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Code Issues Releases Activity

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

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This commit is contained in:
bird_egop 2025-04-13 01:39:38 +03:00
parent b4a85d2839
commit bfaeba0d5f
2 changed files with 81 additions and 4 deletions
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34
X86Disassembler/X86/InstructionDecoder.cs
View File

@ -86,7 +86,7 @@ public class InstructionDecoder
_addressSizePrefix = true;
_position++;
}
else if (prefix >= 0x26 && prefix <= 0x3E && (prefix & 0x7) == 0x6) // Segment override prefix
else if ((prefix >= 0x26 && prefix <= 0x3E && (prefix & 0x7) == 0x6) || prefix == 0x64 || prefix == 0x65) // Segment override prefix
{
_segmentOverridePrefix = true;
switch (prefix)
@ -118,6 +118,21 @@ public class InstructionDecoder
if (_position >= _length)
{
// If we reached the end of the buffer while processing prefixes,
// create an instruction with just the prefix information
if (_segmentOverridePrefix)
{
instruction.Mnemonic = _segmentOverride;
instruction.Operands = "";
// Set the raw bytes
int length = _position - startPosition;
instruction.RawBytes = new byte[length];
Array.Copy(_codeBuffer, startPosition, instruction.RawBytes, 0, length);
return instruction;
}
return null;
}
@ -142,10 +157,21 @@ public class InstructionDecoder
instruction.Operands = "??";
}
// Add segment override prefix to the instruction if present
if (_segmentOverridePrefix && !string.IsNullOrEmpty(instruction.Operands))
{
// If the instruction has memory operands, add the segment override
if (instruction.Operands.Contains("["))
{
// Replace the first '[' with the segment override
instruction.Operands = instruction.Operands.Replace("[", $"{_segmentOverride}:[" );
}
}
// Set the raw bytes
int length = _position - startPosition;
instruction.RawBytes = new byte[length];
Array.Copy(_codeBuffer, startPosition, instruction.RawBytes, 0, length);
int instructionLength = _position - startPosition;
instruction.RawBytes = new byte[instructionLength];
Array.Copy(_codeBuffer, startPosition, instruction.RawBytes, 0, instructionLength);
return instruction;
}

51
X86DisassemblerTests/SegmentOverrideTests.cs Normal file
View File

@ -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);
}
}