diff --git a/src/BizHawk.Client.Common/cheats/GameSharkDecoder.cs b/src/BizHawk.Client.Common/cheats/GameSharkDecoder.cs
index 54ac105888..2ddbe54c89 100644
--- a/src/BizHawk.Client.Common/cheats/GameSharkDecoder.cs
+++ b/src/BizHawk.Client.Common/cheats/GameSharkDecoder.cs
@@ -52,7 +52,8 @@ namespace BizHawk.Client.Common.cheats
 		private static IDecodeResult GameBoy(string code)
 		{
 			// Game Genie
-			if (code.LastIndexOf("-", StringComparison.Ordinal) == 7 && code.IndexOf("-", StringComparison.Ordinal) == 3)
+			if ((code.Length is 11 && code[3] is '-' && code[7] is '-')
+				|| (code.Length is 7 && code[3] is '-'))
 			{
 				return GbGgGameGenieDecoder.Decode(code);
 			}
diff --git a/src/BizHawk.Client.Common/cheats/GbGgGameGenieDecoder.cs b/src/BizHawk.Client.Common/cheats/GbGgGameGenieDecoder.cs
index 849fea2ce3..5433d57fe8 100644
--- a/src/BizHawk.Client.Common/cheats/GbGgGameGenieDecoder.cs
+++ b/src/BizHawk.Client.Common/cheats/GbGgGameGenieDecoder.cs
@@ -1,109 +1,61 @@
 using System;
-using System.Collections.Generic;
+
+using BizHawk.Common.NumberExtensions;
+using BizHawk.Common.StringExtensions;
+
+using static BizHawk.Common.StringExtensions.NumericStringExtensions;
 
 namespace BizHawk.Client.Common.cheats
 {
 	/// <summary>
 	/// Decodes Gameboy and Game Gear Game Genie codes
 	/// </summary>
+	/// <remarks><see href="https://www.devrs.com/gb/files/gg.html"/></remarks>
 	public static class GbGgGameGenieDecoder
 	{
-		private static readonly Dictionary<char, int> _gbGgGameGenieTable = new Dictionary<char, int>
+		public static IDecodeResult Decode(string code)
 		{
-			['0'] = 0,
-			['1'] = 1,
-			['2'] = 2,
-			['3'] = 3,
-			['4'] = 4,
-			['5'] = 5,
-			['6'] = 6,
-			['7'] = 7,
-			['8'] = 8,
-			['9'] = 9,
-			['A'] = 10,
-			['B'] = 11,
-			['C'] = 12,
-			['D'] = 13,
-			['E'] = 14,
-			['F'] = 15
-		};
-
-		public static IDecodeResult Decode(string _code)
-		{
-			if (_code == null)
+			if (code is null) throw new ArgumentNullException(nameof(code));
+			const string ERR_MSG_MALFORMED = "expecting code in the format XXX-XXX or XXX-XXX-XXX";
+			if (code.Length is not (7 or 11)) return new InvalidCheatCode(ERR_MSG_MALFORMED);
+			const char SEP = '-';
+			if (code[3] is not SEP || !(code[0].IsHex() && code[1].IsHex() && code[2].IsHex()
+				&& code[4].IsHex() && code[5].IsHex() && code[6].IsHex()))
 			{
-				throw new ArgumentNullException(nameof(_code));
+				return new InvalidCheatCode(ERR_MSG_MALFORMED);
 			}
-
-			if (_code.LastIndexOf("-", StringComparison.Ordinal) != 7 || _code.IndexOf("-", StringComparison.Ordinal) != 3)
+			DecodeResult result = new() { Size = WatchSize.Byte };
+			Span<char> toParse = stackalloc char[4];
+			if (code.Length is 11)
 			{
-				return new InvalidCheatCode("All Master System Game Genie Codes need to have a dash after the third character and seventh character.");
+				if (code[7] is not SEP || !(code[8].IsHex() && code[9].IsHex() && code[10].IsHex()))
+				{
+					return new InvalidCheatCode(ERR_MSG_MALFORMED);
+				}
+				// code is VVA-AAA-CCC, parse the compare value and fall through
+				toParse[0] = code[8];
+				_ = code[9]; // "undetermined but if you XOR it with encoded nibble A the result must not be any of the values 1 through 7 or else you will receive a bad code message"
+				toParse[1] = code[10];
+				var compareValue = ParseU8FromHex(toParse[..2]);
+				compareValue ^= 0xFF;
+				NumberExtensions.RotateRightU8(ref compareValue, 2);
+				result.Compare = compareValue ^ 0x45;
 			}
-
-			// No cypher on value
-			// Char # |   0   |   1   |   2   |   3   |   4   |   5   |   6   |   7   |   8   |
-			// Bit  # |3|2|1|0|3|2|1|0|3|2|1|0|3|2|1|0|3|2|1|0|3|2|1|0|3|2|1|0|3|2|1|0|3|2|1|0|
-			// maps to|      Value    |A|B|C|D|E|F|G|H|I|J|K|L|XOR 0xF|a|b|c|c|NotUsed|e|f|g|h|
-			// proper |      Value    |XOR 0xF|A|B|C|D|E|F|G|H|I|J|K|L|g|h|a|b|Nothing|c|d|e|f|
-			var result = new DecodeResult { Size = WatchSize.Byte };
-
-			int x;
-
-			// Getting Value
-			if (_code.Length > 0)
-			{
-				_ = _gbGgGameGenieTable.TryGetValue(_code[0], out x);
-				result.Value = x << 4;
-			}
-
-			if (_code.Length > 1)
-			{
-				_ = _gbGgGameGenieTable.TryGetValue(_code[1], out x);
-				result.Value |= x;
-			}
-
-			// Address
-			if (_code.Length > 2)
-			{
-				_ = _gbGgGameGenieTable.TryGetValue(_code[2], out x);
-				result.Value = x << 8;
-			}
-			else
-			{
-				result.Value = -1;
-			}
-
-			if (_code.Length > 3)
-			{
-				_ = _gbGgGameGenieTable.TryGetValue(_code[3], out x);
-				result.Address |= x << 4;
-			}
-
-			if (_code.Length > 4)
-			{
-				_ = _gbGgGameGenieTable.TryGetValue(_code[4], out x);
-				result.Address |= x;
-			}
-
-			if (_code.Length > 5)
-			{
-				_ = _gbGgGameGenieTable.TryGetValue(_code[5], out x);
-				result.Address |= (x ^ 0xF) << 12;
-			}
-
-			// compare need to be full
-			if (_code.Length > 8)
-			{
-				_ = _gbGgGameGenieTable.TryGetValue(_code[6], out x);
-				var comp = x << 2;
-
-				// 8th character ignored
-				_ = _gbGgGameGenieTable.TryGetValue(_code[8], out x);
-				comp |= (x & 0xC) >> 2;
-				comp |= (x & 0x3) << 6;
-				result.Compare = comp ^ 0xBA;
-			}
-
+			// else code is VVA-AAA
+			toParse[0] = code[6];
+			toParse[1] = code[2];
+			toParse[2] = code[4];
+			toParse[3] = code[5];
+			result.Address = ParseU16FromHex(toParse) ^ 0xF000;
+#pragma warning disable CA1846 // Span overload just calls ToString
+			result.Value = ParseU8FromHex(
+#if NET7_0_OR_GREATER
+				code.AsSpan(start: 0, length: 2)
+#else
+				code.Substring(startIndex: 0, length: 2)
+#endif
+			);
+#pragma warning restore CA1846
 			return result;
 		}
 	}
diff --git a/src/BizHawk.Common/Extensions/NumberExtensions.cs b/src/BizHawk.Common/Extensions/NumberExtensions.cs
index 5a59b74679..204e723489 100644
--- a/src/BizHawk.Common/Extensions/NumberExtensions.cs
+++ b/src/BizHawk.Common/Extensions/NumberExtensions.cs
@@ -121,6 +121,14 @@ namespace BizHawk.Common.NumberExtensions
 			return val;
 		}
 
