Write precompensation

Lore & Background

In constant angular velocity (CAV) recording, used by most floppy disk systems and older hard disk systems, the disk spins at a constant speed. Sectors closer to the spindle are packed tighter than outer sectors, requiring a slightly different timing for reliable writing. Consequently, a higher amount of precompensation is needed for data written near the center of the disk. On non-circular media such as magnetic tape, precompensation is usually constant throughout the tape.

Historically, hard disk parameters stored in a PC's CMOS memory included the WPcom number, a marker of the track where stronger precompensation begins. This was required by MFM and RLL hard disk controllers common until the early 1990s. These controllers were external to the drive, often on plug-in cards, and needed parameters like the WPcom number to handle different drive types. This scheme allowed only two precompensation strengths per disk: a lower one for outer tracks and a higher one for inner tracks, sufficient for the low-capacity drives of that era.

After the early 1990s, all common hard disk types—including IDE, SCSI, SATA, and Serial Attached SCSI (SAS)—have a drive-specific controller built into the drive enclosure. These internal controllers know the required precompensation strengths for each part of the disk and ignore any WPcom numbers stored in CMOS. Until the late 1990s, many PC BIOS setup programs still allowed users to set WPcom numbers for older hard disk types, though it was not always made clear that modern drives would ignore the setting. Since then, the WPcom number is no longer offered as a BIOS setting, as precompensation has become a fully automated internal feature. Floppy disk controllers still handle precompensation, but because only a few common floppy drive types exist, all needing the same precompensation, no BIOS setting was ever needed for floppy drives.

Reader's Guide

Write precompensation was a critical parameter in the era of external hard disk controllers, when the controller and drive were separate components. The WPcom number stored in CMOS allowed the controller to apply the correct precompensation strength for inner versus outer tracks, compensating for the tighter packing of sectors near the spindle in CAV recording. This scheme, though limited to only two strengths, was adequate for the simple, low-capacity drives of the time. As drive technology advanced, the controller was integrated into the drive enclosure, making external parameters unnecessary. Modern drives automatically manage precompensation internally, optimizing the write signal for each zone of the disk. The disappearance of the WPcom number from BIOS settings reflects this shift from user-configurable to fully automated control. For floppy disks, the uniformity of drive types meant that precompensation never required a user setting. The legacy of write precompensation lies in its role as a bridge between the nonlinear physics of magnetic recording and the practical need for reliable data storage, a challenge now handled transparently by drive firmware.

Did You Know?

• Write precompensation is required because of the non-linear properties of magnetic recording surfaces.
• A higher amount of precompensation is needed for sectors closer to the center of the disk in CAV recording.
• The WPcom number was a hard disk parameter stored in a PC's CMOS memory for use with MFM and RLL controllers.
• After the early 1990s, all common hard disk types have a drive-specific controller that ignores any WPcom numbers stored in CMOS.