_sub(1)),
+        };
+        self.cycles += cycles as u64;
+        cycles
+    }
+
+    // Trace for testing the cpu
+    pub fn trace(&self, bus: &mut impl Bus) -> String {
+        let opcode: u8 = bus.read(self.pc);
+        let len: u8 = Self::opcode_len(opcode);
+        let mut bytes: String = String::new();
+        for i in 0..len {
+            bytes.push_str(&format!("{:02X} ", bus.read(self.pc.wrapping_add(i as u16))));
+        }
+        format!(
+            "{:04X}  {:<8} A:{:02X} X:{:02X} Y:{:02X} P:{:02X} SP:{:02X} CYC:{}",
+            self.pc, bytes, self.a, self.x, self.y, self.p, self.sp, self.cycles
+        )
+    }
+
+    // Address mode helpers
+    fn address_zp(&mut self, bus: &mut impl Bus) -> (u16, u8) { // Zero Page
+        (self.fetch(bus) as u16, 0)
+    }
+
+    fn address_zpx(&mut self, bus: &mut impl Bus) -> (u16, u8) { // Zero Page, X mode
+        (self.fetch(bus).wrapping_add(self.x) as u16, 0)
+    }
+
+    fn address_zpy(&mut self, bus: &mut impl Bus) -> (u16, u8) { // Zero Page, Y mode
+        (self.fetch(bus).wrapping_add(self.y) as u16, 0)
+    }
+
+    fn address_abs(&mut self, bus: &mut impl Bus) -> (u16, u8) { // Absolute address
+        (self.fetch16(bus), 0)
+    }
+
+    fn address_abx(&mut self, bus: &mut impl Bus) -> (u16, u8) { // Absolute, X address
+        let base: u16 = self.fetch16(bus);
+        let address: u16 = base.wrapping_add(self.x as u16);
+        let extra: u8 = if (base & 0xFF00) != (address & 0xFF00) { 1 } else { 0 };
+        (address, extra)
+    }
+
+    fn address_aby(&mut self, bus: &mut impl Bus) -> (u16, u8) { // Absolute, Y address
+        let base: u16 = self.fetch16(bus);
+        let address: u16 = base.wrapping_add(self.y as u16);
+        let extra: u8 = if (base & 0xFF00) != (address & 0xFF00) { 1 } else { 0 };
+        (address, extra)
+    }
+
+    fn address_izx(&mut self, bus: &mut impl Bus) -> (u16, u8) { // Index-Indirect mode + X
+        let zp: u8 = self.fetch(bus); // Operand: Address on Zero-Page
+        let ptr_address: u8 = zp.wrapping_add(self.x); // We add x to it wrapped
+        let low: u16 = self.read(bus, ptr_address as u16) as u16;
+        let high: u16 = self.read(bus, ptr_address.wrapping_add(1) as u16) as u16;
+        (high << 8 | low, 0) // Return in little endian, 16-bit pointer
+    }
+
+    fn address_izy(&mut self, bus: &mut impl Bus) -> (u16, u8) { // Index-Indirect mode + Y
+        let zp: u8 = self.fetch(bus); // Operand: Address on Zero-Page
+        let low: u16 = self.read(bus, zp as u16) as u16;
+        let high: u16 = self.read(bus, zp.wrapping_add(1) as u16) as u16;
+        let base: u16 = high << 8 | low; // 16-bit pointer to somewhere on WRAM, little-endian
+        let address: u16 = base.wrapping_add(self.y as u16); // Add Y to the 16-bit pointer
+        let extra: u8 = if (base & 0xFF00) != (address & 0xFF00) { 1 } else { 0 };
+        (address, extra)
+    }
+
+    fn address_ind(&mut self, bus: &mut impl Bus) -> (u16, u8) { // used by JMP instruction + hardware bug where the page wraps around
+        let ptr: u16 = self.fetch16(bus); // pointer itself
+        let low: u16 = self.read(bus, ptr) as u16;
+        let high_ptr: u16 = (ptr & 0xFF00) | (ptr.wrapping_add(1) & 0x00FF); // Hardware bug wrap here
+        let high = self.read(bus, high_ptr) as u16;
+        (high << 8 | low, 0) // little-endian Return 
+    }
+
+    fn address_rel(&mut self, bus: &mut impl Bus) -> u16 { // Branch relative
+        let offset: u8 = self.fetch(bus); // signed 8-bit offset from argument
+        self.pc.wrapping_add((offset as i8) as u16) // sign-extend + add, this allows to move the PC backwards
+    }
+
+    // NES 6502 opcode -> functions
+    fn jmp_abs(&mut self, bus: &mut impl Bus) -> u8 {  // 0x4C, 3 cycles
+        self.pc = self.fetch16(bus);
+        3
+    }
+
+    fn ldx_imm(&mut self, bus: &mut impl Bus) -> u8 {  // 0xA2, 2 cycles
+        self.x = self.fetch(bus);
+        self.set_zp_flags(self.x);
+        2
+    }
+
+    fn stx_zp(&mut self, bus: &mut impl Bus) -> u8 {   // 0x86, 3 cycles
+        let (addr, _) = self.address_zp(bus);
+        self.write(bus, addr, self.x);
+        3
+    }
+
+    // NES 6502 opcode length table/matcher
+    fn opcode_len(opcode: u8) -> u8 {
+        match opcode {
+            0x4C => 3, // JMP abs
+            0xA2 => 2, // LDX imm
+            0x86 => 2, // STX zp
+            _ => 1, // fallback, would show the invalid/unknown opcode only
+        }
+    }
+
+}
+
@@ -1,6 +1,18 @@
 mod cartridge;
 mod bus;
+mod cpu;
+
+use crate::bus::{Bus, NesBus};
+use crate::cartridge::Cartridge;
+use crate::cpu::Cpu;

 fn main() {
-    println!("Hello, world!");
+    let cart: Cartridge = Cartridge::from_file("roms/test/cpu/nestest/nestest.nes").unwrap();
+    let mut bus: NesBus = NesBus::new(cart);
+    let mut cpu = Cpu::new();
+    cpu.set_test_start(0xC000, 7);
+    loop {
+        println!("{}", cpu.trace(&mut bus));
+        cpu.step(&mut bus);
+    }
 }