Fixed immediate value formatting in Group1 instruction handlers

X86DisassemblerTests/InstructionTests/Group1SignExtendedHandlerTests.cs

@@ -0,0 +1,56 @@
+using X86Disassembler.X86;
+
+namespace X86DisassemblerTests.InstructionTests;
+
+/// <summary>
+/// Tests for Group 1 sign-extended immediate instructions (0x83 opcode)
+/// </summary>
+public class Group1SignExtendedHandlerTests
+{
+    /// <summary>
+    /// Tests that the disassembler correctly handles ADD ecx, imm8 instruction (0x83 0xC1 0x04)
+    /// </summary>
+    [Fact]
+    public void Disassembler_HandlesAddEcxImm8_Correctly()
+    {
+        // Arrange
+        // ADD ecx, 0x04 (83 C1 04)
+        byte[] codeBuffer = new byte[] { 0x83, 0xC1, 0x04 };
+        var decoder = new InstructionDecoder(codeBuffer, codeBuffer.Length);
+        
+        // Act
+        var instruction = decoder.DecodeInstruction();
+        
+        // Assert
+        Assert.NotNull(instruction);
+        Assert.Equal("add", instruction.Mnemonic);
+        Assert.Equal("ecx, 0x00000004", instruction.Operands);
+    }
+    
+    /// <summary>
+    /// Tests that the disassembler correctly handles the specific sequence from address 0x00001874
+    /// </summary>
+    [Fact]
+    public void Disassembler_HandlesSpecificSequence_Correctly()
+    {
+        // Arrange
+        // This is the sequence from the problematic example:
+        // 83 C1 04 50 E8 42 01 00 00
+        byte[] codeBuffer = new byte[] { 0x83, 0xC1, 0x04, 0x50, 0xE8, 0x42, 0x01, 0x00, 0x00 };
+        var disassembler = new Disassembler(codeBuffer, 0);
+        
+        // Act
+        var instructions = disassembler.Disassemble();
+        
+        // Assert
+        Assert.True(instructions.Count >= 3, $"Expected at least 3 instructions, but got {instructions.Count}");
+        
+        // First instruction should be ADD ecx, 0x04
+        Assert.Equal("add", instructions[0].Mnemonic);
+        Assert.Equal("ecx, 0x00000004", instructions[0].Operands);
+        
+        // Second instruction should be PUSH eax
+        Assert.Equal("push", instructions[1].Mnemonic);
+        Assert.Equal("eax", instructions[1].Operands);
+    }
+}