Add support for FNSTSW instruction

X86Disassembler/X86/Handlers/FloatingPointHandler.cs

@@ -8,12 +8,16 @@ public class FloatingPointHandler : InstructionHandler
     // Floating-point instruction mnemonics based on opcode and ModR/M reg field
     private static readonly string[][] FpuMnemonics = new string[8][];
     
+    // Two-byte floating-point instructions
+    private static readonly Dictionary<ushort, string> TwoByteInstructions = new Dictionary<ushort, string>();
+    
     /// <summary>
     /// Static constructor to initialize the FPU mnemonic tables
     /// </summary>
     static FloatingPointHandler()
     {
         InitializeFpuMnemonics();
+        InitializeTwoByteInstructions();
     }
     
     /// <summary>
@@ -105,6 +109,17 @@ public class FloatingPointHandler : InstructionHandler
         FpuMnemonics[7][7] = "fistp";
     }
     
+    /// <summary>
+    /// Initializes the two-byte floating-point instructions
+    /// </summary>
+    private static void InitializeTwoByteInstructions()
+    {
+        // DF E0 - FNSTSW AX (Store FPU status word to AX without checking for pending unmasked floating-point exceptions)
+        TwoByteInstructions.Add(0xDFE0, "fnstsw");
+        
+        // Add other two-byte instructions as needed
+    }
+    
     /// <summary>
     /// Initializes a new instance of the FloatingPointHandler class
     /// </summary>
@@ -141,6 +156,27 @@ public class FloatingPointHandler : InstructionHandler
             return false;
         }
         
+        // Check for two-byte instructions
+        if (position < Length)
+        {
+            // Create a two-byte opcode by combining the primary opcode with the next byte
+            ushort twoByteOpcode = (ushort)((opcode << 8) | CodeBuffer[position]);
+            
+            // Check if this is a known two-byte instruction
+            if (TwoByteInstructions.TryGetValue(twoByteOpcode, out string? mnemonic) && mnemonic != null)
+            {
+                instruction.Mnemonic = mnemonic;
+                
+                // Special handling for specific instructions
+                if (twoByteOpcode == 0xDFE0) // FNSTSW AX
+                {
+                    instruction.Operands = "ax";
+                    Decoder.SetPosition(position + 1); // Skip the second byte
+                    return true;
+                }
+            }
+        }
+        
         // The opcode index in our tables (0-7 for D8-DF)
         int opcodeIndex = opcode - 0xD8;