DriveSpace vs DoubleSpace in MS-DOS 6.22

The primary difference between DoubleSpace and DriveSpace in MS-DOS 6.22 lies in the underlying compression algorithm, which was completely rewritten due to a high-profile legal dispute. While both utilities served the identical purpose of providing transparent, on-the-fly disk compression to expand hard drive capacity, DoubleSpace relied on technology that infringed on third-party patents, whereas DriveSpace used a newly developed, legally compliant compression engine.

Microsoft introduced DoubleSpace with MS-DOS 6.0 in 1993. Shortly after its release, Stac Electronics sued Microsoft, alleging that DoubleSpace infringed on patents used in their competing product, Stacker. In early 1994, a federal court ruled against Microsoft, awarding Stac Electronics $120 million in damages and issuing an injunction against the distribution of software containing the infringing code.

To comply with the court ruling, Microsoft quickly released MS-DOS 6.21 with all compression capabilities completely removed. Shortly thereafter, Microsoft developed a new compression engine and released MS-DOS 6.22, replacing DoubleSpace with DriveSpace.

Technical and Algorithmic Differences

From an architectural standpoint, the core change between the two utilities is the mathematical compression routine:

  • DoubleSpace (DBLSPACE.EXE): Utilized a data compression technique derived from algorithms patented by Stac Electronics, specifically tailored for hardware- and software-based data streaming.
  • DriveSpace (DRVSPACE.EXE): Featured a proprietary Microsoft compression algorithm designed from the ground up to bypass Stac’s patents while offering roughly equivalent compression ratios and read/write speeds.

Compatibility and User Experience

For the end user, the interface, operation, and overall behavior remained practically identical. Both systems stored data in a hidden, monolithic Compressed Volume File (CVF) on the physical drive, which was mounted as a logical drive at startup.

MS-DOS 6.22’s DriveSpace retained full backward compatibility. It could mount, read, and write to existing DoubleSpace volumes without modification. Furthermore, DriveSpace included a utility that allowed users to permanently convert older DoubleSpace CVFs into the newer DriveSpace format.