feat(bus): add Io Struct

Also, add more information to all panic messages

src/bus/io.zig

@@ -0,0 +1,56 @@
+const std = @import("std");
+const bitfield = @import("bitfield");
+
+const Bitfield = bitfield.Bitfield;
+const Bit = bitfield.Bit;
+
+pub const Io = struct {
+    dispcnt: Dispcnt,
+
+    pub fn init() @This() {
+        return .{
+            .dispcnt = .{ .val = 0x0000_0000 },
+        };
+    }
+
+    pub fn read32(self: *const @This(), addr: u32) u32 {
+        return switch (addr) {
+            0x0400_0000 => @as(u32, self.dispcnt.val),
+            else => std.debug.panic("[I/O:32] tried to read from {X:}", .{addr}),
+        };
+    }
+
+    pub fn read16(self: *const @This(), addr: u32) u16 {
+        return switch (addr) {
+            0x0400_0000 => self.dispcnt.val,
+            else => std.debug.panic("[I/O:16] tried to read from {X:}", .{addr}),
+        };
+    }
+
+    pub fn write16(self: *@This(), addr: u32, halfword: u16) void {
+        switch (addr) {
+            0x0400_000 => self.dispcnt.val = halfword,
+            else => std.debug.panic("[I/O:16] tried to write {X:} to {X:}", .{ halfword, addr }),
+        }
+    }
+
+    pub fn read8(self: *const @This(), addr: u32) u8 {
+        return switch (addr) {
+            0x0400_0000 => @truncate(u8, self.dispcnt.val),
+            else => std.debug.panic("[I/O:8] tried to read from {X:}", .{addr}),
+        };
+    }
+};
+
+const Dispcnt = extern union {
+    bg_mode: Bitfield(u16, 0, 3),
+    frame_select: Bit(u16, 4),
+    hblank_interval_free: Bit(u16, 5),
+    obj_mapping: Bit(u16, 6),
+    forced_blank: Bit(u16, 7),
+    bg_enable: Bitfield(u16, 8, 4),
+    obj_enable: Bit(u16, 12),
+    win_enable: Bitfield(u16, 13, 2),
+    obj_win_enable: Bit(u16, 15),
+    val: u16,
+};