DriveSpace

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 partition contents into one large file in the root directory. 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 add disk compression to MS-DOS 6.0 was influenced by the fact that the competing DR DOS had earlier started to include disk compression software since version 6.0 in 1991.

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 through the undocumented pre-load API[1] 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 for 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.

Shortly after its release, reports of data loss emerged. A company called Blossom Software claimed to have found a bug that could lead to data corruption. The bug occurred when writing files to heavily fragmented disks and was demonstrated by a program called BUST.EXE. The company sold a program called DoubleCheck that could be used to check for the fragmentation condition that could lead to the error. Microsoft's position was that the error only occurred under unlikely conditions, but fixed the problem in MS-DOS 6.2.[2]

The fragmentation condition was related to the way DoubleSpace compresses individual clusters (of size, say, 8K), and fits them on the disk, occupying fewer sectors (size 512 bytes) than the fixed number required without DoubleSpace (16 sectors in this example). This created the possibility of a kind of internal fragmentation issue, where DoubleSpace would be unable to find enough consecutive sectors for storing a compressed cluster even if plenty of space was available.

Other potential causes of data loss included the corruption of DoubleSpace's memory areas by other programs, DoubleSpace's memory areas were not protected, because MS-DOS ran in real mode. Microsoft attempted to remedy this in the MS-DOS 6.2 version of DoubleSpace (via a feature called DoubleGuard that would check for such corruption).

The fact that the compressed contents of a compressed drive was stored in a single file implied the possibility of a user accidentally deleting all his/her data by deleting just that file. This could happen if the user inadvertently got access to the host drive, containing this file. The host drive was usually mapped to the letter H: by the compression driver. However, if the compression driver had failed to load the user might see it as drive C:

Turning off the computer before DoubleSpace could finish updating its data structures could also result in data loss. This problem was compounded by Microsoft making write caching enabled by default in the SMARTDRV disk cache software that came with MS-DOS 6.0. Because of this change, after exiting an application, the MS-DOS prompt might appear before all data had been written to the disk. However, due to the lack of a controlled shutdown procedure (as found in modern operating systems), many users saw the appearance of the MS-DOS prompt as an indication that it was safe to switch off the computer, which was typically the case prior to MS-DOS 6.0. Microsoft addressed this issue in MS-DOS 6.2 where the write caching was still enabled by default, but where the cache would be flushed before allowing the command prompt to reappear.

AddStor, Inc. offered an add-on product called Double Tools for DoubleSpace. It contained a number of tools to enhance the functions of the version of DoubleSpace that came with MS-DOS 6.0. This included various diagnostic features, the ability to have compressed removable media auto-mounted as they were used, as well as support for background defragmentation of DoubleSpace compressed drives.[3] To defragment files in the background, it was possible to let DoubleTools replace the low-level DoubleSpace driver (DBLSPACE.BIN) with one supplied by DoubleTools. Replacing the driver also enabled other enhanced functionality of the product, such as the use of 32-bit code paths when it detected an Intel 80386 or higher CPU, caching capabilities and - in addition to its supporting the use of the Upper Memory Area - also permitted the use of Extended Memory for some of its buffers (reducing the driver's total footprint in conventional and upper memory, albeit at the cost of somewhat reduced speed).[4] Another function was the ability to split a compressed volume over multiple floppy disks, being able to see the entire volume with only the first disk inserted (and being prompted to change discs as necessary). It was also possible to share a compressed volume with a remote computer. Double Tools also had the capability to put a special utility on compressed floppy disks that made it possible to access the compressed data even on computers that didn't have DoubleSpace (or Double Tools).

Vertisoft, the company who developed the DoubleDisk program that Microsoft subsequently licensed and turned into DoubleSpace, developed and sold a DoubleSpace add-on program called SpaceManager, which contained a number of usability enhancements. It also offered improved compression ratios.

Other products, like later versions of Stacker from Stac Electronics, were capable of converting existing DoubleSpace compressed drives into their own format.

DMSDOS, a Linux kernel driver,[5][6] was developed in the late 1990s to support both the reading and writing of DoubleSpace/DriveSpace disks. However, reading and especially writing to compressed filesystems is reliable only in specific versions of the 2.0, 2.1 or 2.2 versions of the kernel.

While DR-DOS supported its own disk compression technology (originally based on SuperStor, later on Stacker), Novell DOS 7 in 1993 and higher introduced an emulation of the undocumented pre-load API in order to provide seamless support for DoubleSpace as well.[1] Since the DR-DOS drivers were DPMS-enabled whereas the MS-DOS ones were not, this did not offer any advantages for DR-DOS users, but allowed easier coexistence or migration due to the possibility of shared use of already existing compressed volumes in multi-boot scenarios. DR-DOS 7.02 and higher also added support for DriveSpace in 1998.[7]

• Cooper, Jim (2001). Special Edition Using MS-DOS 6.22, Third Edition. Que Publishing. ISBN 978-0789725738.
• Tim O'Reilly; Troy Mott; Walter Glenn (1999). Windows 98 in a Nutshell: A Desktop Quick Reference. O'Reilly. ISBN 978-1565924864.

Wikibooks has a book on the topic of: Guide to Windows Commands

• "Microsoft DoubleSpace System Application Programming Interfaces (API) v1.00.05 and DoubleSpace Compressed Volume File (CVF) format" (ARJ). Microsoft. 1993 [1992]. Archived from the original on 2016-03-03.
• DoubleSpace Overview
• Mapping DOS FAT to MDFAT
• DoubleSpace Compressed Volume File Layout
• Microsoft Real-time Compression Interface (MRCI)