Disk compression

A disk compression software utility increases the amount of information that can be stored on a hard disk drive of given size. Unlike a file compression utility, which compresses only specified files—and which requires the user to designate the files to be compressed—an on-the-fly disk compression utility works automatically through resident software without the user needing to be aware of its existence. On-the-fly disk compression is therefore also known as transparent, real-time or online disk compression.

When information needs to be stored to the hard disk, the utility compresses the information. When information needs to be read, the utility decompresses the information. A disk compression utility overrides the standard operating system routines. Since all software applications access the hard disk using these routines, they continue to work after disk compression has been installed.

Disk compression utilities were popular especially in the early 1990s, when microcomputer hard disks were still relatively small (20 to 80 megabytes). Hard drives were also rather expensive at the time, costing roughly 10 USD per megabyte. For the users who bought disk compression applications, the software proved to be in the short term a more economic means of acquiring more disk space as opposed to replacing their current drive with a larger one. A good disk compression utility could, on average, double the available space with negligible speed loss. Disk compression fell into disuse by the late 1990s, as advances in hard drive technology and manufacturing led to increased capacities and lower prices.

Common disk compression solutions

Standalone hardware

Some of the initial disk compression solutions were hardware-assisted and utilized add-on compressor/decompressor coprocessor cards in addition to a software driver. Known solutions include:

* Stacker XT/8 and Stacker AT/16 from Stac Electronics
* Expanz! and Expanz! Plus from InfoChip Systems
* DiskDoubler dd2000 from Datran Corporation
* MRCI (Microsoft Real-Time Compression Interface) from Microsoft

Standalone software

With increasing PC processor power software-only solutions began to reach or even outperform the performance of hardware-assisted solutions in most scenarios. These compression utilities were sold independently. A user had to specifically choose to install and configure the software.

* Squish and Squish Plus from Sundog Software
* Stacker from Stac Electronics
* XtraDrive from Integrated Information Technology (IIT)
* SuperStor and SuperStor Pro from AddStor
* DoubleDisk and DoubleDisk Gold from Vertisoft Systems
* JAM (JAM.SYS) from JAM Software, Kiev, Ukraine
* DiskDoubler from Salient Software
* Double Density from Data Becker

Bundled software

The idea of bundling disk compression into new machines appealed to resellers and users. Resellers liked that they could claim more storage space; users liked that they did not have to configure the software. Bundled utilities included (in chronological order):

* DR DOS 6.0 (1991), from Digital Research, included a custom version of AddStor's ''SuperStor''.
* PalmDOS 1.0 (1992), from Novell (after acquiring Digital Research), included a DPMS-enabled version of ''SuperStor''.
* MS-DOS 6.0 (1993), from Microsoft, included '' DoubleSpace'', which was based in part on Vertisoft Systems' ''DoubleDisk''. MS-DOS 6.0 introduced the so-called DOS preload API, an undocumented challenge–response type interface which allowed the disk compression software (DBLSPACE.BIN) to be loaded automatically even before CONFIG.SYS was processed.
* PC DOS 6.1 (1993), from IBM, while the original release did not include a disk compressor, a custom version of ''SuperStor/DS'' (after acquiring AddStor) was included in a later release, also introducing the preload API in the PC DOS line.
* MS-DOS 6.2 (1993), from Microsoft, included an improved version of ''DoubleSpace''.
* Novell DOS 7 (1993), from Novell, included a DPMS-enabled version of ''Stacker 3.12'' (STACKER.BIN) from Stac Electronics, which also supported compressed data exchange with remote NetWare and PNW servers also running Stacker in order to avoid unnecessary recompression when copying files. It also introduced support for the DOS 6 preload API in order to maintain compatibility with Microsoft's DoubleSpace in multi-boot scenarios. A similar mechanism was used to load the optional single- or multi-user SECURITY component.
* PTS-DOS Extended 6.4 (1994), from PhysTechSoft, included a disk compression component named ''Folder'' (FOLDER.SYS)
* PC DOS 6.3 (1994), from IBM, included a version of ''SuperStor/DS''.
* MS-DOS 6.22 (1994), from Microsoft, included '' DriveSpace'', which was ''DoubleSpace'' with a different compression algorithm (DRVSPACE.BIN), after MS-DOS 6.21 had to ship without any disk compression software at all for legal reasons.
* PC DOS 7.0 (1995), from IBM, included a DPMS-enabled version of ''Stacker 4.02'' from Stac Electronics.
* Windows 95 (1995), from Microsoft, included an updated version of ''DriveSpace 2