Tagged Command Queuing (TCQ) is a technology built into certain ATA and SCSI hard drives. It allows the operating system to send multiple read and write requests to a hard drive. ATA TCQ is not identical in function to the more efficient Native Command Queuing (NCQ) used by SATA drives. SCSI TCQ does not suffer from the same limitations as ATA TCQ. Without TCQ, an operating system was limited to sending one request at a time. To boost performance, the OS had to determine the order of the requests based on its own possibly incorrect perspective of the hard drive activity (otherwise known as I/O scheduling). With TCQ, the drive can make its own decisions about how to order the requests (and in turn relieve the operating system from having to do so). Thus TCQ can improve the overall performance of a hard drive if it is implemented correctly. Overview For increased efficiency the sectors should be serviced in order of proximity to the current head position, not the order received. The queue is constantly receiving new requests, fulfilling and removing existing requests, and re-ordering the queue according to the current pending read/write requests and the changing position of the head. The exact reordering algorithm may depend upon the controller and the drive itself, but the host computer simply makes requests as needed, leaving the controller to handle the details. This queuing mechanism is sometimes referred to as "elevator seeking", as the image of a modern elevator in a building servicing multiple calls and processing them to minimise travel illustrates the idea well. If the buttons for floors 5, 2, and 4 are pressed in that order with the elevator starting on floor 1, an old elevator would go to the floors in the order requested. A modern elevator processes the requests to stop at floors in the logical order 2, 4, and 5, without unnecessary travel. Non-queueing disk drives service the requests in the order received, like an old elevator; queueing drives service requests in the most efficient order. This may improve performance slightly in a system used by a single user, but may dramatically increase performance in a system with many users making widely varied requests on the disk surface. Comparison of SCSI TCQ, ATA TCQ, and SATA NCQ SCSI TCQ SCSI TCQ was the first popular version of TCQ and is still popular today. It allows tasks