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Troubleshooting Disk Compression Programs
DocumentID: 659290
Revision Date: 29-Feb-96 8:29:48 PM

The information in this document applies to:
WordPerfect® 6.0 for DOS

Problem

Solutions: What is a disk compression program?
On-the-fly compression programs (also known as real-time compression programs) work in the background, shrinking and expanding files as they are accessed in a way that is totally transparent to the user, and more importantly, to DOS. These programs use hardware, software, or both, to make entire drives or portions of drives compressed environments. In the process, they give the appearance of doubling the size of available space on a variety of fixed and removable media.

In plain English, disk compression programs gives you more available disk space by condensing the existing data on your hard drive. When you use the programs and files that have been condensed, they are decompressed "on-the-fly".

What are some of the most common disk compression programs?
1.      Stacker, by Stac Electronics
2.      SuperStor, by AddStor (sold separately and included with DR DOS 6.0)
3.      DoubleDisk, by Verisoft Systems
4.      Expanz!, by InfoChip Systems
5.      DoubleSpace by Microsoft (included with MS-DOS 6.0)

Lossless Data Compression - How it Works:
1.      Once you've loaded a data compression device driver into memory, it intercepts data going to and from the hard drive or diskette. When you invoke your application's File Save command, data compression utilities channel the data into a 2,048-byte history buffer in your PC's RAM. Some hardware-assisted compression systems contain their own RAM for this purpose.

2.      As the buffer fills with bytes of data, the device driver scans the succession of binary information for matching strings of characters. It sends the first instances of characters straight to DOS to write to disk. The first time the word the goes through, for instance, the compression utility will send through the five characters that make up the string (the opening space, three letters, and the final space).

3.      As with a regular File Save procedure, DOS checks your file allocation table (FAT) and finds the address of a cluster to store the data. DOS translates the FAT address into logical sector numbers that the ROM BIOS will recognize, and the ROM BIOS begins to write the data to the disk.

4.      When the compression device driver sees a repeated instance of a character string, it creates a two-part code to send to the disk in its place. This code (also called a token) has two parts: The first indicates the byte offset between this instance of the word and the first instance--a kind of "pointer"; the second indicates the length of the byte string. The token replacing the word is much shorter than the word itself. The word the for instance, comprises 40 bits--that's five bytes. The token replacing it, however, is almost a quarter of its size at 11 bits--seven for the address, and four for the byte length.

5.      When you retrieve a compressed file, DOS initially makes most of the calls. First, it checks the FAT address of the clusters that make up the file and translates the address into a logical sector number that the ROM BIOS can recognize. Then DOS calls the compressing device driver. Instead of going straight into working memory as usual, the file first passes through the compression driver's buffer.

6.      The compression algorithm reads the data passing through the buffer. When it finds the cluster address and byte string length that make up a compressed data token, it inserts the relevant string into the file in its buffer, then passes it to working memory for application software to read. Because of this final decrypting stage, DOS alone can't read compressed data off a diskette. This is why booting up without the compression program's drivers does not allow you to access the data on the disk. If you boot up with the drivers, you should then be able to successfully access the compressed data.

How Disk Compression Programs Affect Your Hard Drive
When installing a Disk Compression program, the hard drive appears to be divided into two partitions with one partition left uncompressed and one partition compressed. In reality, the compressed drive is actually a large, hidden file that occupies all of the space on the uncompressed drive that was specified for the compressed drive. Stacker names this hidden file STACVOL.DSK or STACVOL.000, SuperStor names it SSPARTSS.SWP, and DoubleDisk names it DBLSPACE.000.

For example, if your C: drive is a 100M hard drive and you tell the compression software to compress 90M and leave 10M uncompressed, it really creates a 90M file which contains the compressed data. This file is addressed as a logical drive. For convenience, the drive designations are logically swapped. In this example, you would have your C: drive and the logical D: drive. These would be swapped so you would access the compressed drive as C: and the uncompressed portion as D:.

NOTE: If this hidden file is damaged or deleted, all of the data on the compressed "drive" is lost. It is very important that these files not be deleted, renamed, moved, etc. That is why they are assigned the hidden, system, and (usually) Read-Only attributes.

When the computer is turned on, the computer will only recognize the uncompressed portion of the disk which it calls C: (the compressed portion of the drive is not even recognized yet.)

