Refactored ModRMDecoder class into smaller, more focused components. Created RegisterMapper and SIBDecoder classes to improve maintainability.

X86Disassembler/X86/RegisterMapper.cs

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+namespace X86Disassembler.X86;
+
+using Operands;
+
+/// <summary>
+/// Handles mapping between register indices and register enums
+/// </summary>
+public static class RegisterMapper
+{
+    /// <summary>
+    /// Maps the register index from the ModR/M byte to the RegisterIndex enum value
+    /// </summary>
+    /// <param name="modRMRegIndex">The register index from the ModR/M byte (0-7)</param>
+    /// <returns>The corresponding RegisterIndex enum value</returns>
+    public static RegisterIndex MapModRMToRegisterIndex(int modRMRegIndex)
+    {
+        // The mapping from ModR/M register index to RegisterIndex enum is:
+        // 0 -> A (EAX)
+        // 1 -> C (ECX)
+        // 2 -> D (EDX)
+        // 3 -> B (EBX)
+        // 4 -> Sp (ESP)
+        // 5 -> Bp (EBP)
+        // 6 -> Si (ESI)
+        // 7 -> Di (EDI)
+        return modRMRegIndex switch
+        {
+            0 => RegisterIndex.A,  // EAX
+            1 => RegisterIndex.C,  // ECX
+            2 => RegisterIndex.D,  // EDX
+            3 => RegisterIndex.B,  // EBX
+            4 => RegisterIndex.Sp, // ESP
+            5 => RegisterIndex.Bp, // EBP
+            6 => RegisterIndex.Si, // ESI
+            7 => RegisterIndex.Di, // EDI
+            _ => RegisterIndex.A   // Default to EAX
+        };
+    }
+
+    /// <summary>
+    /// Maps the register index from the ModR/M byte to the RegisterIndex8 enum value
+    /// </summary>
+    /// <param name="modRMRegIndex">The register index from the ModR/M byte (0-7)</param>
+    /// <returns>The corresponding RegisterIndex8 enum value</returns>
+    public static RegisterIndex8 MapModRMToRegisterIndex8(int modRMRegIndex)
+    {
+        // The mapping from ModR/M register index to RegisterIndex8 enum is direct:
+        // 0 -> AL, 1 -> CL, 2 -> DL, 3 -> BL, 4 -> AH, 5 -> CH, 6 -> DH, 7 -> BH
+        return (RegisterIndex8)modRMRegIndex;
+    }
+    
+    /// <summary>
+    /// Maps a RegisterIndex8 enum value to the corresponding RegisterIndex enum value for base registers
+    /// </summary>
+    /// <param name="regIndex8">The RegisterIndex8 enum value</param>
+    /// <returns>The corresponding RegisterIndex enum value</returns>
+    public static RegisterIndex MapRegister8ToBaseRegister(RegisterIndex8 regIndex8)
+    {
+        // Map 8-bit register indices to their corresponding 32-bit register indices
+        return regIndex8 switch
+        {
+            RegisterIndex8.AL => RegisterIndex.A,
+            RegisterIndex8.CL => RegisterIndex.C,
+            RegisterIndex8.DL => RegisterIndex.D,
+            RegisterIndex8.BL => RegisterIndex.B,
+            RegisterIndex8.AH => RegisterIndex.A,
+            RegisterIndex8.CH => RegisterIndex.C,
+            RegisterIndex8.DH => RegisterIndex.D,
+            RegisterIndex8.BH => RegisterIndex.B,
+            _ => RegisterIndex.A // Default to EAX
+        };
+    }
+    
+    /// <summary>
+    /// Gets the register name based on the register index and size
+    /// </summary>
+    /// <param name="regIndex">The register index as RegisterIndex enum</param>
+    /// <param name="size">The register size (16, 32, or 64 bits)</param>
+    /// <returns>The register name</returns>
+    public static string GetRegisterName(RegisterIndex regIndex, int size)
+    {
+        return size switch
+        {
+            16 => Constants.RegisterNames16[(int)regIndex],
+            32 => Constants.RegisterNames32[(int)regIndex],
+            64 => Constants.RegisterNames32[(int)regIndex], // For now, reuse 32-bit names for 64-bit
+            _ => "unknown"
+        };
+    }
+    
+    /// <summary>
+    /// Gets the 8-bit register name based on the RegisterIndex8 enum value
+    /// </summary>
+    /// <param name="regIndex8">The register index as RegisterIndex8 enum</param>
+    /// <returns>The 8-bit register name</returns>
+    public static string GetRegisterName(RegisterIndex8 regIndex8)
+    {
+        return regIndex8.ToString().ToLower();
+    }
+}