DoubleSpace

DriveSpace (initially known as DoubleSpace) is a disk compression utility supplied with MS-DOS starting from version 6.0. The purpose of DriveSpace is to increase the amount of data the user could store on disks, by transparently compressing and decompressing data on-the-fly. It is primarily intended for use with hard drives, but use for floppy disks is also supported.

In the most common usage scenario, the user would have one hard drive in the computer, with all the space allocated to one partition (usually as drive C:). The software would compress the entire drive contents into one large file in the root partition. On booting the system, the driver would allocate this large file as drive C:, enabling files to be accessed as normal.

Microsoft's decision to develop DoubleSpace and add it to MS-DOS was probably influenced by the fact that DOS-based operating systems from other manufacturers (IBM and Novell) had started including disk compression software in their products.

Instead of developing its own product from scratch, Microsoft licensed the technology for the DoubleDisk product developed by Vertisoft and adapted it to become DoubleSpace. For instance, the loading of the driver controlling the compression/decompression (DBLSPACE.BIN) became more deeply integrated into the operating system (being loaded even before the CONFIG.SYS file).

Microsoft had originally sought to license the technology from Stac Electronics, which had a similar product called Stacker, but these negotiations had failed. Microsoft was later successfully sued for patent infringement by Stac Electronics for violating some of its compression patents. During the court case Stac Electronics claimed that Microsoft had refused to pay any money when it attempted to license Stacker, offering only the possibility of Stac Electronics to develop enhancement products.

A few computer programs, particularly games, were incompatible with DoubleSpace because they effectively bypassed the DoubleSpace driver. DoubleSpace also consumed a significant amount of conventional memory, making it difficult to run memory-intensive programs.

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