DriveSpace

Drive Space (formerly Double Space ) is a system program under MS- DOS, Version 6.0/6.2 (under the name Double Space) and from version 6.22, as well as under higher versions also an integral part of Microsoft Windows 9x, version 95, with a FAT16 - formatted disk can be compressed. It is a so-called on-line compression because the compression / decompression online, so at runtime automatically in the background, is performed. The system program has been integrated into DOS version 6.0/6.2 as Double Space, due to a court order for a patent dispute in version 6.21 removed and then re-implemented in version 6.22 under the name Drive Space. For the Windows versions from Windows 95 was added a graphical user interface for managing, as well as a 32- bit drivers.

Operation

In this compressed disk, the data is compressed into a single file (eg DRVSPACE.000 ). In this case, the storage space required is reduced in several steps, so that the effective capacity of the disk increases. In the best case, the space savings should be about 50 percent as specified by Microsoft, there are practically but usually 30 to 40 percent on a Windows system drive. The different compression rate depends, as with ZIP files, the amount of unused memory or similar sequences of bytes within a file. For Word files in the old format (*. DOC), the savings is particularly high, with many image file formats ( such as *. JPG) it is hardly noticeable.

When a compressed volume is set, gets the drive on which the file is located with the compressed data, a new drive letter. The original drive letter receives the compressed "virtual" disk, whose content is stored in the file of uncompressed "Host Drive." This shift from drive letter is necessary because preserving the file paths in the system registry and in the initialization file. There also remain at least the most necessary for the computer to start DOS files that are needed before the device driver for compressed drives will be loaded if it is the compressed volume is the system drive. In all other cases can the media almost completely compressed. A decompression of the drive is only possible if remaining on the original (host ) volume of temporary space.

Configuration

The configuration is only possible with fixed disk sizes. Remains much room on the uncompressed drive, leaving little space on the compressed volume. In the opposite case the space for the compressed volume rises to the host disk. A change these limits requires creation of a new Disk Organization and therefore requires also computation time, even if only for unused free space between the virtual and the host disk is moved. A dynamic adjustment during operation, as in the online compression modern file systems, is not possible.

An existing compressed volume ( eg file on a removable disk) can be at runtime integrated into the existing operating system ( "Load" and "Unload" called ).

The configuration is possible to ME with a Windows utility under Windows 95, which graphically represents both drives in dependence on each other, indicating the presumed level of compression.

Program versions

The first version of Double Space suffered mainly from poor implementation of the utility. Most users were accustomed to the computer after the termination of an application just turn it off, there was no provision under DOS a controlled shutdown. Since the online compression, however, was already equipped with a buffer ( write cache ), were data that has not been written from the cache to the hard disk will be lost. In addition, some had their own computer games optimized to speed drivers that were incompatible with the standard drivers from Microsoft. There was also a high consumption of conventional memory. The program was therefore limited in application. The problem with the cache could only be eliminated through third-party software, the company AddStor, Inc. offered for an add-on under the name Double Tools for Double Space.

The disk verification ScanDisk was added, and the possibility compressed volume, if necessary, to remove. From DOS 6.22 was the program Drive Space, but was substantially identical to Double Space. Configuration is via the console program DBLSPACE full representation possible for the Windows 3.x must first be completed.

It is no longer a console tool that Drive Space is started under a Windows graphical interface under Windows.

Drive Space 3 is for the first time with Microsoft Plus! available for Windows 95 as a paid add-on product. Here the effectiveness of compression was further enhanced by better algorithms ( HiPack and Ultra Pack ) again. In Windows 95B, the improved program is also integrated without additional software into the operating system. On the technical implementation and in related disadvantages (see section below) nothing changed, the importance of this form of online compression dwindled steadily with the advent of large disks at bargain prices.

Under Windows ME hard drives can not be compressed. Drive Space had almost completely lost its meaning. The last time, however, can be compressed or removable media, for it but the operating system is already a native support for zip files, which are also effective in the handling easier, more compatible and usually.

Disadvantages

Against the use of Drive Space speak the demand for computing power, system-related inaccuracy of the specification of the remaining memory and compatibility issues when transferring the compressed volume between different operating systems. In addition, in a write error equal to all data will be lost if the file should not be repairable. This can already be done in an operating system crash under certain circumstances. The importance of Drive Space waned with the advent of hard drive capacity at reasonable prices very quickly.

Alternative programs

Separately programs that met the same purpose, were Stacker of Stac Electronics and Double Density of Data Becker. Because of patent disputes with Stac Electronics Microsoft Doublespace had to rename Drive Space and reimplement. Due to the integration of Drive Space in the operating system, this previously available separate solutions but have quickly lost in importance again.

With the dwindling importance of online compression during the selling period of Windows 98 won simpler compression methods for archiving files in importance. Zip programs are no longer tied to a particular volume, which thus created Zip archives are available on any computer and also on removable media. In modern file systems, such as those used by Windows NT - based systems, the online compression is gaining importance again since compression takes place here at the file level and the compression attribute therefore part is for example the NTFS specification. An error of a file can therefore no longer destroy the entire drive structure and the compatibility between different media and operating systems is guaranteed at all times.