DoubleSpace

DoubleSpace was the original name of the disk compression software that was supplied with MS-DOS starting from version 6.0. The purpose of DoubleSpace was to increase the amount of data the user could store on disks, by transparently compressing and decompressing data on-the-fly. It was primarily intended for use with hard drives, but use for floppy disks was also supported.

Overview

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 letter C). When the user ran DoubleSpace for the first time, it would offer to compress the selected drive. The initial compression was a lengthy process, which could take hours to complete. After the process, all the files previously on the drive would have been compressed and stored in one big file, usually called DBLSPACE.000. The compression process would also enable a special DoubleSpace driver to be loaded as part of MS-DOS during boot. Upon loading this driver would change the drive letter of the actual drive on the disk to H (which was also known as the Host Drive) and would then map the drive letter C to the compressed drive 'contained' in DBLSPACE.000, compressing and decompressing data as needed. DoubleSpace could also be used to create a blank compressed drive out of space from one of the physical drives in the system.

DoubleSpace was not the first program of its class. For instance, Stacker from Stac Electronics had existed for years as an add-on product. Microsoft's decision to create 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.

Reception and incompatibilities

Initially, DoubleSpace was surrounded by an air of mystery. Many had difficulties understanding how it was possible to store more data on the disk than it could actually contain. This led to much speculation, with some users thinking DoubleSpace changed the way data was stored physically on the disk, and there were rumors that using DoubleSpace would reduce the longevity of the disks or the computer itself. This was of course a misunderstanding: DoubleSpace used purely logical/software means to obtain its results and was not different from other compression tools like PKZIP; except that it handled the compression/decompression transparently to the user.

A few computer programs, particularly games, were incompatible with DoubleSpace because of the way they accessed the disk: in effect bypassing the DoubleSpace driver. DoubleSpace also consumed a significant amount of conventional memory, making it difficult to run memory-intensive programs.

Some users reported data loss believed to be caused by DoubleSpace. Some of these cases were attributed to cases where the memory used by DoubleSpace was corrupted by other programs, and Microsoft attempted to remedy this in the MS-DOS 6.2 version of DoubleSpace. Another situation that led to data loss was the accidental deletion of the file on the host drive that contained the compressed data: in effect, a user could delete all his/her data by deleting just one file.

Another problem was attributed to users turning off their computer before DoubleSpace had finished writing a "virtual file" back to the file DBLSPACE.000. This was compounded by Microsoft concurrently releasing an option that allowed the MS-DOS prompt to reappear for use before MS-DOS had finished writing a file back to disk.

Later versions

MS-DOS 6.2

DOS 6.2 featured a new and improved version of DoubleSpace. The ability to remove DoubleSpace was added. The program SCANDISK introduced in this release was able to scan the non-compressed and compressed drives, including checks of the internal DoubleSpace structures. Security features (known as DoubleGuard) were added to prevent memory corruption from leading to data loss. The memory footprint of the DoubleSpace driver was reduced compared to the version shipped in MS-DOS 6.0.

MS-DOS 6.21

Following a successful lawsuit by Stac Electronics regarding demonstrated patent infringement, Microsoft released MS-DOS 6.21 without DoubleSpace. A court injunction also prevented any further distribution of the previous versions of MS-DOS that included DoubleSpace.

MS-DOS 6.22

MS-DOS 6.22 contained a reimplemented version of the disk compression software, but this time released under the name DriveSpace. The software was essentially identical to the MS-DOS 6.2 version of DoubleSpace from a user point-of-view, and was compatible with previous versions.

DriveSpace in Windows 95

Windows 95 had full support of DoubleSpace/DriveSpace via a native 32-bit driver for accessing the compressed drives, along with a graphical version of the software tools. MS-DOS DriveSpace users could upgrade to Windows 95 without any troubles. Furthermore, the Microsoft Plus! for Windows 95 pack contained version 3 of DriveSpace. This version introduced new compression formats (HiPack and UltraPack) with different performance characteristics for even greater compression ratios along with a tool that could recompress the files on the disk using the different formats, depending on how frequently the files were used etc. One could upgrade from DriveSpace 2 to DriveSpace 3 but not vice versa. One could however decompress a DriveSpace 3 drive. The DOS device driver of DriveSpace 3 had a memory footprint of around 150 KB because of all these new features. This caused difficulty for users rebooting into the MS-DOS mode of Windows 95 for running games, because of the reduced amount of conventional memory available.

DriveSpace 3 also shipped with Windows 95 OSR2 but many features were disabled unless Plus! was also installed, also DriveSpace could not be used with FAT32 making it of little use on PCs with large hard drives.

DriveSpace in Windows 98

Windows 98 shipped with DriveSpace 3 as part of the operating system. Functionality was the same as in Windows 95 with Plus!.

DriveSpace in Windows Me

Because of the removal of real mode support and the decreasing popularity of DriveSpace, DriveSpace in Windows Me had only limited support. DriveSpace no longer supported hard disk compression, but still supported reading and writing compressed removable media, although the only DriveSpace operation supported beside that was deleting and reallocating compressed drives.

Compression for other common Windows file systems

* FAT32 is not supported by DriveSpace tools.
* NTFS has its own compression technology ("compact") native to Windows NT-based operating systems instead of DriveSpace.

External links

* [http://support.microsoft.com/kb/q154997/ FAT32 Compatibility]
* [http://www.examcram2.com/articles/article.asp?p=381919&seqNum=2 DriveSpace not compatible with newer OS]