The CONFIG.SYS file is read from this uncompressed portion of the hard drive. Once the disk compression device (STACKER.COM for Stacker, SSTORDRV.SYS for SuperStor, DBLSPACE.SYS for DoubleSpace) is loaded, the hard drive can then recognize the compressed portion of the hard drive.

Next, the CONFIG.SYS file reads the device driver which swaps the drive letters (SSWAP.COM for Stacker, DEVSWAP.COM for SuperStor).

Notice that D: is now the uncompressed portion of the hard drive and C: is the compressed portion of the hard drive. Only the drive letters have changed, no data has been moved or altered. Also note that any device drivers that are listed before the disk compression drivers in the CONFIG.SYS file must be located on the uncompressed drive. Any devices listed after the disk compression drivers can be located on the compressed drive.

NOTE: The device driver that swaps the drive letters (SSWAP.COM or DEVSWAP.COM) driver is not a required device. If the user so chooses, he may remove this device driver and work with the uncompressed portion of the hard drive still labeled C: and the compressed portion of the drive labeled D:. This is not a change that you should make or recommend, just be aware of the possibility. If you take out this driver, all paths, pointers, *.INI files, menus, etc. would have to be changed to reflect the change in drive name.

The purpose of this driver is to make the disk compression program "invisible" to the user. Once the disk compression program is installed, the drive swapping device allows the user to do all work on the C:\ drive -- the same way the user did before installing the disk compression program. In fact, some users aren't even aware of the existence of the D: uncompressed drive.

What if I accidentally boot up without these drivers?
Contrary to popular belief, booting without these drivers will not reformat, damage, or in any other way destroy any data. The only effect will be the inability to access the compressed portion of the hard drive, which itself can be difficult to remedy.

In effect, booting without the disk compression drivers will only allow you to access the uncompressed disk drive, which may be 1M or less in size. Without the drivers, the compressed portion of the hard drive is not recognizable by DOS. DOS will only know that there is a huge hidden file on the C:\ drive. To access the compressed portion of the hard drive, the devices would need to be restored to the CONFIG.SYS file and the machine rebooted. This could be difficult since the customer's text editor programs (including Edlin) are most likely located on the compressed drive, which is now inaccessible. One possibility is to use the TYPE command to type out the CONFIG.SYS file on the screen, and then use the COPY CON command and recreate the CONFIG.SYS file totally, reinserting the Disk Compression drivers. Another option would be to rename the existing CONFIG.SYS file, and then create a very simple CONFIG.SYS file with only the disk compression drivers. The customer could then access a text editor to remedy the problem.

The bottom line is this: Don't remove any drivers that you or the user do not recognize. It is very possible that the user is not aware that a disk compression program has been installed on the system.

General Stacker Information:
Stacker 2.0 is available in two versions: a software-only system intended for PCs without available slot space (especially laptop, notebook, and portable systems), and a hardware/software combination that includes a coprocessor board for XT, AT, and IBM PS/2-compatible Micro Channel systems. In all versions, Stacker works behind the scenes, compressing and decompressing files as you open, close, move, and edit them. Stacker doubles your available hard disk space by halving the sizes of the files and programs you store on it.

The software-only version is just as capable as the hardware, but it will make storing files to your hard disk and retrieving them again a bit slower than usual.

Stacker will work with DOS Versions 3.0 and later, but if you use any version of DOS earlier than 4.01, you're limited to hard disk partitions no larger than 32M. In that case, you should strongly consider upgrading to DOS 5.0, which supports large partitions.

Finally, you should be aware that Stacker won't work with your hard disk unless it's formatted with 512K sectors. Most hard disks are, unless you use a third-party hard disk partitioning product. If your disk uses a non-DOS formatting scheme, you'll have to reformat and repartition it before installing Stacker.

Stacker 3.0 adds Windows utilities and improves its compression routines. Stacker 3.1 adds MS DOS 6.0 compatibility (although no incompatibilities have been noted with Stacker 3.0 and MS DOS 6.0). Stac Electronics also offers Stacker SE which includes a conversion option to convert from MS DOS 6.0 DoubleSpace to Stacker.

