chore: implement a non-working mode 0

src/core/nds7/Bus.zig

@@ -110,7 +110,7 @@ fn _write(self: *@This(), comptime T: type, comptime mode: Mode, address: u32, v
             0b00 => writeInt(T, self.wram[aligned_addr & 0x0000_FFFF ..][0..byte_count], value),
             else => self.shr_wram.write(T, .nds7, aligned_addr, value),
         },
-        0x0380_0000...0x0380_FFFF => writeInt(T, self.wram[aligned_addr & 0x0000_FFFF ..][0..byte_count], value),
+        0x0380_0000...0x03FF_FFFF => writeInt(T, self.wram[aligned_addr & 0x0000_FFFF ..][0..byte_count], value),
         0x0400_0000...0x04FF_FFFF => io.write(self, T, aligned_addr, value),
         0x0600_0000...0x06FF_FFFF => self.vram.write(T, .nds7, aligned_addr, value),
         else => log.warn("unexpected: write(T: {}, addr: 0x{X:0>8}, value: 0x{X:0>8})", .{ T, address, value }),

src/core/nds7/io.zig

@@ -63,7 +63,7 @@ pub fn read(bus: *const Bus, comptime T: type, address: u32) T {
             0x0400_0214 => bus.io.irq.raw,
 
             0x0410_0000 => bus.io.shr.ipc.recv(.nds7),
-            else => warn("unexpected: read(T: {}, addr: 0x{X:0>8}) {} ", .{ T, address, T }),
+            else => 0, // warn("unexpected: read(T: {}, addr: 0x{X:0>8}) {} ", .{ T, address, T }),
         },
         u16 => switch (address) {
             0x0400_0004 => bus.io.ppu.?.nds7.dispstat.raw,
@@ -76,7 +76,7 @@ pub fn read(bus: *const Bus, comptime T: type, address: u32) T {
             0x0400_0130 => bus.io.shr.keyinput.load(.Monotonic),
             0x0400_0180 => @truncate(bus.io.shr.ipc._nds7.sync.raw),
             0x0400_0184 => @truncate(bus.io.shr.ipc._nds7.cnt.raw),
-            else => warn("unexpected: read(T: {}, addr: 0x{X:0>8}) {} ", .{ T, address, T }),
+            else => 0, // warn("unexpected: read(T: {}, addr: 0x{X:0>8}) {} ", .{ T, address, T }),
         },
         u8 => switch (address) {
             // DMA Transfers
@@ -89,7 +89,7 @@ pub fn read(bus: *const Bus, comptime T: type, address: u32) T {
             0x0400_0241 => bus.io.shr.wramcnt.raw,
 
             0x0400_0300 => @intFromEnum(bus.io.postflg),
-            else => warn("unexpected: read(T: {}, addr: 0x{X:0>8}) {} ", .{ T, address, T }),
+            else => 0, // warn("unexpected: read(T: {}, addr: 0x{X:0>8}) {} ", .{ T, address, T }),
         },
         else => @compileError(T ++ " is an unsupported bus read type"),
     };
@@ -110,7 +110,7 @@ pub fn write(bus: *Bus, comptime T: type, address: u32, value: T) void {
             0x0400_0214 => bus.io.irq.raw &= ~value,
 
             0x0400_0188 => bus.io.shr.ipc.send(.nds7, value),
-            else => log.warn("unexpected: write(T: {}, addr: 0x{X:0>8}, value: 0x{X:0>8})", .{ T, address, value }),
+            else => {}, // log.warn("unexpected: write(T: {}, addr: 0x{X:0>8}, value: 0x{X:0>8})", .{ T, address, value }),
         },
         u16 => switch (address) {
             // DMA Transfers
@@ -123,7 +123,7 @@ pub fn write(bus: *Bus, comptime T: type, address: u32, value: T) void {
             0x0400_0184 => bus.io.shr.ipc.setIpcFifoCnt(.nds7, value),
 
