diff --git a/Assets/gamedb/gamedb_n64.txt b/Assets/gamedb/gamedb_n64.txt
index ecc5fc9b51..4ceb1ec2e0 100644
--- a/Assets/gamedb/gamedb_n64.txt
+++ b/Assets/gamedb/gamedb_n64.txt
@@ -1,3 +1,4 @@
+; these are for the .z64 (no byte swapping) format -- EmuHawk automatically converts .v64 and .n64 to that format before hashing
 167C3C433DEC1F1EB921736F7D53FAC8CB45EE31	G	007 - GoldenEye (Europe)	N64		Glide_fb_smart=true;Glide64mk2_enable_hacks_for_game=8;Glide64mk2_filtering=1;Glide64mk2_fb_smart=true;Jabo_Clear_Frame=2
 2A5DADE32F7FAD6C73C659D2026994632C1B3174	G	007 - GoldenEye (Japan)	N64		Glide_fb_smart=true;Glide64mk2_enable_hacks_for_game=8;Glide64mk2_filtering=1;Glide64mk2_fb_smart=true
 ABE01E4AEB033B6C0836819F549C791B26CFDE83	G	007 - GoldenEye (USA)	N64		RiceEnableHacksForGame=19;RiceZHack=true;RiceFrameBufferOption=1;RiceScreenUpdateSettingHack=4;Glide_fb_smart=true;Glide64mk2_enable_hacks_for_game=8;Glide64mk2_filtering=1;Glide64mk2_fb_smart=true;Jabo_Clear_Frame=2
diff --git a/src/BizHawk.Client.Common/RomGame.cs b/src/BizHawk.Client.Common/RomGame.cs
index e00093f7ce..51be2e127d 100644
--- a/src/BizHawk.Client.Common/RomGame.cs
+++ b/src/BizHawk.Client.Common/RomGame.cs
@@ -96,16 +96,7 @@ namespace BizHawk.Client.Common
 				RomData = DeInterleaveSMD(RomData);
 			}
 
-			if (file.Extension == ".z64" || file.Extension == ".n64" || file.Extension == ".v64")
-			{
-				// Use a simple magic number to detect N64 rom format, then byteswap the ROM to ensure a consistent endianness/order
-				RomData = RomData[0] switch
-				{
-					0x37 => EndiannessUtils.ByteSwap16(RomData), // V64 format (byte swapped)
-					0x40 => EndiannessUtils.ByteSwap32(RomData), // N64 format (word swapped)
-					_ => RomData // Z64 format (no swap), or something unexpected; in either case do nothing
-				};
-			}
+			if (file.Extension is ".n64" or ".v64" or ".z64") N64RomByteswapper.ToZ64Native(RomData); //TODO don't use file extension for N64 rom detection (yes that means detecting all formats before converting to Z64)
 
 			// note: this will be taking several hashes, of a potentially large amount of data.. yikes!
 			GameInfo = Database.GetGameInfo(RomData, file.Name);
diff --git a/src/BizHawk.Client.Common/lua/LuaHelperLibs/BitLuaLibrary.cs b/src/BizHawk.Client.Common/lua/LuaHelperLibs/BitLuaLibrary.cs
index 76bf02cd12..afd57927b2 100644
--- a/src/BizHawk.Client.Common/lua/LuaHelperLibs/BitLuaLibrary.cs
+++ b/src/BizHawk.Client.Common/lua/LuaHelperLibs/BitLuaLibrary.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Buffers.Binary;
 using System.ComponentModel;
 
 // ReSharper disable UnusedMember.Global
@@ -99,26 +100,16 @@ namespace BizHawk.Client.Common
 		[LuaMethodExample("local usbitbyt = bit.byteswap_16( 100 );")]
 		[LuaMethod("byteswap_16", "Byte swaps 'short', i.e. bit.byteswap_16(0xFF00) would return 0x00FF")]
 		public static ushort Byteswap16(ushort val)
-		{
-			return (ushort)((val & 0xFFU) << 8 | (val & 0xFF00U) >> 8);
-		}
+			=> BinaryPrimitives.ReverseEndianness(val);
 
