Write precompensation (abbreviated WPcom in the literature) is a technical aspect of hard disk design. It is the use of a stronger magnetic field to write data in sectors that are closer to the center of the disk. In constant angular velocity recording, in which the disk spins at a constant speed no matter where the data is written, the sectors closest to the spindle are packed tighter than the outer sectors and so require a stronger magnetic field to write the data.
In the past one of the hard disk parameters stored in a PC’s CMOS memory is the WPcom number, a marker of the track where precompensation begins. Modern hard disks rarely access the CMOS memory and internally store the WPcom number if write precompensation is used.
As discussed in the section on zoned bit recording, older hard disks used the same number of sectors per track. This meant that older disks had a varying bit density as you moved from the outside edge to the inner part of the platter. Many of these older disks required that an adjustment be made when writing the inside tracks, and a setting was placed in the BIOS to allow the user to specify at what track number this compensation was to begin.
This entire matter is no longer relevant to modern hard disks, but the BIOS setting remains for compatibility reasons. Write precompensation is not done with today’s drives; even if it were, the function would be implemented within the integrated controller and would be transparent to the user.
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