             0x0400_0208 => bus.io.ime = value & 1 == 1,
-            else => log.warn("unexpected: write(T: {}, addr: 0x{X:0>8}, value: 0x{X:0>4})", .{ T, address, value }),
+            else => {}, // log.warn("unexpected: write(T: {}, addr: 0x{X:0>8}, value: 0x{X:0>4})", .{ T, address, value }),
         },
         u8 => switch (address) {
             // DMA Transfers
@@ -133,7 +133,16 @@ pub fn write(bus: *Bus, comptime T: type, address: u32, value: T) void {
             0x0400_0100...0x0400_010F => log.warn("TODO: impl timer", .{}),
 
             0x0400_0208 => bus.io.ime = value & 1 == 1,
-            else => log.warn("unexpected: write(T: {}, addr: 0x{X:0>8}, value: 0x{X:0>2})", .{ T, address, value }),
+
+            0x0400_0301 => switch ((value >> 6) & 0b11) {
+                0b00 => bus.io.haltcnt = .execute,
+                0b10 => bus.io.haltcnt = .halt,
+                else => |val| {
+                    const tag: Haltcnt = @enumFromInt(val);
+                    log.err("TODO: Implement {}", .{tag});
+                },
+            },
+            else => {}, // log.warn("unexpected: write(T: {}, addr: 0x{X:0>8}, value: 0x{X:0>2})", .{ T, address, value }),
         },
         else => @compileError(T ++ " is an unsupported bus write type"),
     }

src/core/nds9/Bus.zig

@@ -18,6 +18,7 @@ const log = std.log.scoped(.nds9_bus);
 
 main: *[4 * MiB]u8,
 wram: *Wram,
+makeshift_palram: *[2 * KiB]u8,
 scheduler: *Scheduler,
 
 io: io.Io,
@@ -31,6 +32,7 @@ pub fn init(allocator: Allocator, scheduler: *Scheduler, ctx: SharedCtx) !@This(
     return .{
         .main = ctx.main,
         .wram = ctx.wram,
+        .makeshift_palram = try allocator.create([2 * KiB]u8),
         .ppu = try Ppu.init(allocator, ctx.vram),
         .scheduler = scheduler,
         .io = io.Io.init(ctx.io),
@@ -42,6 +44,8 @@ pub fn init(allocator: Allocator, scheduler: *Scheduler, ctx: SharedCtx) !@This(
 pub fn deinit(self: *@This(), allocator: Allocator) void {
     self.ppu.deinit(allocator);
     self.bios.deinit(allocator);
+
+    allocator.destroy(self.makeshift_palram);
 }
 
 pub fn reset(_: *@This()) void {
@@ -72,6 +76,7 @@ fn _read(self: *@This(), comptime T: type, comptime mode: Mode, address: u32) T
         0x0200_0000...0x02FF_FFFF => readInt(T, self.main[aligned_addr & 0x003F_FFFF ..][0..byte_count]),
         0x0300_0000...0x03FF_FFFF => self.wram.read(T, .nds9, aligned_addr),
         0x0400_0000...0x04FF_FFFF => io.read(self, T, aligned_addr),
+        0x0500_0000...0x05FF_FFFF => readInt(T, self.makeshift_palram[aligned_addr & (2 * KiB - 1) ..][0..@sizeOf(T)]),
         0x0600_0000...0x06FF_FFFF => self.ppu.vram.read(T, .nds9, aligned_addr),
         0xFFFF_0000...0xFFFF_FFFF => self.bios.read(T, address),
         else => warn("unexpected: read(T: {}, addr: 0x{X:0>8}) {} ", .{ T, address, T }),
@@ -102,6 +107,7 @@ fn _write(self: *@This(), comptime T: type, comptime mode: Mode, address: u32, v
         0x0200_0000...0x02FF_FFFF => writeInt(T, self.main[aligned_addr & 0x003F_FFFF ..][0..byte_count], value),
         0x0300_0000...0x03FF_FFFF => self.wram.write(T, .nds9, aligned_addr, value),
         0x0400_0000...0x04FF_FFFF => io.write(self, T, aligned_addr, value),
+        0x0500_0000...0x05FF_FFFF => writeInt(T, self.makeshift_palram[aligned_addr & (2 * KiB - 1) ..][0..@sizeOf(T)], value),
         0x0600_0000...0x06FF_FFFF => self.ppu.vram.write(T, .nds9, aligned_addr, value),
         0xFFFF_0000...0xFFFF_FFFF => self.bios.write(T, address, value),
         else => log.warn("unexpected: write(T: {}, addr: 0x{X:0>8}, value: 0x{X:0>8})", .{ T, address, value }),

