Implement separate FSTSW handlers and fix test encodings

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

@@ -0,0 +1,78 @@
+using X86Disassembler.X86.Operands;
+
+namespace X86Disassembler.X86.Handlers.FloatingPoint.Control;
+
+/// <summary>
+/// Handler for FSTSW m2byte instruction (0x9B 0xDD /7) - Store FPU status word with wait prefix to memory
+/// </summary>
+public class FstswMemHandler : InstructionHandler
+{
+    /// <summary>
+    /// Initializes a new instance of the FstswMemHandler class
+    /// </summary>
+    /// <param name="decoder">The instruction decoder that owns this handler</param>
+    public FstswMemHandler(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)
+    {
+        // FSTSW m2byte starts with the WAIT prefix (0x9B)
+        if (opcode != 0x9B) return false;
+
+        // Check if we can read the next two bytes
+        if (!Decoder.CanReadByte())
+            return false;
+
+        // Check if the next bytes are 0xDD followed by ModR/M with reg field = 7
+        var (nextByte, modRM) = Decoder.PeakTwoBytes();
+
+        // The first byte must be 0xDD for FSTSW m2byte
+        if (nextByte != 0xDD)
+            return false;
+            
+        // Check if ModR/M byte has reg field = 7
+        byte regField = ModRMDecoder.GetRegFromModRM(modRM);
+        
+        // The reg field must be 7 for FSTSW m2byte
+        return regField == 7;
+    }
+    
+    /// <summary>
+    /// Decodes an FSTSW m2byte 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)
+    {
+        // Skip the WAIT prefix (0x9B) - we already read it in CanHandle
+        if (!Decoder.CanReadByte())
+            return false;
+
+        // Read the second byte (0xDD)
+        byte secondByte = Decoder.ReadByte();
+        if (secondByte != 0xDD)
+            return false;
+            
+        // Set the instruction type
+        instruction.Type = InstructionType.Fstsw;
+        
+        // Use ModRMDecoder to read and decode the ModR/M byte for 16-bit memory operand
+        var (mod, reg, rm, memoryOperand) = ModRMDecoder.ReadModRM16();
+        
+        // Set the structured operands
+        instruction.StructuredOperands = 
+        [
+            memoryOperand
+        ];
+        
+        return true;
+    }
+}