+		public static void RotateRightU8(ref byte b, int shift)
+		{
+			byte temp = b;
+			temp <<= 8 - shift;
+			b >>= shift;
+			b |= temp;
+		}
+
 		public static int RoundToInt(this double d) => (int) Math.Round(d);
 
 		public static int RoundToInt(this float f) => (int) Math.Round(f);
diff --git a/src/BizHawk.Common/Extensions/NumericStringExtensions.cs b/src/BizHawk.Common/Extensions/NumericStringExtensions.cs
index 7db7040c3c..755108c245 100644
--- a/src/BizHawk.Common/Extensions/NumericStringExtensions.cs
+++ b/src/BizHawk.Common/Extensions/NumericStringExtensions.cs
@@ -1,3 +1,5 @@
+using System;
+using System.Globalization;
 using System.Linq;
 
 namespace BizHawk.Common.StringExtensions
@@ -31,6 +33,26 @@ namespace BizHawk.Common.StringExtensions
 		/// </returns>
 		public static string OnlyHex(this string? raw) => string.IsNullOrWhiteSpace(raw) ? string.Empty : string.Concat(raw.Where(IsHex)).ToUpperInvariant();
 
+#if NET7_0_OR_GREATER
+		public static ushort ParseU16FromHex(ReadOnlySpan<char> str)
+			=> ushort.Parse(str, NumberStyles.AllowHexSpecifier, CultureInfo.InvariantCulture);
+
+		public static byte ParseU8FromHex(ReadOnlySpan<char> str)
+			=> byte.Parse(str, NumberStyles.AllowHexSpecifier, CultureInfo.InvariantCulture);
+#else
+		public static ushort ParseU16FromHex(ReadOnlySpan<char> str)
+			=> ParseU16FromHex(str.ToString());
+
+		public static ushort ParseU16FromHex(string str)
+			=> ushort.Parse(str, NumberStyles.AllowHexSpecifier, CultureInfo.InvariantCulture);
+
+		public static byte ParseU8FromHex(ReadOnlySpan<char> str)
+			=> ParseU8FromHex(str.ToString());
+
+		public static byte ParseU8FromHex(string str)
+			=> byte.Parse(str, NumberStyles.AllowHexSpecifier, CultureInfo.InvariantCulture);
+#endif
+
 		/// <returns><see langword="true"/> iff <paramref name="str"/> is not <see langword="null"/> and all chars of <paramref name="str"/> are digits</returns>
 		public static bool IsUnsigned(this string? str) => !string.IsNullOrWhiteSpace(str) && str.All(IsUnsigned);
 	}