Can't auto load the files in core directory
When you are extending the core controller your class name needs to end with "Controller". So you can't name your class CMBase it would have to be CMBase_Controller.
So if the value in $config['subclass_prefix'] is "CM" you class needs to look like
class CMController extends CI_Controller {//...}Then once you have done that you can do
class AnyClassName extends CMController {//...}
Have you tried looking $config['subclass_prefix'] = 'MY_'; applications folder. Can only extend MY_Controller etc. http://ellislab.com/codeigniter/user-guide/general/controllers.html
https://github.com/EllisLab/CodeIgniter/wiki/MY-Controller
I think your missing the _ underscore part ie MY_