DISK FORMAT FOR SECONDARY STORAGE SYSTEM

This document is a 1984 United States Patent (No. 4,434,487) assigned to Digital Equipment Corporation describing a disk mass storage format designed to improve reliability and performance in data processing systems.

The patent details a three-layered disk architecture (physical, logical, and functional) and introduces specific mechanisms for handling defective disk media:

• Bad Block Replacement: The system reserves spare sectors across the disk. If a sector becomes defective, access is "revectored" to a designated replacement sector.

• Three-Level Revectoring:

  • Primary: Revectoring is implied by the bad block's position, pointing to a replacement sector on the same track.
  • Secondary: If primary replacement isn't possible, an arbitrary replacement block is used, with its address determined by reading multiple copies of the header information stored in the data field of the bad block.
  • Tertiary: If secondary mechanisms fail, a fallback search is performed using a Replacement and Caching Table (RCT) stored on the disk.

• "Forced Error" Indicator: The patent describes a method to logically mark sectors that are corrupted but otherwise on usable media. By writing a unique error code (the one's complement of the error detecting code) in the data field, the system can notify the host of corrupted data without unnecessarily marking the physical medium as defective.

The document also outlines hierarchical logical addressing (cylinders, groups, and tracks) to optimize rotational latency and provides flowcharts for error recovery and data management procedures.