function checkAll(form_id)
{
    var field=document.getElementById(form_id).elements;
        for (i = 0; i < field.length; i++)
                field[i].checked = true ;
}

function uncheckAll(form_id)
{
    var field=document.getElementById(form_id).elements;
        for (i = 0; i < field.length; i++)
                field[i].checked = false ;
}