Stacking your drive:
When you install Stacker, you have two options to decide on how to stack your drive. The first leaves your existing data alone and uses some or all of the free space on your disk for the Stacker volume, where the compressed files will ultimately reside. This option is a good idea only for a brand-new disk or a disk that is less than a third full. If you take this route, you'll end up with an unstacked hard disk volume as the C: drive and a larger Stacked, or compressed, volume as drive D:.

For most users, the second option makes the most sense. It compresses all the existing files and programs, and makes all but a section of the drive a Stacker volume for compressed data.

After you've selected an option, Stacker will ask you how large you want the uncompressed area on your hard disk to be. This area will be where essential DOS files such as AUTOEXEC.BAT, CONFIG.SYS, and any drivers or TSRs loaded at boot-up must reside. Accepting the default size (1M) is a good idea; you can always go back later and make the uncompressed area larger, but you can't make it smaller.

After all this, Stacker will use its modified copy of Symantec's Norton Speed Disk to defragment your disk and make all free space contiguous. Stacker then creates a large hidden file and begins compressing and transferring your files into it. Ultimately, the hidden file will occupy all of your disk except the area you've chosen to remain uncompressed.

Troubleshooting Stacker:
The following is an example of a typical CONFIG.SYS file with Stacker installed:

DEVICE=C:\WINDOWS\HIMEM.SYS
FILES=40
BUFFERS=10
SHELL=C:\COMMAND.COM C:\ /P /E:256
DEVICE=C:\STACKER\STACKER.COM C:\STACVOL.DSK
DEVICE=C:\STACKER\SSWAP.COM C:\STACVOL.DSK /SYNC

The device drivers STACKER.COM and SSWAP.COM in the CONFIG.SYS file indicate the presence of Stacker on the system.

Stacker will maintain copies of your AUTOEXEC.BAT and CONFIG.SYS in both the compressed (Stacked) volume and the uncompressed volume. By using the /SYNC switch after the SSWAP.COM device, Stacker keeps both copies of these files identical.

The CONFIG.SYS file is read from the uncompressed drive. However, since the SSWAP.COM device switches the drive letters and the AUTOEXEC.BAT file is read after the CONFIG.SYS file, the AUTOEXEC.BAT will be read from the compressed (Stacker) drive.

As long as the /SYNC command is used after SSWAP.COM, you can edit the AUTOEXEC.BAT and CONFIG.SYS files on either drive and be assured that the changes will take effect upon rebooting the system.

Stacker swaps the drives in order to make the program transparent to the user. Customers may remove this device so that the drives are not swapped. If this device is removed, the customer would have to change each of the programs on the Stacked volume to recognize the drive change (from C: to D: for example). For example, if the customer originally had WordPerfect for Windows on drive C and then installed Stacker, WPWin would then be on drive D:. If he left the SSWAP.COM command in the CONFIG.SYS, WPWin would then be on the C: drive and no modifications would be necessary. However, if the customer removed SSWAP.COM, he would then need to edit WPWin's .SET, .INI, and .ENV files to reflect the change to drive D:.

If you eliminate the device STACKER.COM and reboot the computer, the compressed drive will not be recognized. Only the non-compressed drive will be recognized. If you replace the devices in CONFIG.SYS and reboot, you should then be able to access the compressed (Stacker) volume. This may be difficult since the customer's text editing programs, including Edlin, are probably located on the compressed drive. You can use the command TYPE C:\CONFIG.SYS to type the CONFIG.SYS file to the screen, and then use COPY CON CONFIG.SYS and recreate the file, reinserting the drivers in the process. Rebooting should then restore access to the compressed drive.

Stacker creates a hidden file in the root directory of your boot drive. The file is named STACVOL.DSK. It has the attributes Hidden, System, Read-Only. This is your Stacker volume. The size of this file will be equal to the amount of remaining disk space beyond the 1M Stacker allows for the uncompressed drive. By no means should you modify this file. If this file is damaged or deleted, the entire Stacker drive along with all of the programs and data on it will be lost.

If you use startup options to route virtual files, temporary files, or printer files to the non-compressed drive, you should make sure that the non-compressed drive is large enough to accommodate them. By default, Stacker sets the non-compressed drive to 1M. This may not be large enough to accommodate a lot of devices, TSRs, and files from other programs.

