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Heads
A device called a head reads and writes data in a hard drive by manipulating the magnetic medium that composes the surface of an associated disk platter. Naturally, a platter has 2 sides and thus 2 surfaces on which data can be manipulated; usually there are 2 heads per platter, one per side. (Sometimes the term side is substituted for head, since platters might be separated from their head assemblies, as with the removable media of a floppy drive.)
The CHS addressing supported in old BIOS code used eight bits for up to 256 heads counted as head 0 up to 255 (hex. FFh). However, some long-forgotten[citation needed] software supported only 255 heads, causing no problems for physical disks at this time with fewer heads. In essence erroneous 255 is still in use for virtual 255×63 geometries. This historical oddity can affect the maximum disk size in old BIOS INT 13h code as well as old PC DOS or similar operating systems:
(512 byte/sector)×255×63=8032.5 MB, but actually (512 byte/sector)×256×63=8064 MB yields what is known as 8 GB limit.[5] In this context relevant definition of 8 GB = 8192 MB is another incorrect limit, because it would require CHS 512×256×64 with 64 sectors per track.
[edit]Tracks
The tracks are the thin concentric circular strips of sectors. At least one head is required to read a single track. With respect to disk geometries the terms track and cylinder are closely related. For a single or double sided floppy disk track is the common term; and for more than two heads cylinder is the common term. Strictly speaking a track is a given CH combination consisting of SPT sectors, while a cylinder consists of SPT×H sectors.
Tracks and cylinders are counted from 0, i.e., track 0 is the first (outer-most) track on floppy or other cylindrical disks. Old BIOS code supported ten bits in CHS addressing with up to 1024 cylinders (1024=2**10). Adding six bits for sectors and eight bits for heads results in the 24 bits supported by BIOS interrupt 13h. Subtracting the disallowed sector number 0 in 1024×256 tracks corresponds to 128 MB for a sector size of 512 bytes (128 MB=1024×256×(512 byte/sector)); and 8192-128=8064 confirms the (roughly) 8 GB limit.[6] |
Всего записей: 14007 | Зарегистр. 27-04-2013 | Отправлено: 20:56 13-01-2012 | Исправлено: dialmak, 20:59 13-01-2012 |
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