<P> From an implementation perspective, the drive maintains a buffer of pending read / write requests, along with the associated cylinder number of the request . (Lower cylinder numbers generally indicate that the cylinder is closer to the spindle, and higher numbers indicate the cylinder is farther away .) </P> <P> When a new request arrives while the drive is idle, the initial arm / head movement will be in the direction of the cylinder where the data is stored, either in or out . As additional requests arrive, requests are serviced only in the current direction of arm movement until the arm reaches the edge of the disk . When this happens, the direction of the arm reverses, and the requests that were remaining in the opposite direction are serviced, and so on . </P> <P> One variation of this method ensures all requests are serviced in only one direction, that is, once the head has arrived at the outer edge of the disk, it returns to the beginning and services the new requests in this one direction only (or vice versa). This is known as the "Circular Elevator Algorithm" or C - SCAN . Although the time of the return seek is wasted, this results in more equal performance for all head positions, as the expected distance from the head is always half the maximum distance, unlike in the standard elevator algorithm where cylinders in the middle will be serviced as much as twice as often as the innermost or outermost cylinders . </P> <P> Other variations include: </P>

4. what does c stand for in case of disk scheduling algorithms