 		[LuaMethodExample("local uibitbyt = bit.byteswap_32( 1000 );")]
 		[LuaMethod("byteswap_32", "Byte swaps 'dword'")]
 		public static uint Byteswap32(uint val)
-		{
-			return (val & 0x000000FFU) << 24 | (val & 0x0000FF00U) << 8 |
-				(val & 0x00FF0000U) >> 8 | (val & 0xFF000000U) >> 24;
-		}
+			=> BinaryPrimitives.ReverseEndianness(val);
 
 		[LuaMethodExample("local ulbitbyt = bit.byteswap_64( 10000 );")]
-		[LuaMethod("byteswap_64", "Byte swaps 'long'")]
+		[LuaMethod("byteswap_64", "Byte swaps 'long' (NOTE: You may get unexpected results for large numbers! In current Lua engines, all numbers are double-precision floating-point, even if you intended to use integers.)")]
 		public static ulong Byteswap64(ulong val)
-		{
-			return (val & 0x00000000000000FFUL) << 56 | (val & 0x000000000000FF00UL) << 40 |
-				(val & 0x0000000000FF0000UL) << 24 | (val & 0x00000000FF000000UL) << 8 |
-				(val & 0x000000FF00000000UL) >> 8 | (val & 0x0000FF0000000000UL) >> 24 |
-				(val & 0x00FF000000000000UL) >> 40 | (val & 0xFF00000000000000UL) >> 56;
-		}
+			=> BinaryPrimitives.ReverseEndianness(val);
 	}
 }
diff --git a/src/BizHawk.Common/EndiannessUtils.cs b/src/BizHawk.Common/EndiannessUtils.cs
index 2da5288a27..fd64c38d00 100644
--- a/src/BizHawk.Common/EndiannessUtils.cs
+++ b/src/BizHawk.Common/EndiannessUtils.cs
@@ -5,29 +5,10 @@ namespace BizHawk.Common
 {
 	public static unsafe class EndiannessUtils
 	{
-		/// <summary>reverses pairs of octets: <c>0xAABBIIJJPPQQYYZZ</c> &lt;=> <c>0xBBAAJJIIQQPPZZYY</c></summary>
-		public static byte[] ByteSwap16(byte[] a)
-		{
-			var l = a.Length;
-			var copy = new byte[l];
-			Array.Copy(a, copy, l);
-			MutatingByteSwap16(copy);
-			return copy;
-		}
-
-		/// <summary>reverses groups of 4 octets: <c>0xAABBCCDDWWXXYYZZ</c> &lt;=> <c>0xDDCCBBAAZZYYXXWW</c></summary>
-		public static byte[] ByteSwap32(byte[] a)
-		{
-			var l = a.Length;
-			var copy = new byte[l];
-			Array.Copy(a, copy, l);
-			MutatingByteSwap32(copy);
-			return copy;
-		}
-
 		/// <summary>reverses pairs of octets in-place: <c>0xAABBIIJJPPQQYYZZ</c> &lt;=> <c>0xBBAAJJIIQQPPZZYY</c></summary>
-		public static void MutatingByteSwap16(byte[] a)
+		public static void MutatingByteSwap16(Span<byte> a)
 		{
+#if true //TODO benchmark (both methods work correctly); also there is another method involving shifting the (output copy of the) array over by 1 byte then manually writing every second byte
 			var l = a.Length;
 			Debug.Assert(l % 2 == 0);
 			fixed (byte* p = &a[0]) for (var i = 0; i < l; i += 2)
@@ -36,11 +17,17 @@ namespace BizHawk.Common
 				p[i] = p[i + 1];
 				p[i + 1] = b;
 			}
+#else
+			Debug.Assert(a.Length % 2 == 0);
+			var shorts = MemoryMarshal.Cast<byte, ushort>(a);
+			for (var i = 0; i < shorts.Length; i++) shorts[i] = BinaryPrimitives.ReverseEndianness(shorts[i]);
+#endif
 		}
 
