如何给ext的下拉框设默认值？

Question

[code="java"]
var isbige = [["是", "1"], ["否", "0"]];
var isbigestore = new Ext.data.SimpleStore({
fields : ['name', 'isbigedian'],
data : isbige
});

        var isbigecombo = new Ext.form.ComboBox({
            fieldLabel : '是否高低字节',

            xtype : 'combo',
            store : isbigestore,
            mode : 'local',
            hiddenName : 'ISBIGEDIAN',
            emptyText : '请选择',
            triggerAction : 'all',
            valueField : 'isbigedian',
            displayField : 'name',
            width : 125,
            readOnly : true,
            allowBlank : false,
            editable : false,
            typeAhead : true,
            selectOnFocus : true,
            anchor : '85%'
        });

[/code]要求用户添加时下拉框的默认值为是，该如何实现那？