If the non-compressed drive is large enough, routing temporary and virtual files may speed up your programs, since these files won't then be continuously compressed and uncompressed. However, the speed increase may not be dramatic, and this option is not a "cure-all" for speed issues.

The Stacker command SCHECK or CHECK will perform the same function on a compressed drive as the DOS command CHKDSK does on a non-compressed drive. It can also be used to determine the overall compression ratio of the drive. SCHECK will also give you estimates of the amount of additional data that can be stored on the Stacker drive. The bytes free value in the Stacker Drive column shows the maximum amount of additional data that can be stored on the Stacker drive. It represents the "best case" for how much data you can store on the Stacker drive.

Note: In Stacker 1.x and 2.0, the disk checking utility is called SCHECK; in Stacker 3.0 and newer, the name was changed to CHECK.

      The bytes free value in the STACVOL File column shows the actual amount of hard disk space remaining for storing compressed data on the Stacker drive. It is a "worst case" projection, i.e., the minimum amount of data that will fit on the Stacker drive if the data does not compress at all.

      The projected bytes free value is an estimate that lies somewhere between the maximum and minimum values and is based on the overall compression ratio of the data currently stored on the Stacker drive.

The SWAPMAP utility from Stacker identifies which drive was the boot drive before being switched by the SSWAP driver.

DOS commands such as DIR, COPY, and DELETE work normally with Stacker volumes.

In most cases, Stacker will only leave 1M of disk space for the uncompressed drive. If you have a Windows swap file on the machine, Stacker will ask you if you want to reserve enough space on the uncompressed drive for the swap file. If you answer yes, you can create the swap file on the uncompressed drive after Stacker has been installed.

When you check the amount of free disk space on a Stacked volume, the number shown is based on the compression ratio of the data already stored on the stacked drive. For example, if all of the data on the Stacked drive has been compressed at a 2:1 ratio, the estimate of available space on the stacker drive will be based on a 2:1 compression ratio. If you add a program to the stacked drive that compresses at a 1.5:2 ratio, it may appear that the program is taking up more space than it should.

For example, say that Stacker reports that there are 12M available on the Stacked volume, based on a 2:1 compression ratio. You decide to copy a bunch of zipped files to this drive. These files do not compress at all (i.e., a 1:1 compression ratio). The stacked volume would be full after you copy 6M of these files.

Troubleshooting Stacker and Windows
If "Bad Or Missing " errors occur while the CONFIG.SYS is loading, it is likely these device drivers are called prior to the Stacker device drivers making the system aware of "stacked" volumes. There are two solutions to this problem:
      1.      Make a copy of the device drivers that are not loading to the uncompressed physical C: drive. Edit the CONFIG.SYS so the device driver lines call the files from the physical C: drive.
      2.      Move the device drivers lines so they are listed after the Stacker device drivers.

Never use SMARTDrive or any other disk caching software to cache a "stacked" volume.
      
Make sure SMARTDRV.EXE is loading after SSWAP.COM. This will ensure that SMARTDrive is caching only the physical drives, and not the logical "stacked" volumes.
      NOTE: The SMARTDrive command line can be edited to allow caching of "stacked" volumes by adding a + command (where is a "stacked" volume). If caching has been enabled on a "stacked" volume, disable it. This can cause file damage.

If the message "Insufficient Disk Space" is encountered when adding applications to a "stacked" volume, it is possible that the disk is full, even if MS-DOS CHKDSK reports sufficient disk space. The MS-DOS DIR command is the only accurate check of disk space on a "stacked" volume.

When SSWAP.COM is used to swap "stacked" volume drive designations with uncompressed volume drive designations, Windows Setup will issue a message which warns the user that Setup cannot edit the CONFIG.SYS and AUTOEXEC.BAT. These two files will have to be edited manually, using the CONFIG.WIN and AUTOEXEC.WIN files (found in the Windows directory) as a reference for what should be added to the existing CONFIG.SYS and AUTOEXEC.BAT.

Stacker installation will delete empty subdirectories. If Windows is already set up, prior to the Stacker installation, your Windows TEMP subdirectory will be deleted. The user should re-create the TEMP subdirectory prior to using Windows again.

Do not attempt to create a permanent swap file on a "stacked" volume. Permanent swap files can be created on uncompressed volumes.

