ParkanPlayground/X86Disassembler/X86/Handlers/Sub/SubImmFromRm32SignExtendedHandler.cs

namespace X86Disassembler.X86.Handlers.Sub;

/// <summary>
/// Handler for SUB r/m32, imm8 (sign-extended) instruction (0x83 /5)
/// </summary>
public class SubImmFromRm32SignExtendedHandler : InstructionHandler
{
    /// <summary>
    /// Initializes a new instance of the SubImmFromRm32SignExtendedHandler 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 SubImmFromRm32SignExtendedHandler(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)
    {
        if (opcode != 0x83)
            return false;

        // Check if the reg field of the ModR/M byte is 5 (SUB)
        if (!Decoder.CanReadByte())
            return false;

        byte modRM = CodeBuffer[Decoder.GetPosition()];
        byte reg = (byte) ((modRM & 0x38) >> 3);

        return reg == 5; // 5 = SUB
    }

    /// <summary>
    /// Decodes a SUB r/m32, imm8 (sign-extended) 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)
    {
        // Set the mnemonic
        instruction.Mnemonic = "sub";

        // Check if we have enough bytes for the ModR/M byte
        if (!Decoder.CanReadByte())
        {
            return false;
        }

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

        // Check if we have enough bytes for the immediate value
        if (!Decoder.CanReadByte())
        {
            return false;
        }

        // Read the immediate value as a signed byte and sign-extend it to 32 bits
        int imm32 = (sbyte) Decoder.ReadByte();
        
        // Format the immediate value - use a consistent approach for all operands
        // For negative values, show the full 32-bit representation
        string immStr = imm32 < 0 
            ? $"0x{(uint)imm32:X8}" 
            : $"0x{(byte)imm32:X2}";

        // Set the operands
        instruction.Operands = $"{destOperand}, {immStr}";

        return true;
    }
}