Fixed segment override prefix handling for 0x64 (FS) opcode with tests

X86DisassemblerTests/SegmentOverrideTests.cs

@@ -0,0 +1,51 @@
+namespace X86DisassemblerTests;
+
+using System;
+using Xunit;
+using X86Disassembler.X86;
+
+/// <summary>
+/// Tests for segment override prefixes
+/// </summary>
+public class SegmentOverrideTests
+{
+    /// <summary>
+    /// Tests that the FS segment override prefix (0x64) is correctly recognized
+    /// </summary>
+    [Fact]
+    public void FsSegmentOverride_IsRecognized()
+    {
+        // Arrange
+        // FS segment override prefix (0x64) followed by MOV [0], ESP (89 25 00 00 00 00)
+        byte[] codeBuffer = new byte[] { 0x64, 0x89, 0x25, 0x00, 0x00, 0x00, 0x00 };
+        var decoder = new InstructionDecoder(codeBuffer, codeBuffer.Length);
+        
+        // Act
+        var instruction = decoder.DecodeInstruction();
+        
+        // Assert
+        Assert.NotNull(instruction);
+        Assert.Equal("mov", instruction.Mnemonic);
+        Assert.Equal("esp, dword ptr fs:[0x00000000]", instruction.Operands);
+    }
+    
+    /// <summary>
+    /// Tests that the FS segment override prefix (0x64) is correctly recognized when it's the only byte
+    /// </summary>
+    [Fact]
+    public void FsSegmentOverride_Alone_IsRecognized()
+    {
+        // Arrange
+        // Just the FS segment override prefix (0x64)
+        byte[] codeBuffer = new byte[] { 0x64 };
+        var decoder = new InstructionDecoder(codeBuffer, codeBuffer.Length);
+        
+        // Act
+        var instruction = decoder.DecodeInstruction();
+        
+        // Assert
+        Assert.NotNull(instruction);
+        Assert.Equal("fs", instruction.Mnemonic);
+        Assert.Equal("", instruction.Operands);
+    }
+}