The time to read or write each sector will be same however The data density is

The time to read or write each sector will be same

This preview shows page 84 - 86 out of 140 pages.

longer than those close to the hub. The time to read or write each sector will be same, however. The data density is obviously higher on the innermost cylinders, and some disk designs require a change in the drive current to the read-write heads for the inner tracks. This is handled by the disk controller hardware and is not visible to the user (or the implementer of an operating system). The difference in data density between inner and outer tracks means a sacrifice in capacity, and more sophisticated systems exist. Floppy disk designs that rotate at higher speeds when the heads are over the outer tracks have been tried. This allows more sectors on those tracks, increasing disk capacity. Such disks are not supported by any system for which MINIX 3 is currently available, however. Modern large hard drives also have more sectors per track on outer tracks than on inner tracks. These are IDE (Integrated Drive Electronics) drives, and the sophisticated processing done by the drive's built-in electronics masks the details. To the operating system they appear to have a simple geometry with the same number of sectors on each track. The drive and controller electronics are as important as the mechanical hardware. The main element of the disk controller is a specialized integrated circuit, really a small microcomputer. Once this would have been on a card plugged into the computer's backplane, but on modern systems, the disk controller is on the parentboard. For a modern hard disk this disk controller circuitry may be simpler than for a floppy disk, since a hard drive has a powerful electronic controller integrated into the drive itself. A device feature that has important implications for the disk driver is the possibility of a controller doing seeks on two or more drives at the same time. These are known as overlapped seeks. While the controller and software are waiting for a seek to complete on one drive, the controller can initiate a seek on another drive. Many controllers can also read or write on one drive while seeking on one or more other drives, but a floppy disk controller cannot read or write on two drives at the same time. (Reading or writing requires the controller to move bits on a microsecond time scale, so one transfer uses up most of its computing power.) The situation is different for hard disks with integrated controllers, and in a system with more than one of these hard drives they can operate simultaneously, at least to the extent of transferring between the disk and the controller's buffer memory. Only one transfer between the controller and the system memory is
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possible at once, however. The ability to perform two or more operations at the same time can reduce the average access time considerably. [Page 279] One thing to be aware of in looking at the specifications of modern hard disks is that the geometry specified, and used by the driver software, is almost always different from the physical format. In fact, if you look up the "recommended setup parameters" for a large
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