 		/// <summary>reverses groups of 4 octets in-place: <c>0xAABBCCDDWWXXYYZZ</c> &lt;=> <c>0xDDCCBBAAZZYYXXWW</c></summary>
-		public static void MutatingByteSwap32(byte[] a)
+		public static void MutatingByteSwap32(Span<byte> a)
 		{
+#if true //TODO benchmark (both methods work correctly)
 			var l = a.Length;
 			Debug.Assert(l % 4 == 0);
 			fixed (byte* p = &a[0]) for (var i = 0; i < l; i += 4)
@@ -52,6 +39,11 @@ namespace BizHawk.Common
 				p[i + 1] = p[i + 2];
 				p[i + 2] = b;
 			}
+#else
+			Debug.Assert(a.Length % 4 == 0);
+			var ints = MemoryMarshal.Cast<byte, uint>(a);
+			for (var i = 0; i < ints.Length; i++) ints[i] = BinaryPrimitives.ReverseEndianness(ints[i]);
+#endif
 		}
 	}
 }
diff --git a/src/BizHawk.Emulation.Common/N64RomByteswapper.cs b/src/BizHawk.Emulation.Common/N64RomByteswapper.cs
new file mode 100644
index 0000000000..a802b1a0f0
--- /dev/null
+++ b/src/BizHawk.Emulation.Common/N64RomByteswapper.cs
@@ -0,0 +1,22 @@
+using System;
+
+using BizHawk.Common;
+
+namespace BizHawk.Emulation.Common
+{
+	/// <summary>Uses a simple magic number to detect N64 rom format, then byteswaps the ROM to ensure a consistent endianness/order</summary>
+	/// <remarks>http://n64dev.org/romformats.html</remarks>
+	public static class N64RomByteswapper
+	{
+		/// <remarks>not actually magic, just always the same in commercial carts? https://n64brew.dev/wiki/ROM_Header works all the same</remarks>
+		private static readonly byte[] MAGIC_BYTES = { 0x80, 0x37, 0x12, 0x40 };
+
+		/// <summary>ensures <paramref name="rom"/> is in the native (<c>.z64</c>) format, mutating it in-place if necessary</summary>
+		public static void ToZ64Native(Span<byte> rom)
+		{
+			if (rom[0] == MAGIC_BYTES[1]) EndiannessUtils.MutatingByteSwap16(rom); // byte-swapped (.v64)
+			else if (rom[0] == MAGIC_BYTES[3]) EndiannessUtils.MutatingByteSwap32(rom); // rare little-endian .n64
+			// else already native (.z64)
+		}
+	}
+}
diff --git a/src/BizHawk.Tests/Common/EndiannessUtils/EndiannessUtilsTests.cs b/src/BizHawk.Tests/Common/EndiannessUtils/EndiannessUtilsTests.cs
index 05038f872f..0d96a33391 100644
--- a/src/BizHawk.Tests/Common/EndiannessUtils/EndiannessUtilsTests.cs
+++ b/src/BizHawk.Tests/Common/EndiannessUtils/EndiannessUtilsTests.cs
@@ -1,4 +1,4 @@
-using System.Linq;
+using System;
 
 using BizHawk.Common;
 
@@ -14,17 +14,23 @@ namespace BizHawk.Tests.Common
 		[TestMethod]
 		public void TestByteSwap16()
 		{
-			var a = new byte[] { 0x23, 0x01, 0x67, 0x45, 0xAB, 0x89, 0xEF, 0xCD };
+			var b = new byte[] { 0x23, 0x01, 0x67, 0x45, 0xAB, 0x89, 0xEF, 0xCD }.AsSpan();
+			var a = b.ToArray().AsSpan();
 			EndiannessUtils.MutatingByteSwap16(a);
 			Assert.IsTrue(a.SequenceEqual(expected));
+			EndiannessUtils.MutatingByteSwap16(a);
+			Assert.IsTrue(a.SequenceEqual(b));
 		}
 
 		[TestMethod]
 		public void TestByteSwap32()
 		{
-			var a = new byte[] { 0x67, 0x45, 0x23, 0x01, 0xEF, 0xCD, 0xAB, 0x89 };
+			var b = new byte[] { 0x67, 0x45, 0x23, 0x01, 0xEF, 0xCD, 0xAB, 0x89 }.AsSpan();
+			var a = b.ToArray().AsSpan();
 			EndiannessUtils.MutatingByteSwap32(a);
 			Assert.IsTrue(a.SequenceEqual(expected));
+			EndiannessUtils.MutatingByteSwap32(a);
+			Assert.IsTrue(a.SequenceEqual(b));
 		}
 	}
 }