A separate generic controller card was used, so that the operating system had to know the exact physical "geometry" of the specific drive attached to the controller, to correctly address data blocks. Sector finally selects which data block in this track is to be addressed, as the track is subdivided into several equally-sized portions, each of which is an arc of (360/n) degrees, where n is the number of sectors in the track.ĬHS addresses were exposed, instead of simple linear addresses (going from 0 to the total block count on disk - 1), because early hard drives didn't come with an embedded disk controller, that would hide the physical layout. Combined, cylinder and head intersect to a circular line, or more precisely: a circular strip of physical data blocks called track. Cylinder is a cylindrical intersection through the stack of platters in a disk, centered around the disk's spindle. Head selects a circular surface: a platter in the disk (and one of its two sides). It is a 3D-coordinate system made out of a vertical coordinate head, a horizontal (or radial) coordinate cylinder, and an angular coordinate sector. Cylinder, head, and sector of a hard drive.Ĭylinder-head-sector ( CHS) is an early method for giving addresses to each physical block of data on a hard disk drive.
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