CreateSystemThunk.cs

List<Action> _systemThunkHanders = new List<Action>();
public uint CreateSystemThunk(Action handler, ushort popStack)
{
    // Capture address of this thunk
    ushort address = _systemCodeGenPos;

    // Store the handler
    ushort thunkIndex = (ushort)_systemThunkHanders.Count;
    _systemThunkHanders.Add(handler);

    // Get memory buffer
    byte[] mem = _globalHeap.GetBuffer(_systemCodeSelector);

    // MOV AX, thunk index
    mem[_systemCodeGenPos++] = 0xb8;
    mem[_systemCodeGenPos++] = (byte)(thunkIndex & 0xFF);
    mem[_systemCodeGenPos++] = (byte)(thunkIndex >> 8);

    // INT 80h
    mem[_systemCodeGenPos++] = 0xCD;
    mem[_systemCodeGenPos++] = 0x80;

    // RETF
    mem[_systemCodeGenPos++] = 0xCA;
    mem[_systemCodeGenPos++] = (byte)(popStack & 0xFF);
    mem[_systemCodeGenPos++] = (byte)(popStack >> 8);

    // Return seg:offset address of thunk
    return (uint)(_systemCodeSelector << 16 | address);
}