MS-DOS 6.22 Disk Write Verification Utility

In MS-DOS 6.22, the primary utility that verifies disk write operations at the physical sector level is the internal VERIFY command. This article explains how the VERIFY command operates, how it confirms data integrity at the sector level through disk controller checks, and how it compares to companion utilities such as COPY /V and ScanDisk.

The VERIFY Command in MS-DOS 6.22

The VERIFY command is an internal MS-DOS utility designed to ensure that data written to a storage medium (such as a floppy disk or hard drive) is physically recorded without sector-level errors. When enabled, MS-DOS instructs the storage subsystem to validate every write operation immediately after it occurs.

How Sector-Level Verification Works

Rather than performing a slow, byte-by-byte comparison between memory and the disk surface, MS-DOS 6.22 leverages low-level BIOS disk services (specifically Interrupt 13h) and the disk controller's hardware capabilities:

  1. Write Operation: MS-DOS sends a request to write a cluster of data across specific physical sectors.
  2. Verify Signal: With verification active, MS-DOS issues a verify command to the controller immediately following the write.
  3. CRC/ECC Validation: The disk controller attempts to read the sectors that were just written and recalculates their Cyclic Redundancy Check (CRC) or Error-Correcting Code (ECC) checksums.
  4. Error Handling: If the calculated checksum does not match the value recorded in the sector header, the controller returns a hardware error to DOS, triggering the standard "Abort, Retry, Fail?" prompt.

Because validation relies on sector readability and checksum integrity rather than comparing data contents against RAM, it operates strictly at the physical sector level.

Syntax and Usage

The VERIFY utility is executed directly from the MS-DOS command prompt or within batch files:

  • Enable Verification: VERIFY ON instructs MS-DOS to verify all subsequent sector write operations.
  • Disable Verification: VERIFY OFF turns off automatic sector verification, which is the default operating state to maximize disk performance.
  • Check Status: Typing VERIFY with no parameters displays whether the setting is currently ON or OFF.

The /V Switch in File Commands

MS-DOS 6.22 also integrates this verification routine directly into file management commands:

  • COPY /V: Forces MS-DOS to perform sector-level verification for the target file, even if global verification is set to OFF.
  • XCOPY /V: Applies sector verification to batch file copy operations across subdirectories.

VERIFY vs. ScanDisk

While the VERIFY command monitors real-time disk write operations, MS-DOS 6.22 also includes ScanDisk (SCANDISK.EXE), an external utility designed for diagnostic purposes. Unlike VERIFY, ScanDisk performs comprehensive surface scans of existing sectors, detecting defective sectors, reallocating data, and marking physically damaged tracks directly in the File Allocation Table (FAT).