Fixed floating point comparison handlers for FCOM ST(i) and FCOMP ST(i) instructions

X86Disassembler/X86/Handlers/FloatingPoint/Comparison/FcomStHandler.cs

@@ -0,0 +1,73 @@
+namespace X86Disassembler.X86.Handlers.FloatingPoint.Comparison;
+
+using X86Disassembler.X86.Operands;
+
+/// <summary>
+/// Handler for FCOM ST(i) instruction (D8 D0-D7) - compares ST(0) with ST(i)
+/// </summary>
+public class FcomStHandler : InstructionHandler
+{
+    /// <summary>
+    /// Initializes a new instance of the FcomStHandler class
+    /// </summary>
+    /// <param name="decoder">The instruction decoder that owns this handler</param>
+    public FcomStHandler(InstructionDecoder decoder)
+        : base(decoder)
+    {
+    }
+
+    /// <summary>
+    /// Checks if this handler can decode the given opcode
+    /// </summary>
+    /// <param name="opcode">The opcode to check</param>
+    /// <returns>True if this handler can decode the opcode</returns>
+    public override bool CanHandle(byte opcode)
+    {
+        // FCOM ST(i) is D8 D0-D7 (compares ST(0) with ST(i))
+        if (opcode != 0xD8) return false;
+
+        if (!Decoder.CanReadByte())
+        {
+            return false;
+        }
+
+        var opcodeSecond = Decoder.PeakByte();
+
+        // this is a special case of a handler, only handling FCOM with ST(i)
+        if (opcodeSecond < 0xD0 || opcodeSecond > 0xD7)
+            return false;
+
+        return true;
+    }
+    
+    /// <summary>
+    /// Decodes a FCOM ST(i) instruction - compares ST(0) with ST(i)
+    /// </summary>
+    /// <param name="opcode">The opcode of the instruction</param>
+    /// <param name="instruction">The instruction object to populate</param>
+    /// <returns>True if the instruction was successfully decoded</returns>
+    public override bool Decode(byte opcode, Instruction instruction)
+    {
+        if (!Decoder.CanReadByte())
+        {
+            return false;
+        }
+        
+        var stIndex = (FpuRegisterIndex)(Decoder.ReadByte() - 0xD0);
+
+        // Set the instruction type
+        instruction.Type = InstructionType.Fcom;
+        
+        // Create the FPU register operands
+        var st0Operand = OperandFactory.CreateFPURegisterOperand(FpuRegisterIndex.ST0);
+        var stiOperand = OperandFactory.CreateFPURegisterOperand(stIndex);
+        
+        // Set the structured operands
+        instruction.StructuredOperands = 
+        [
+            stiOperand  // The instruction is FCOM ST(i), which compares ST(0) with ST(i)
+        ];
+
+        return true;
+    }
+}