asm-decoder-homework-1.cs

using System.Text;

var RegFieldEncoding = new Dictionary<(int reg, int w), string>()
{
    { (0b000, 0), "al" },
    { (0b000, 1), "ax" },

    { (0b001, 0), "cl" },
    { (0b001, 1), "cx" },

    { (0b010, 0), "dl" },
    { (0b010, 1), "dx" },

    { (0b011, 0), "bl" },
    { (0b011, 1), "bx" },

    { (0b100, 0), "ah" },
    { (0b100, 1), "sp" },

    { (0b101, 0), "ch" },
    { (0b101, 1), "bp" },

    { (0b110, 0), "dh" },
    { (0b110, 1), "si" },

    { (0b111, 0), "bh" },
    { (0b111, 1), "di" },
};

//var filename = "listing_0037_single_register_mov";
var filename = args[0];

var decodedContents = new StringBuilder();
decodedContents.AppendLine("; ================================================");
decodedContents.AppendLine($"; {filename}");
decodedContents.AppendLine("; ================================================");
decodedContents.AppendLine();
decodedContents.AppendLine("bits 16");
decodedContents.AppendLine();

var asmBytes = await File.ReadAllBytesAsync(filename);
for (var i = 0; i < asmBytes.Length; i += 2)
{
    ProcessInstruction(asmBytes[i..(i + 2)]);
}

Console.Write(decodedContents);
// End

void ProcessInstruction(byte[] bytes)
{
    const byte MOV = 0b100010;
    if (bytes[0] >> 2 == MOV)
    {
        // move the 7th bit to position 0 and check it
        var d = bytes[0] >> 1 & 1;
        var w = bytes[0] & 1;


        var mod = bytes[1] >> 6;

        switch (mod)
        {
            case 0b00: // memory mode, no displacement follows
                break;
            case 0b01: // memory mode, 8-bit displacement follows
                break;
            case 0b10: // memory mode, 16-bit displacement follows
                break;
            case 0b11: // register mode, no displacement 
                var reg = bytes[1] >> 3 & 0b111;
                var rm = bytes[1] & 0b111;

                var regField = RegFieldEncoding[(reg, w)];
                var rmField = RegFieldEncoding[(rm, w)];

                // if d == 1, swap the fields
                if (d == 1)
                {
                    (regField, rmField) = (rmField, regField);
                }

                decodedContents.AppendLine($"mov {rmField}, {regField}");
                break;
        }
    }
}