6soz/src/system/nes/bus.zig

const std = @import("std");
const common = @import("common.zig");
const Region = common.Region;
const Mirroring = common.Mirroring;
const Ppu = @import("ppu/root.zig").Ppu;
const Apu = @import("apu/root.zig").Apu;
const Controller = @import("controller.zig").Controller;
const Cartridge = @import("cartridge.zig").Cartridge;

pub const Bus = @This();

ram: [0x800]u8 = [_]u8{0} ** 0x800,
ciram: [0x800]u8 = [_]u8{0} ** 0x800,

ppu: Ppu,
apu: Apu,
controllers: [2]Controller = .{ Controller.init(), Controller.init() },
cartridge: ?Cartridge = null,

open_bus: u8 = 0,

// OAM DMA state machine
dma_page: u8 = 0,
dma_addr: u8 = 0,
dma_active: bool = false,
dma_dummy: bool = true,

pub fn init(region: Region, cart: ?Cartridge) Bus {
    return .{
        .ppu = Ppu.init(region),
        .apu = Apu.init(region),
        .cartridge = cart,
    };
}

pub fn reset(self: *Bus) void {
    self.ppu.reset();
    self.apu.reset();
    self.controllers[0].reset();
    self.controllers[1].reset();
    self.dma_active = false;
    self.dma_dummy = true;
    self.dma_addr = 0;
}

pub fn mapNametableAddress(mirroring_mode: Mirroring, address: u16) u11 {
    const v = (address - 0x2000) & 0x0fff;
    return switch (mirroring_mode) {
        .horizontal => @truncate(((v >> 1) & 0x0400) | (v & 0x03ff)),
        .vertical => @truncate(v & 0x07ff),
        .single_screen_lower => @truncate(v & 0x03ff),
        .single_screen_upper => @truncate(0x0400 | (v & 0x03ff)),
        .four_screen => @truncate(v & 0x07ff),
    };
}

pub const PpuBus = struct {
    bus: *Bus,

    pub fn read(self: *PpuBus, address: u16) u8 {
        if (address < 0x2000) {
            if (self.bus.cartridge) |*c| {
                return c.ppuRead(address) orelse 0;
            }
            return 0;
        } else if (address < 0x3f00) {
            const mir = if (self.bus.cartridge) |*c| c.mirroring() else .horizontal;
            const ciram_idx = mapNametableAddress(mir, address);
            return self.bus.ciram[ciram_idx];
        } else {
            return self.bus.ppu.palette.read(address);
        }
    }

    pub fn write(self: *PpuBus, address: u16, value: u8) void {
        if (address < 0x2000) {
            if (self.bus.cartridge) |*c| {
                _ = c.ppuWrite(address, value);
            }
        } else if (address < 0x3f00) {
            const mir = if (self.bus.cartridge) |*c| c.mirroring() else .horizontal;
            const ciram_idx = mapNametableAddress(mir, address);
            self.bus.ciram[ciram_idx] = value;
        } else {
            self.bus.ppu.palette.write(address, value);
        }
    }
};

pub inline fn getPpuBus(self: *Bus) PpuBus {
    return PpuBus{ .bus = self };
}

pub fn read(self: *Bus, address: u16) u8 {
    const val: u8 = switch (address) {
        0x0000...0x1fff => self.ram[address & 0x07ff],
        0x2000...0x3fff => blk: {
            var ppu_bus = self.getPpuBus();
            break :blk self.ppu.cpuRead(&ppu_bus, address) orelse self.open_bus;
        },
        0x4000...0x4014 => self.open_bus,
        0x4015 => self.apu.readWithOpenBus(address, self.open_bus) orelse self.open_bus,
        0x4016 => self.controllers[0].read(),
        0x4017 => self.controllers[1].read(),
        0x4018...0x401f => self.open_bus,
        0x4020...0xffff => blk: {
            if (self.cartridge) |*c| {
                break :blk c.cpuRead(address) orelse self.open_bus;
            }
            break :blk self.open_bus;
        },
    };
    self.open_bus = val;
    return val;
}

pub fn write(self: *Bus, address: u16, value: u8) void {
    self.open_bus = value;
    switch (address) {
        0x0000...0x1fff => self.ram[address & 0x07ff] = value,
        0x2000...0x3fff => {
            var ppu_bus = self.getPpuBus();
            _ = self.ppu.cpuWrite(&ppu_bus, address, value);
        },
        0x4000...0x4013, 0x4015, 0x4017 => self.apu.write(address, value),
        0x4014 => {
            self.dma_page = value;
            self.dma_addr = 0;
            self.dma_active = true;
            self.dma_dummy = true;
        },
        0x4016 => {
            self.controllers[0].write(value);
            self.controllers[1].write(value);
        },
        0x4018...0x401f => {},
        0x4020...0xffff => {
            if (self.cartridge) |*c| {
                _ = c.cpuWrite(address, value);
            }
        },
    }
}

/// Clocks one CPU cycle: steps APU and 3 PPU dots.
pub fn tick(self: *Bus) void {
    self.apu.clockCpu();
    self.ppu.registers.tickCpuCycle();

    var ppu_bus = self.getPpuBus();
    self.ppu.clock(&ppu_bus);
    self.ppu.clock(&ppu_bus);
    self.ppu.clock(&ppu_bus);
}

/// Executes one DMA CPU cycle step.
pub fn stepDma(self: *Bus) void {
    if (self.dma_dummy) {
        self.dma_dummy = false;
        self.tick();
        return;
    }

    const cpu_addr: u16 = (@as(u16, self.dma_page) << 8) | self.dma_addr;
    const data = self.read(cpu_addr);
    self.ppu.registers.oam[self.ppu.registers.oam_addr] = data;
    self.ppu.registers.oam_addr +%= 1;
    self.dma_addr +%= 1;

    if (self.dma_addr == 0) {
        self.dma_active = false;
    }
    self.tick();
}

pub fn nmiLine(self: *const Bus) bool {
    return self.ppu.nmiLine();
}

pub fn irqLine(self: *const Bus) bool {
    const apu_irq = self.apu.irqAsserted();
    const cart_irq = if (self.cartridge) |*c| c.irqLine() else false;
    return apu_irq or cart_irq;
}

// consolidated bus memory mirroring test suite
test "bus internal RAM and CIRAM nametable mirroring" {
    var bus = Bus.init(.ntsc, null);
    bus.write(0x0005, 0x99);
    try std.testing.expectEqual(@as(u8, 0x99), bus.read(0x0805));

    var ppu_bus = bus.getPpuBus();
    ppu_bus.write(0x2000, 0xaa);
    try std.testing.expectEqual(@as(u8, 0xaa), ppu_bus.read(0x2400));
}