diff --git a/src/core/io.zig b/src/core/io.zig
index 51d1a37..9b7403c 100644
--- a/src/core/io.zig
+++ b/src/core/io.zig
@@ -204,6 +204,33 @@ const IpcFifoCnt = extern union {
     raw: u32,
 };
 
+pub const masks = struct {
+    const Bus9 = @import("nds9/Bus.zig");
+    const Bus7 = @import("nds7/Bus.zig");
+
+    pub inline fn ipcFifoSync(bus: anytype, value: anytype) @TypeOf(value) {
+        comptime verifyBusType(@TypeOf(bus));
+        const T = @TypeOf(value);
+        const mask: T = 0xF;
+
+        return value & ~mask | @as(T, @intCast(bus.io.shared.ipc_fifo.sync.raw & mask));
+    }
+
+    pub inline fn ipcFifoCnt(bus: anytype, value: anytype) @TypeOf(value) {
+        comptime verifyBusType(@TypeOf(bus));
+        const T = @TypeOf(value);
+        const mask: T = 0x0303;
+
+        return value & ~mask | @as(T, @intCast(bus.io.shared.ipc_fifo.cnt.raw & mask));
+    }
+
+    fn verifyBusType(comptime BusT: type) void {
+        std.debug.assert(@typeInfo(BusT) == .Pointer);
+        std.debug.assert(@typeInfo(BusT).Pointer.size == .One);
+        std.debug.assert(@typeInfo(BusT).Pointer.child == Bus9 or @typeInfo(BusT).Pointer.child == Bus7);
+    }
+};
+
 pub const nds7 = struct {
     pub const IntEnable = extern union {
         raw: u32,
diff --git a/src/core/nds7/io.zig b/src/core/nds7/io.zig
index 4dcbf03..7dae8c5 100644
--- a/src/core/nds7/io.zig
+++ b/src/core/nds7/io.zig
@@ -5,8 +5,7 @@ const Bit = @import("bitfield").Bit;
 
 const Bus = @import("Bus.zig");
 const SharedIo = @import("../io.zig").Io;
-const writeToAddressOffset = @import("../io.zig").writeToAddressOffset;
-const valueAtAddressOffset = @import("../io.zig").valueAtAddressOffset;
+const masks = @import("../io.zig").masks;
 
 const log = std.log.scoped(.nds7_io);
 
@@ -51,13 +50,8 @@ pub fn write(bus: *Bus, comptime T: type, address: u32, value: T) void {
             else => log.warn("unexpected: write(T: {}, addr: 0x{X:0>8}, value: 0x{X:0>8})", .{ T, address, value }),
         },
         u16 => switch (address) {
-            0x0400_0180 => bus.io.shared.ipc_fifo.sync.raw = blk: {
-                const ret = value & ~@as(u16, 0xF) | (bus.io.shared.ipc_fifo.sync.raw & 0xF);
-                log.debug("IPCFIFOSYNC <- 0x{X:0>8}", .{ret});
-
-                break :blk ret;
-            },
-            0x0400_0184 => bus.io.shared.ipc_fifo.cnt.raw = value,
+            0x0400_0180 => bus.io.shared.ipc_fifo.sync.raw = masks.ipcFifoSync(bus, value),
+            0x0400_0184 => bus.io.shared.ipc_fifo.cnt.raw = masks.ipcFifoCnt(bus, value),
             else => log.warn("unexpected: write(T: {}, addr: 0x{X:0>8}, value: 0x{X:0>8})", .{ T, address, value }),
         },
         u8 => switch (address) {
diff --git a/src/core/nds9/io.zig b/src/core/nds9/io.zig
index 38e1bc2..1421830 100644
--- a/src/core/nds9/io.zig
+++ b/src/core/nds9/io.zig
@@ -5,8 +5,7 @@ const Bit = @import("bitfield").Bit;
 
 const Bus = @import("Bus.zig");
 const SharedIo = @import("../io.zig").Io;
-const writeToAddressOffset = @import("../io.zig").writeToAddressOffset;
-const valueAtAddressOffset = @import("../io.zig").valueAtAddressOffset;
+const masks = @import("../io.zig").masks;
 
 const log = std.log.scoped(.nds9_io);
 
@@ -54,13 +53,8 @@ pub fn write(bus: *Bus, comptime T: type, address: u32, value: T) void {
     switch (T) {
         u32 => switch (address) {
             0x0400_0000 => bus.ppu.io.dispcnt_a.raw = value,
-            0x0400_0180 => bus.io.shared.ipc_fifo.sync.raw = blk: {
-                const ret = value & ~@as(u32, 0xF) | (bus.io.shared.ipc_fifo.sync.raw & 0xF);
-                log.debug("IPCFIFOSYNC <- 0x{X:0>8}", .{ret});
-
-                break :blk ret;
-            },
-            0x0400_0184 => bus.io.shared.ipc_fifo.cnt.raw = value,
+            0x0400_0180 => bus.io.shared.ipc_fifo.sync.raw = masks.ipcFifoSync(bus, value),
+            0x0400_0184 => bus.io.shared.ipc_fifo.cnt.raw = masks.ipcFifoCnt(bus, value),
             0x0400_0188 => bus.io.shared.ipc_fifo.send(.arm9, value) catch |e| std.debug.panic("IPC FIFO Error: {}", .{e}),
 
             0x0400_0240 => {
@@ -79,13 +73,8 @@ pub fn write(bus: *Bus, comptime T: type, address: u32, value: T) void {
             else => log.warn("unexpected: write(T: {}, addr: 0x{X:0>8}, value: 0x{X:0>8})", .{ T, address, value }),
         },
         u16 => switch (address) {
-            0x0400_0180 => bus.io.shared.ipc_fifo.sync.raw = blk: {
-                const ret = value & ~@as(u16, 0xF) | (bus.io.shared.ipc_fifo.sync.raw & 0xF);
-                log.debug("IPCFIFOSYNC <- 0x{X:0>8}", .{ret});
-
-                break :blk ret;
-            },
-            0x0400_0184 => bus.io.shared.ipc_fifo.cnt.raw = value,
+            0x0400_0180 => bus.io.shared.ipc_fifo.sync.raw = masks.ipcFifoSync(bus, value),
+            0x0400_0184 => bus.io.shared.ipc_fifo.cnt.raw = masks.ipcFifoCnt(bus, value),
             0x0400_0208 => bus.io.shared.ime = value & 1 == 1,
 
             else => log.warn("unexpected: write(T: {}, addr: 0x{X:0>8}, value: 0x{X:0>8})", .{ T, address, value }),