src/core/nds9/io.zig

@@ -62,6 +62,8 @@ pub fn read(bus: *const Bus, comptime T: type, address: u32) T {
             0x0400_02A8, 0x0400_02AC => @truncate(bus.io.div.remainder >> shift(u64, address)),
             0x0400_02B4 => @truncate(bus.io.sqrt.result),
 
+            0x0400_1000 => bus.ppu.engines[1].dispcnt.raw,
+
             0x0410_0000 => bus.io.shr.ipc.recv(.nds9),
 
             0x0400_4000, 0x0400_4008 => 0x0000_0000, // Lets software know this is NOT a DSi
@@ -84,6 +86,20 @@ pub fn read(bus: *const Bus, comptime T: type, address: u32) T {
             0x0400_0280 => @truncate(bus.io.div.cnt.raw),
             0x0400_02B0 => @truncate(bus.io.sqrt.cnt.raw),
 
+            0x0400_1008 => bus.ppu.engines[1].bg[0].cnt.raw,
+            0x0400_100A => bus.ppu.engines[1].bg[1].cnt.raw,
+            0x0400_100C => bus.ppu.engines[1].bg[2].cnt.raw,
+            0x0400_100E => bus.ppu.engines[1].bg[3].cnt.raw,
+
+            0x0400_1010 => bus.ppu.engines[1].bg[0].hofs.raw,
+            0x0400_1012 => bus.ppu.engines[1].bg[0].vofs.raw,
+            0x0400_1014 => bus.ppu.engines[1].bg[1].hofs.raw,
+            0x0400_1016 => bus.ppu.engines[1].bg[1].vofs.raw,
+            0x0400_1018 => bus.ppu.engines[1].bg[2].hofs.raw,
+            0x0400_101A => bus.ppu.engines[1].bg[2].vofs.raw,
+            0x0400_101C => bus.ppu.engines[1].bg[3].hofs.raw,
+            0x0400_101E => bus.ppu.engines[1].bg[3].vofs.raw,
+
             else => warn("unexpected: read(T: {}, addr: 0x{X:0>8}) {} ", .{ T, address, T }),
         },
         u8 => switch (address) {
@@ -108,6 +124,7 @@ const subset = @import("../../util.zig").subset;
 pub fn write(bus: *Bus, comptime T: type, address: u32, value: T) void {
     switch (T) {
         u32 => switch (address) {
+
             // DMA Transfers
             0x0400_00B0...0x0400_00DC => log.warn("TODO: impl DMA", .{}),
             0x0400_00E0...0x0400_00EC => log.warn("TODO: impl DMA fill", .{}),
@@ -179,6 +196,20 @@ pub fn write(bus: *Bus, comptime T: type, address: u32, value: T) void {
 
