ParkanPlayground/X86Disassembler/X86/Handlers/Sub/SubImmFromRm16Handler.cs

namespace X86Disassembler.X86.Handlers.Sub;

/// <summary>
/// Handler for SUB r/m16, imm16 instruction (0x81 /5 with 0x66 prefix)
/// </summary>
public class SubImmFromRm16Handler : InstructionHandler
{
    /// <summary>
    /// Initializes a new instance of the SubImmFromRm16Handler class
    /// </summary>
    /// <param name="codeBuffer">The buffer containing the code to decode</param>
    /// <param name="decoder">The instruction decoder that owns this handler</param>
    /// <param name="length">The length of the buffer</param>
    public SubImmFromRm16Handler(byte[] codeBuffer, InstructionDecoder decoder, int length)
        : base(codeBuffer, decoder, length)
    {
    }

    /// <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)
    {
        // Check if the opcode is 0x81 and we have a 0x66 prefix
        return opcode == 0x81 && Decoder.HasOperandSizeOverridePrefix();
    }

    /// <summary>
    /// Decodes a SUB r/m16, imm16 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)
    {
        int position = Decoder.GetPosition();

        if (position >= Length)
        {
            return false;
        }

        // Extract the fields from the ModR/M byte
        var (mod, reg, rm, destOperand) = ModRMDecoder.ReadModRM();

        // Check if this is a SUB instruction (reg field must be 5)
        if (reg != RegisterIndex.Di)
        {
            return false;
        }

        // Set the mnemonic
        instruction.Mnemonic = "sub";

        // For mod == 3, the r/m field specifies a register
        string destination;
        if (mod == 3)
        {
            // Get the register name (16-bit)
            destination = ModRMDecoder.GetRegisterName(rm, 16);
        }
        else
        {
            // Replace "dword" with "word" in the memory operand
            destination = destOperand.Replace("dword", "word");
        }

        // Get the current position after processing the ModR/M byte
        position = Decoder.GetPosition();

        // Check if we have enough bytes for the immediate value
        if (position + 1 >= Length)
        {
            return false;
        }

        // Read the immediate value (16-bit)
        ushort immediate = Decoder.ReadUInt16();

        // Set the operands (note: we use 32-bit register names to match the disassembler's output)
        if (mod == 3)
        {
            // For register operands, use the 32-bit register name
            string reg32Name = destination
                .Replace("ax", "eax")
                .Replace("bx", "ebx")
                .Replace("cx", "ecx")
                .Replace("dx", "edx")
                .Replace("sp", "esp")
                .Replace("bp", "ebp")
                .Replace("si", "esi")
                .Replace("di", "edi");

            instruction.Operands = $"{reg32Name}, 0x{immediate:X4}";
        }
        else
        {
            // For memory operands, keep the memory operand as is
            instruction.Operands = $"{destination}, 0x{immediate:X4}";
        }

        return true;
    }
}