Abstract: An out-of-order buffer includes an out-of-order queue and a controlling circuit. The out-of-order queue includes a request sequence table and a request storage device. The controlling circuit receives and temporarily stores the plural requests into the out-of-order queue. After the plural requests are transmitted to plural corresponding target devices, the controlling circuit retires the plural requests. The request sequence table contains m×n indicating units. The request sequence table contains m entry indicating rows. Each of the m entry indicating rows contains n indicating units. The request storage device includes m storage units corresponding to the m entry indicating rows in the request sequence table. The state of indicating whether one request is stored in the corresponding storage unit of the m storage units is recoded in the request sequence table. The storage sequence of the plural requests is recoded in the request sequence table.