             0x0400_0304 => bus.ppu.io.powcnt.raw = value,
 
+            0x0400_1008 => bus.ppu.engines[1].bg[0].cnt.raw = value,
+            0x0400_100A => bus.ppu.engines[1].bg[1].cnt.raw = value,
+            0x0400_100C => bus.ppu.engines[1].bg[2].cnt.raw = value,
+            0x0400_100E => bus.ppu.engines[1].bg[3].cnt.raw = value,
+
+            0x0400_1010 => bus.ppu.engines[1].bg[0].hofs.raw = value,
+            0x0400_1012 => bus.ppu.engines[1].bg[0].vofs.raw = value,
+            0x0400_1014 => bus.ppu.engines[1].bg[1].hofs.raw = value,
+            0x0400_1016 => bus.ppu.engines[1].bg[1].vofs.raw = value,
+            0x0400_1018 => bus.ppu.engines[1].bg[2].hofs.raw = value,
+            0x0400_101A => bus.ppu.engines[1].bg[2].vofs.raw = value,
+            0x0400_101C => bus.ppu.engines[1].bg[3].hofs.raw = value,
+            0x0400_101E => bus.ppu.engines[1].bg[3].vofs.raw = value,
+
             else => log.warn("unexpected: write(T: {}, addr: 0x{X:0>8}, value: 0x{X:0>4})", .{ T, address, value }),
         },
         u8 => switch (address) {
@@ -400,7 +431,7 @@ pub const DispcntA = extern union {
     tile_obj_1d_boundary: Bitfield(u32, 20, 2),
     bitmap_obj_1d_boundary: Bit(u32, 22),
     obj_during_hblank: Bit(u32, 23),
-    character_base: Bitfield(u32, 24, 3),
+    char_base: Bitfield(u32, 24, 3),
     screen_base: Bitfield(u32, 27, 3),
     bg_ext_pal_enable: Bit(u32, 30),
     obj_ext_pal_enable: Bit(u32, 31),
@@ -476,3 +507,23 @@ pub const Dispstat = extern union {
     lyc: Bitfield(u16, 7, 9),
     raw: u16,
 };
+
+pub const Bgcnt = extern union {
+    priority: Bitfield(u16, 0, 2),
+    char_base: Bitfield(u16, 2, 4),
+    mosaic_enable: Bit(u16, 6),
+    colour_mode: Bit(u16, 7),
+    screen_base: Bitfield(u16, 8, 5),
+    display_overflow: Bit(u16, 13),
+    size: Bitfield(u16, 14, 2),
+    raw: u16,
+};
+
+/// Write Only
+const BackgroundOffset = extern union {
+    offset: Bitfield(u16, 0, 9),
+    raw: u16,
+};
+
+pub const Hofs = BackgroundOffset;
+pub const Vofs = BackgroundOffset;

src/core/ppu.zig

@@ -21,7 +21,7 @@ pub const Ppu = struct {
 
     vram: *Vram,
 
-    engines: struct { EngineA, EngineB } = .{ .{}, .{} },
+    engines: struct { EngineA, EngineB },
 
     io: Io = .{},
 
@@ -44,20 +44,22 @@ pub const Ppu = struct {
     pub fn init(allocator: Allocator, vram: *Vram) !@This() {
         return .{
             .fb = try FrameBuffer.init(allocator),
+            .engines = .{ try EngineA.init(allocator), try EngineB.init(allocator) },
             .vram = vram,
         };
     }
 
     pub fn deinit(self: @This(), allocator: Allocator) void {
         self.fb.deinit(allocator);
+        inline for (self.engines) |eng| eng.deinit(allocator);
     }
 
     pub fn drawScanline(self: *@This(), bus: *System.Bus9) void {
         if (self.io.powcnt.engine2d_a.read())
-            self.engines[0].drawScanline(bus, &self.fb, &self.io);
+            self.engines[0].drawScanline(bus, &self.fb);
 
         if (self.io.powcnt.engine2d_b.read())
-            self.engines[1].drawScanline(bus, &self.fb, &self.io);
+            self.engines[1].drawScanline(bus, &self.fb);
     }
 
     /// HDraw -> HBlank

src/core/ppu/engine.zig

@@ -11,14 +11,15 @@ const Ppu = @import("../ppu.zig").Ppu;
 const width = @import("../ppu.zig").screen_width;
 const height = @import("../ppu.zig").screen_height;
 
+const KiB = 0x400;
+
 const EngineKind = enum { a, b };
 
 pub const EngineA = Engine(.a);
 pub const EngineB = Engine(.b);
 
 fn Engine(comptime kind: EngineKind) type {
-    const log = std.log.scoped(.engine2d);
+    const log = std.log.scoped(.engine2d); // TODO: specify between 2D-A and 2D-B
-    _ = log; // TODO: specify between 2D-A and 2D-B
 
     // FIXME: don't commit zig crimes
     const Type = struct {
@@ -30,25 +31,79 @@ fn Engine(comptime kind: EngineKind) type {
     return struct {
         dispcnt: Type(DispcntA, DispcntB) = .{ .raw = 0x0000_0000 },
 
