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6 512e and 4Kn Disk Formats
2 Background
The legacy sector format contains a Gap section, a Sync section, an Address Mark section, a Data section
and Error Correction Code (ECC) section as shown in Figure 1.
Figure 1 Legacy sector format
The sector layout is structured as follows:
 Gap: Separates sectors.
 Sync: Sync marks the beginning of the sector and also provides timing alignment.
 Address Mark: Not only stores information to identify the sector’s number and location but also
provides sector status.
 Data: Is designated to store user data.
 ECC: Error correction codes that are used to repair and recover gap, sync, address mark and data
fields which may be corrupted during reads or writes.
The 512-byte sector has non-data-related overhead for the Gap, Sync, and Address Mark sections for
every 512-byte. Reducing the amount of space used for error correction code improves format efficiency.
As the demand for hard drive capacity growth has increased, format efficiency with 512-byte sectors has
become a limiting design element. To keep up with the capacity growth demand it was imperative for the
hard drive industry had to innovate new methods to improve the error correction efficiency.
512-byte sectors were optimal when managing small, discrete amounts of data. However, applications
common in modern computing systems manage data in large blocks, much larger in fact than the legacy
512-byte sector size. The migration to larger sectors has become a fundamental need relative to gaining
improvements in error correction and achieving format efficiencies.
3 Long-term benefits of 4K sectors
All hard drive manufacturers have decided to transition to Advanced Format, the industry must adapt to
and embrace this change to minimize potential negative side effects. Short-term benefits to end users are
not dramatic in terms of immediate capacity increases, however, the migration to 4K-sized sectors will
most definitely provide quicker paths to higher areal densities and hard drive capacities, as well as more
robust error correction.
The new Advanced Format standard of a 4K-byte sector essentially combines eight legacy 512-byte
sectors into a single 4K-byte sector. The Advanced Format standard uses the same number of bytes for
Gap, Sync and Address Mark, but increases the ECC. This yields a format efficiency of 97 percent, almost a
10 percent improvement. Together, the benefits of improved format efficiency and more robust error