Split FINIT/FNINIT handlers for proper instruction recognition

X86Disassembler/X86/Handlers/FloatingPoint/Control/FinitHandler.cs

@@ -1,9 +1,7 @@
 namespace X86Disassembler.X86.Handlers.FloatingPoint.Control;
 
-using X86Disassembler.X86.Operands;
-
 /// <summary>
-/// Handler for FINIT instruction (DB E3)
+/// Handler for FINIT instruction with WAIT prefix (0x9B 0xDB 0xE3) - Initialize FPU after checking for pending unmasked floating-point exceptions
 /// </summary>
 public class FinitHandler : InstructionHandler
 {
@@ -23,41 +21,51 @@ public class FinitHandler : InstructionHandler
     /// <returns>True if this handler can decode the opcode</returns>
     public override bool CanHandle(byte opcode)
     {
-        // FINIT is DB E3
+        // FINIT with WAIT prefix starts with 0x9B
-        if (opcode != 0xDB) return false;
+        if (opcode != 0x9B) return false;
 
+        // Check if we can read the next two bytes
         if (!Decoder.CanReadByte())
-        {
             return false;
-        }
 
-        // Check if the next byte is E3
+        // Check if the next bytes are 0xDB 0xE3 (for FINIT with WAIT)
-        byte nextByte = Decoder.PeakByte();
+        var (nextByte, thirdByte) = Decoder.PeakTwoBytes();
-        return nextByte == 0xE3;
+
+        // The sequence must be 9B DB E3 for FINIT with WAIT
+        return nextByte == 0xDB && thirdByte == 0xE3;
     }
     
     /// <summary>
-    /// Decodes a FINIT instruction
+    /// Decodes a FINIT instruction with WAIT prefix
     /// </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)
     {
+        // Skip the WAIT prefix (0x9B) - we already read it in CanHandle
         if (!Decoder.CanReadByte())
-        {
             return false;
-        }
 
-        // Read the second byte of the opcode
+        // Read the second byte (0xDB)
         byte secondByte = Decoder.ReadByte();
+        if (secondByte != 0xDB)
+            return false;
+            
+        // Read the third byte (0xE3)
+        if (!Decoder.CanReadByte())
+            return false;
+
+        byte thirdByte = Decoder.ReadByte();
+        if (thirdByte != 0xE3)
+            return false;
         
         // Set the instruction type
         instruction.Type = InstructionType.Finit;
-
+        
         // FINIT has no operands
         instruction.StructuredOperands = [];
-
+        
         return true;
     }
 }

X86Disassembler/X86/Handlers/FloatingPoint/Control/FninitHandler.cs

@@ -0,0 +1,63 @@
+namespace X86Disassembler.X86.Handlers.FloatingPoint.Control;
+
+using X86Disassembler.X86.Operands;
+
+/// <summary>
+/// Handler for FNINIT instruction (DB E3) - Initialize FPU without checking for pending unmasked exceptions
+/// </summary>
+public class FninitHandler : InstructionHandler
+{
+    /// <summary>
+    /// Initializes a new instance of the FninitHandler class
+    /// </summary>
+    /// <param name="decoder">The instruction decoder that owns this handler</param>
+    public FninitHandler(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)
+    {
+        // FNINIT is DB E3
+        if (opcode != 0xDB) return false;
+
+        if (!Decoder.CanReadByte())
+        {
+            return false;
+        }
+
+        // Check if the next byte is E3
+        byte nextByte = Decoder.PeakByte();
+        return nextByte == 0xE3;
+    }
+    
+    /// <summary>
+    /// Decodes a FNINIT instruction
+    /// </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;
+        }
+
+        // Read the second byte of the opcode
+        byte secondByte = Decoder.ReadByte();
+        
+        // Set the instruction type
+        instruction.Type = InstructionType.Fninit;
+
+        // FINIT has no operands
+        instruction.StructuredOperands = [];
+
+        return true;
+    }
+}

X86Disassembler/X86/Handlers/InstructionHandlerFactory.cs

@@ -437,7 +437,8 @@ public class InstructionHandlerFactory
         // DB opcode handlers (control instructions)
         _handlers.Add(new FloatingPoint.Control.FclexHandler(_decoder));           // FNCLEX (DB E2)
         _handlers.Add(new FloatingPoint.Control.FclexWaitHandler(_decoder));       // FCLEX (9B DB E2)
-        _handlers.Add(new FloatingPoint.Control.FinitHandler(_decoder));           // FINIT (DB E3)
+        _handlers.Add(new FloatingPoint.Control.FninitHandler(_decoder));          // FNINIT (DB E3)
+        _handlers.Add(new FloatingPoint.Control.FinitHandler(_decoder));           // FINIT (9B DB E3)
         
         // DB opcode handlers (comparison instructions)
         _handlers.Add(new FloatingPoint.Comparison.FucomiHandler(_decoder));       // FUCOMI (DB E8-EF)