tagged queueing

hardware A method allowing a device or controller to

process commands received from a device driver out of order.

It requires that the device driver attaches a tag to each

command which the controller or device can later use to

identify the response to the command.

Tagged queueing can speed up processing considerably if a

controller serves devices of very different speeds, such as an

SCSI controller serving a mix of CD-ROMs and high-speed

disks. In such cases if a request to fetch data from the

CD-ROM is shortly followed by a request to read from the disk,

the controller doesn't have to wait for the CD-ROM to fetch

the data, it can instead instruct the disk to fetch the data

and return the value to the device driver, while the CD-ROM is

probably still seeking.

(1997-07-04)