hard disk you are likely to find it specified as 16383 cylinders 16 heads and

Hard disk you are likely to find it specified as

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hard disk, you are likely to find it specified as 16383 cylinders, 16 heads, and 63 sectors per track, no matter what the size of the disk. These numbers correspond to a disk size of 8 GB, but are used for all disks this size or larger. The designers of the original IBM PC ROM BIOS allotted a 6-bit field for the sector count, 4 bits to specify the head, and 14 bits to select a cylinder. With 512 byte sectors this comes out to 8 GB. So if you try to install a large hard drive into a very old computer you may find you can access only 8 GB, even though you have a much bigger disk. The usual way around this limitation is to use logical block addressing in which disk sectors are just numbered consecutively starting at zero, without regard to the disk geometry. The geometry of a modern disk is a fiction, anyway. On a modern disk the surface is divided into 20 or more zones. Zones closer to the center of the disk have fewer sectors per track than zones nearer the periphery. Thus sectors have approximately the same physical length no matter where they are located on the disk, making more efficient use of the disk surface. Internally, the integrated controller addresses the disk by calculating the zone, cylinder, head, and sector. But this is never visible to the user, and the details are rarely found in published specifications. The bottom line is, there is no point to using cylinder, head, sector addressing of a disk unless you are working with a very old computer that does not support logical block addressing. Also, it does not make sense to buy a new 400 GB drive for the PC-XT you bought in 1983; you will get no more than 8 GB use out of it. This is a good place to mention a confusing point about disk capacity specifications. Computer professionals are accustomed to using powers of 2a Kilobyte (KB) is 2 10 = 1024 bytes, a Megabyte (MB) is 2 20 = 1024 2 bytes, etc., to express the size of memory devices. A Gigabyte (GB), then, should be 1024 3 , or 2 30 bytes. However, disk manufacturers have adopted the habit of using the term "Gigabyte" to mean 10 9 , which (on paper) instantly increases the size of their products. Thus the 8 GB limit mentioned above is an 8.4 GB disk in the language of the disk salesman. Recently there has been a move toward using the term Gibibyte (GiB) to mean 2 30 . However, in this text the authors, being set in their ways and in protest of the hijacking of tradition for advertising purposes, will continue to use terms like Megabyte and Gigabyte to mean what they have always meant. [Page 280]
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3.7.2. RAID Although modern disks are much faster than older ones, improvements in CPU performance have far exceeded improvements in disk performance. It has occurred to various people over the years that parallel disk I/O might be helpful. Thus has come about a new class of I/O device called a RAID, an acronym for Redundant Array of Independent Disks. Actually, the designers of RAID (at Berkeley) originally used the acronym RAID to stand for "Redundant Array of Inexpensive Disks" to contrast this design with a SLED (Single Large Expensive Disk). However, when RAID became
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