How MS-DOS 6.22 Marks Bad Sectors in the FAT

MS-DOS 6.22 relies on the File Allocation Table (FAT) file system—typically FAT12 for floppy disks and FAT16 for hard drives—to track disk storage and handle media defects. When the operating system identifies physically damaged or unreadable sectors, it flags the entire parent cluster containing those sectors directly within the allocation table using a reserved hexadecimal value. This article explains how MS-DOS 6.22 detects bad sectors, writes the corresponding values to both copies of the FAT, updates file chains, and prevents future read and write operations on unreliable media areas.

The Allocation Unit and Cluster-Level Tagging

In MS-DOS 6.22, disk storage is divided into clusters, which are fixed groups of contiguous sectors (typically ranging from 512 bytes to 32 kilobytes). The operating system does not allocate or isolate individual 512-byte sectors within the file system; instead, it tracks media health at the cluster level. If even a single sector within a cluster fails a Cyclic Redundancy Check (CRC) or read verification test, MS-DOS marks the entire cluster as unusable.

The Bad Cluster Marker Values

Each entry in the File Allocation Table maps directly to a specific cluster number on the drive. To prevent the DOS kernel from writing data to damaged media, a reserved marker is written directly into that cluster's entry:

• FAT12: The value 0xFF7 is written to the 12-bit entry.
• FAT16: The value 0xFFF7 is written to the 16-bit entry.

When the MS-DOS input/output system searches the FAT for available storage (represented by 0x0000), it interprets the 0xFFF7 code as a defective cluster and immediately skips it during the allocation sequence.

Detection and Marking by Utilities

MS-DOS 6.22 primarily relies on two disk utilities to discover defects and update the FAT:

1. FORMAT: During a standard format, the utility attempts to read and verify every sector on the partition. When a read error occurs, FORMAT flags the corresponding cluster entry in the FAT as 0xFFF7. These entries are preserved so that subsequent operations know to avoid them from the moment the file system is created.
2. SCANDISK: MS-DOS 6.22 introduced SCANDISK as the default diagnostic utility, replacing the older surface-scan capabilities of CHKDSK. When SCANDISK performs a surface scan and encounters an unreadable sector:
   • If the cluster is unallocated, it simply writes 0xFFF7 into the FAT.
   • If the cluster is currently occupied by a file, SCANDISK reads whatever data can still be salvaged, copies that data to a newly allocated healthy cluster, adjusts the file's allocation chain in the FAT to point to the new cluster, and then marks the original defective cluster with 0xFFF7.

Synchronization of Primary and Secondary FATs

To safeguard against file system corruption, FAT file systems maintain two identical copies of the table: FAT1 (primary) and FAT2 (backup), located consecutively near the start of the partition. Whenever a utility marks a bad cluster with 0xFFF7, the change is mirrored immediately across both FAT tables. This ensures that if the operating system or a repair utility references FAT2, the bad cluster remains flagged and out of circulation.

Persistence of Bad Sector Flags

Once a cluster is marked as 0xFFF7, MS-DOS treats the mark as permanent. Standard file operations (copying, moving, or deleting) will not overwrite or clear this entry. The cluster remains removed from the total pool of available disk space reported by commands such as DIR and CHKDSK until a low-level format is performed or a specialized disk utility explicitly tests and clears the cluster marker.