Added proper REPNE prefix handling and comprehensive string instruction tests

X86Disassembler/X86/PrefixDecoder.cs

@@ -11,6 +11,7 @@ public class PrefixDecoder
     private bool _segmentOverridePrefix;
     private bool _lockPrefix;
     private bool _repPrefix;
+    private bool _repnePrefix;
     private string _segmentOverride = string.Empty;
     
     /// <summary>
@@ -31,6 +32,7 @@ public class PrefixDecoder
         _segmentOverridePrefix = false;
         _lockPrefix = false;
         _repPrefix = false;
+        _repnePrefix = false;
         _segmentOverride = string.Empty;
     }
     
@@ -70,11 +72,16 @@ public class PrefixDecoder
             _lockPrefix = true;
             return true;
         }
-        else if (prefix == 0xF2 || prefix == 0xF3) // REP/REPNE prefix
+        else if (prefix == 0xF3) // REP prefix
         {
             _repPrefix = true;
             return true;
         }
+        else if (prefix == 0xF2) // REPNE prefix
+        {
+            _repnePrefix = true;
+            return true;
+        }
         
         return false;
     }
@@ -125,14 +132,23 @@ public class PrefixDecoder
     }
     
     /// <summary>
-    /// Checks if the REP/REPNE prefix is present
+    /// Checks if the REP prefix is present
     /// </summary>
-    /// <returns>True if the REP/REPNE prefix is present</returns>
+    /// <returns>True if the REP prefix is present</returns>
     public bool HasRepPrefix()
     {
         return _repPrefix;
     }
     
+    /// <summary>
+    /// Checks if the REPNE prefix is present
+    /// </summary>
+    /// <returns>True if the REPNE prefix is present</returns>
+    public bool HasRepnePrefix()
+    {
+        return _repnePrefix;
+    }
+    
     /// <summary>
     /// Applies the segment override prefix to the operands string if applicable
     /// </summary>
@@ -154,13 +170,17 @@ public class PrefixDecoder
     }
     
     /// <summary>
-    /// Applies the REP prefix to the mnemonic if applicable
+    /// Applies the REP/REPNE prefix to the mnemonic if applicable
     /// </summary>
     /// <param name="mnemonic">The mnemonic</param>
-    /// <returns>The mnemonic with REP prefix applied</returns>
+    /// <returns>The mnemonic with REP/REPNE prefix applied</returns>
     public string ApplyRepPrefix(string mnemonic)
     {
-        if (_repPrefix && !mnemonic.StartsWith("rep"))
+        if (_repnePrefix && !mnemonic.StartsWith("repne"))
+        {
+            return $"repne {mnemonic}";
+        }
+        else if (_repPrefix && !mnemonic.StartsWith("rep"))
         {
             return $"rep {mnemonic}";
         }

X86DisassemblerTests/StringInstructionHandlerTests.cs

@@ -11,10 +11,10 @@ using X86Disassembler.X86.Handlers.String;
 public class StringInstructionHandlerTests
 {
     /// <summary>
-    /// Tests the RepMovsHandler for decoding REP MOVS instruction
+    /// Tests the StringInstructionHandler for decoding REP MOVS instruction
     /// </summary>
     [Fact]
-    public void RepMovsHandler_DecodesRepMovs_Correctly()
+    public void StringInstructionHandler_DecodesRepMovs_Correctly()
     {
         // Arrange
         // REP MOVS (F3 A5)
@@ -29,4 +29,104 @@ public class StringInstructionHandlerTests
         Assert.Equal("rep movs", instruction.Mnemonic);
         Assert.Equal("dword ptr [edi], dword ptr [esi]", instruction.Operands);
     }
+    
+    /// <summary>
+    /// Tests the StringInstructionHandler for decoding REPNE SCAS instruction
+    /// </summary>
+    [Fact]
+    public void StringInstructionHandler_DecodesRepneScas_Correctly()
+    {
+        // Arrange
+        // REPNE SCAS (F2 AF)
+        byte[] codeBuffer = new byte[] { 0xF2, 0xAF };
+        var decoder = new InstructionDecoder(codeBuffer, codeBuffer.Length);
+        
+        // Act
+        var instruction = decoder.DecodeInstruction();
+        
+        // Assert
+        Assert.NotNull(instruction);
+        Assert.Equal("repne scas", instruction.Mnemonic);
+        Assert.Equal("eax, dword ptr [edi]", instruction.Operands);
+    }
+    
+    /// <summary>
+    /// Tests the StringInstructionHandler for decoding MOVS instruction without prefix
+    /// </summary>
+    [Fact]
+    public void StringInstructionHandler_DecodesMovs_Correctly()
+    {
+        // Arrange
+        // MOVS (A5)
+        byte[] codeBuffer = new byte[] { 0xA5 };
+        var decoder = new InstructionDecoder(codeBuffer, codeBuffer.Length);
+        
+        // Act
+        var instruction = decoder.DecodeInstruction();
+        
+        // Assert
+        Assert.NotNull(instruction);
+        Assert.Equal("movs", instruction.Mnemonic);
+        Assert.Equal("dword ptr [edi], dword ptr [esi]", instruction.Operands);
+    }
+    
+    /// <summary>
+    /// Tests the StringInstructionHandler for decoding STOS instruction without prefix
+    /// </summary>
+    [Fact]
+    public void StringInstructionHandler_DecodesStosb_Correctly()
+    {
+        // Arrange
+        // STOSB (AA)
+        byte[] codeBuffer = new byte[] { 0xAA };
+        var decoder = new InstructionDecoder(codeBuffer, codeBuffer.Length);
+        
+        // Act
+        var instruction = decoder.DecodeInstruction();
+        
+        // Assert
+        Assert.NotNull(instruction);
+        Assert.Equal("stos", instruction.Mnemonic);
+        Assert.Equal("byte ptr [edi], al", instruction.Operands);
+    }
+    
+    /// <summary>
+    /// Tests the StringInstructionHandler for decoding LODS instruction without prefix
+    /// </summary>
+    [Fact]
+    public void StringInstructionHandler_DecodesLodsd_Correctly()
+    {
+        // Arrange
+        // LODSD (AD)
+        byte[] codeBuffer = new byte[] { 0xAD };
+        var decoder = new InstructionDecoder(codeBuffer, codeBuffer.Length);
+        
+        // Act
+        var instruction = decoder.DecodeInstruction();
+        
+        // Assert
+        Assert.NotNull(instruction);
+        Assert.Equal("lods", instruction.Mnemonic);
+        Assert.Equal("eax, dword ptr [esi]", instruction.Operands);
+    }
+    
+    /// <summary>
+    /// Tests the StringInstructionHandler for decoding SCAS instruction without prefix
+    /// </summary>
+    [Fact]
+    public void StringInstructionHandler_DecodesScasb_Correctly()
+    {
+        // Arrange
+        // SCASB (AE)
+        byte[] codeBuffer = new byte[] { 0xAE };
+        var decoder = new InstructionDecoder(codeBuffer, codeBuffer.Length);
+        
+        // Act
+        var instruction = decoder.DecodeInstruction();
+        
+        // Assert
+        Assert.NotNull(instruction);
+        Assert.Equal("scas", instruction.Mnemonic);
+        Assert.Equal("al, byte ptr [edi]", instruction.Operands);
+    }
 }