function __construct(){
    parent::__construct();
    $this->load->model('m_login');
}

function index(){
    $this->load->view('v_login');
}

function aksi_login(){
    $username = $this->input->post('username');
    $password = $this->input->post('password');
    $where = array(
        'username' => $username,
        'password' => md5($password)
        );
    $cek = $this->m_login->cek_login("admin",$where)->num_rows();
    if($cek > 0){
        $data_session = array(
            'username' => $cek['username'],
            'nama'     => $cek['nama']
            'status'   => "login"
            );
        $this->session->set_userdata($data_session);
        redirect(base_url("admin"));
    }else{
        echo "Username dan password salah !";
    }
}
}

<?php
class M_login extends CI_Model{
function cek_login($table,$where){
    return $this->db->get_where($table,$where);
    }
}