-        pub fn drawScanline(self: *@This(), bus: *Bus, fb: *FrameBuffer, io: *Ppu.Io) void {
+        bg: [4]bg.Text = .{ .{}, .{}, .{}, .{} },
+
+        // TODO: Rename
+        scanline: Scanline,
+
+        pub fn init(allocator: std.mem.Allocator) !@This() {
+            return .{
+                .scanline = try Scanline.init(allocator),
+            };
+        }
+
+        pub fn deinit(self: @This(), allocator: std.mem.Allocator) void {
+            self.scanline.deinit(allocator);
+        }
+
+        pub fn drawScanline(self: *@This(), bus: *Bus, fb: *FrameBuffer) void {
             const disp_mode = self.dispcnt.display_mode.read();
 
             switch (disp_mode) {
                 0 => { // Display Off
                     const buf = switch (kind) {
-                        .a => if (io.powcnt.display_swap.read()) fb.top(.back) else fb.btm(.back),
+                        .a => if (bus.ppu.io.powcnt.display_swap.read()) fb.top(.back) else fb.btm(.back),
-                        .b => if (io.powcnt.display_swap.read()) fb.btm(.back) else fb.top(.back),
+                        .b => if (bus.ppu.io.powcnt.display_swap.read()) fb.btm(.back) else fb.top(.back),
                     };
 
                     @memset(buf, 0xFF); // set everything to white
                 },
-                1 => @panic("TODO: standard graphics display (text mode, etc)"),
+                1 => {
+                    const bg_mode = self.dispcnt.bg_mode.read();
+                    const bg_enable = self.dispcnt.bg_enable.read();
+                    const scanline: u32 = bus.ppu.io.nds9.vcount.scanline.read();
+
+                    switch (bg_mode) {
+                        //  BG0     BG1     BG2     BG3
+                        //  Text/3D Text    Text    Text
+                        0 => {
+                            // TODO: Fetch Sprites
+
+                            for (0..4) |layer| {
+                                // TODO: Draw Sprites
+
+                                inline for (0..4) |i| {
+                                    if (layer == self.bg[i].cnt.priority.read() and (bg_enable >> i) & 1 == 1)
+                                        self.drawBackground(i, bus);
+                                }
+                            }
+
+                            // TODO: IN ZBA we calculate the base address of the emu framebuffer to pass in
+
+                            const buf = switch (kind) {
+                                .a => if (bus.ppu.io.powcnt.display_swap.read()) fb.top(.back) else fb.btm(.back),
+                                .b => if (bus.ppu.io.powcnt.display_swap.read()) fb.btm(.back) else fb.top(.back),
+                            };
+
+                            const scanline_buf = blk: {
+                                const rgba_ptr: *[width * height]u32 = @ptrCast(@alignCast(buf));
+                                break :blk rgba_ptr[width * scanline ..][0..width];
+                            };
+
+                            self.renderTextMode(scanline_buf);
+                        },
+                        else => |mode| {
+                            log.err("TODO: Implement Mode {}", .{mode});
+                            @panic("fatal error");
+                        },
+                    }
+                },
                 2 => { // VRAM display
                     if (kind == .b) return;
                     // TODO: Master Brightness can still affect this mode
 
-                    const scanline: u32 = io.nds9.vcount.scanline.read();
+                    const scanline: u32 = bus.ppu.io.nds9.vcount.scanline.read();
-                    const buf = if (io.powcnt.display_swap.read()) fb.top(.back) else fb.btm(.back);
+                    const buf = if (bus.ppu.io.powcnt.display_swap.read()) fb.top(.back) else fb.btm(.back);
 
