Short for tagged command queuing, a method of processing requests from a host device to a hard drive that is storing the data requested. TCQ was first introduced with the SCSI-2 standard as a way to allow hard drives to accept multiple concurrent commands from a host PC. When commands arrive at the drive’s buffer, they are tagged with an identifier and then reordered by the drive s microprocessor to minimize the distance the drive’s read head needs to move laterally along the platter looking for data. For example, if a command is looking for data in one section of the drive and a following queued command is looking for data in a neighboring area, the SCSI host adapter can reorder the commands to make the two occur sequentially. This is a different system than SCSI-1 or IDE/ATA, which will allow only a single command to be outstanding at a time to any device and processes requests serially.
Tagged command queuing also is called command queuing and reordering.
Also see What’s Inside a Hard Drive? in the Did You Know . . . ? section of Webopedia.