                     const scanline_buf = blk: {
                         const rgba_ptr: *[width * height]u32 = @ptrCast(@alignCast(buf));
@@ -69,9 +124,210 @@ fn Engine(comptime kind: EngineKind) type {
                 },
             }
         }
+
+        fn drawBackground(self: *@This(), comptime layer: u2, bus: *Bus) void {
+            const screen_base = blk: {
+                const bgcnt_off: u32 = self.bg[layer].cnt.screen_base.read();
+                const dispcnt_off: u32 = if (kind == .a) self.dispcnt.screen_base.read() else 0;
+
+                break :blk (2 * KiB) * bgcnt_off + (64 * KiB) * dispcnt_off;
+            };
+
+            const char_base = blk: {
+                const bgcnt_off: u32 = self.bg[layer].cnt.char_base.read();
+                const dispcnt_off: u32 = if (kind == .a) self.dispcnt.char_base.read() else 0;
+
+                break :blk (16 * KiB) * bgcnt_off + (64 * KiB) * dispcnt_off;
+            };
+
+            const is_8bpp = self.bg[layer].cnt.colour_mode.read();
+            const size = self.bg[layer].cnt.size.read();
+
+            // In 4bpp: 1 byte represents two pixels so the length is (8 x 8) / 2
+            // In 8bpp: 1 byte represents one pixel so the length is 8 x 8
+            const tile_len: u32 = if (is_8bpp) 0x40 else 0x20;
+            const tile_row_offset: u32 = if (is_8bpp) 0x8 else 0x4;
+
+            const vofs: u32 = self.bg[layer].vofs.offset.read();
+            const hofs: u32 = self.bg[layer].hofs.offset.read();
+
+            const y = vofs + bus.ppu.io.nds9.vcount.scanline.read();
+
+            for (0..width) |_i| {
+                const i: u32 = @intCast(_i);
+                const x = hofs + i;
+
+                // TODO: Windowing
+
+                // Grab the Screen Entry from VRAM
+                const entry_addr = 0x0600_0000 + screen_base + tilemapOffset(size, x, y);
+                const entry: bg.Screen.Entry = @bitCast(bus.read(u16, entry_addr));
+
+                // Calculate the Address of the Tile in the designated Charblock
+                // We also take this opportunity to flip tiles if necessary
+                const tile_id: u32 = entry.tile_id.read();
+
+                // Calculate row and column offsets. Understand that
+                // `tile_len`, `tile_row_offset` and `col` are subject to different
+                // values depending on whether we are in 4bpp or 8bpp mode.
+                const row = @as(u3, @truncate(y)) ^ if (entry.v_flip.read()) 7 else @as(u3, 0);
+                const col = @as(u3, @truncate(x)) ^ if (entry.h_flip.read()) 7 else @as(u3, 0);
+                const tile_addr = char_base + (tile_id * tile_len) + (row * tile_row_offset) + if (is_8bpp) col else col >> 1;
+
+                const tile = bus.read(u8, 0x0600_0000 + tile_addr);
+                // If we're in 8bpp, then the tile value is an index into the palette,
+                // If we're in 4bpp, we have to account for a pal bank value in the Screen entry
+                // and then we can index the palette
+                const pal_id: u16 = if (!is_8bpp) get4bppTilePalette(entry.pal_bank.read(), col, tile) else tile;
+
+                if (pal_id != 0) self.drawBackgroundPixel(layer, i, bus.read(u16, @as(u32, 0x0500_0000) + pal_id * 2));
+            }
+        }
+
+        inline fn get4bppTilePalette(pal_bank: u4, col: u3, tile: u8) u8 {
+            const nybble_tile = tile >> ((col & 1) << 2) & 0xF;
+            if (nybble_tile == 0) return 0;
+
+            return (@as(u8, pal_bank) << 4) | nybble_tile;
+        }
+
+        fn renderTextMode(self: *@This(), frame_buf: []u32) void {
+            for (self.scanline.top(), 0..) |maybe_top, i| {
+                const maybe_btm = self.scanline.btm()[i];
+                _ = maybe_btm;
+
+                const bgr555 = switch (maybe_top) {
+                    .set => |px| px,
+                    else => 0xAAAA,
+                };
+
+                frame_buf[i] = rgba888(bgr555);
+            }
+
+            self.scanline.reset();
+        }
+
+        // TODO: Comment this + get a better understanding
+        fn tilemapOffset(size: u2, x: u32, y: u32) u32 {
+            // Current Row: (y % PIXEL_COUNT) / 8
+            // Current COlumn: (x % PIXEL_COUNT) / 8
+            // Length of 1 row of Screen Entries: 0x40
+            // Length of 1 Screen Entry: 0x2 is the size of a screen entry
+            @setRuntimeSafety(false);
+
+            return switch (size) {
+                0 => (x % 256 / 8) * 2 + (y % 256 / 8) * 0x40, // 256 x 256
+                1 => blk: {
+                    // 512 x 256
+                    const offset: u32 = if (x & 0x1FF > 0xFF) 0x800 else 0;
+                    break :blk offset + (x % 256 / 8) * 2 + (y % 256 / 8) * 0x40;
+                },
+                2 => blk: {
+                    // 256 x 512
+                    const offset: u32 = if (y & 0x1FF > 0xFF) 0x800 else 0;
+                    break :blk offset + (x % 256 / 8) * 2 + (y % 256 / 8) * 0x40;
+                },
+                3 => blk: {
+                    // 512 x 512
+                    const offset: u32 = if (x & 0x1FF > 0xFF) 0x800 else 0;
+                    const offset_2: u32 = if (y & 0x1FF > 0xFF) 0x800 else 0;
+                    break :blk offset + offset_2 + (x % 256 / 8) * 2 + (y % 512 / 8) * 0x40;
+                },
             };
         }
 
+        fn drawBackgroundPixel(self: *@This(), comptime layer: u2, i: u32, bgr555: u16) void {
+            _ = layer;
+
+            self.scanline.top()[i] = Scanline.Pixel.from(.Background, bgr555);
+        }
+    };
+}
+
+const Scanline = struct {
+    const Pixel = union(enum) {
+        // TODO: Rename
+        const Layer = enum { Background, Sprite };
+
+        set: u16,
+        obj_set: u16,
+        unset: void,
+        hidden: void,
+
+        fn from(comptime layer: Layer, bgr555: u16) Pixel {
+            return switch (layer) {
+                .Background => .{ .set = bgr555 },
+                .Sprite => .{ .obj_set = bgr555 },
+            };
+        }
+
+        pub fn isSet(self: @This()) bool {
+            return switch (self) {
+                .set, .obj_set => true,
+                .unset, .hidden => false,
+            };
+        }
+    };
+
+    layers: [2][]Pixel,
+    buf: []Pixel,
+
+    fn init(allocator: std.mem.Allocator) !@This() {
+        const buf = try allocator.alloc(Pixel, width * 2); // Top & Bottom Scanline
+        @memset(buf, .unset);
+
+        return .{
+            // Top & Bototm Layers
+            .layers = [_][]Pixel{ buf[0..][0..width], buf[width..][0..width] },
+            .buf = buf,
+        };
+    }
+
+    fn reset(self: *@This()) void {
+        @memset(self.buf, .unset);
+    }
+
+    fn deinit(self: @This(), allocator: std.mem.Allocator) void {
+        allocator.free(self.buf);
+    }
+
+    fn top(self: *@This()) []Pixel {
+        return self.layers[0];
+    }
+
+    fn btm(self: *@This()) []Pixel {
+        return self.layers[1];
+    }
+};
+
+const bg = struct {
+    const Text = struct {
+        const io = @import("../nds9/io.zig");
+
+        /// Read / Write
+        cnt: io.Bgcnt = .{ .raw = 0x0000 },
+        /// Write Only
+        hofs: io.Hofs = .{ .raw = 0x0000 },
+        /// Write Only
+        vofs: io.Vofs = .{ .raw = 0x0000 },
+    };
+
+    const Screen = struct {
+        const Entry = extern union {
+            const Bitfield = @import("bitfield").Bitfield;
+            const Bit = @import("bitfield").Bit;
+
+            tile_id: Bitfield(u16, 0, 10),
+            h_flip: Bit(u16, 10),
+            v_flip: Bit(u16, 11),
+            pal_bank: Bitfield(u16, 12, 4),
+            raw: u16,
+        };
+    };
+
+    const Affine = @compileError("TODO: Implement Affine Backgrounds");
+};
+
 inline fn rgba888(bgr555: u16) u32 {
     const b: u32 = bgr555 >> 10 & 0x1F;
     const g: u32 = bgr555 >> 5 & 0x1F;