Hard disk defragmentation utilities such as Norton Speed Disk and PCTools Compress should never be used on a "stacked" volume. Stacker 2.0 includes a utility called SDEFRAG.EXE that is safe to use on a "stacked" volume. Do not run SDEFRAG.EXE while Windows is running. Refer to the Stacker documentation for more information.

SuperStor Troubleshooting
The following is an example of a typical CONFIG.SYS file with SuperStor installed:

DEVICE=C:\HIMEM.SYS
DEVICE=\SSTORDRV.SYS
DEVICE=\DEVSWAP.COM
FILES=40
BUFFERS=15
SHELL=C:\COMMAND.COM C:\ /P /E:256
INSTALL=C:\DOS\SHARE.EXE

SuperStor will install the drivers SSTORDRV.SYS and DEVSWAP.COM to the CONFIG.SYS file. SSTORDRV.SYS is the main SuperStor system driver, and DEVSWAP.COM is the driver used to swap the drive letters.

The utility SSTOR can be used to remove a SuperStor drive, generate statistics about a SuperStor drive, and to obtain help regarding the status of SuperStor.

The SuperStor utility SSUTIL is used to (1) analyze and verify the integrity of a SuperStor disk (2) repair damage to a SuperStor disk, (3) optimize a SuperStor disk, and (4) shrink or expand the size of a SuperStor disk.

A Windows permanent swap file is not compatible with a SuperStor drive. The permanent swap file must be placed on a disk managed by DOS (an uncompressed disk). Temporary Windows swap files may be used on a SuperStor drive.

When you install SuperStor, SuperStor creates a hidden file in the root directory of your boot drive. The file is named SSPARTSS.SWP. It has the attributes Hidden, System, Read-Only. This is the SuperStor volume. The size of this file will be equal to the amount of remaining disk space beyond the disk space allowed for the uncompressed drive. By no means should you modify this file. If this file is damaged or deleted, the entire SuperStor drive along with all of the programs and data on it will be lost.

SuperStor maintains a copy of the CONFIG.SYS file on both the compressed and uncompressed drives. If you make editing changes to the CONFIG.SYS file that is on the uncompressed drive, the CONFIG.SYS file on the compressed drive is not updated. However, if you make modifications to the CONFIG.SYS file on the compressed drive, the CONFIG.SYS file on the uncompressed drive will be updated.

SuperStor only maintains a copy of the AUTOEXEC.BAT file on the compressed drive. Unless the user copies the AUTOEXEC.BAT file to the uncompressed drive, no AUTOEXEC.BAT file will be found there. Therefore, if the SuperStor drivers are removed from the CONFIG.SYS file, two problems exist (1) no AUTOEXEC.BAT file will be found upon boot-up, and (2) the compressed drive will not be accessible. If the customer restores the drivers, the computer should boot up correctly and have access to the compressed drive.

SuperStor swaps the drives in order to make the program transparent to the user. Customers may remove this device so that the drives are not swapped. If this device is removed, the customer would have to change each of the programs on the compressed volume to recognize the drive change (from C: to D: for example). For example, if the customer originally had WordPerfect for Windows on drive C and then installed SuperStor, WPWin would then be on drive D:. If he left the DEVSWAP.COM command in the CONFIG.SYS, WPWin would then be on the C: drive and no modifications would be necessary. However, if the customer removed DEVSWAP.COM, he would then have to edit WPWin's .SET files and .INI files to reflect the change to drive D:.

If you eliminate the device SSTORDRV.SYS and reboot the computer, the compressed drive will not be recognized. Only the uncompressed drive will be recognized. If you replace the devices in CONFIG.SYS and reboot, you should then be able to access the compressed volume. This may be difficult since the customer's text editing programs, including Edlin, are probably located on the compressed drive. You can use the command TYPE C:\CONFIG.SYS to type the CONFIG.SYS file to the screen, and then use COPY CON CONFIG.SYS to recreate the file, reinserting the drivers in the process. Rebooting should then restore access to the compressed drive.

SuperStor creates a hidden file in the root directory of your boot drive. The file is named SSPARTSS.SWP. It has the attributes Hidden, System, Read Only. This is your compressed volume. The size of this file will be equal to the amount of remaining disk space beyond the portion of the drive SuperStor allows for the uncompressed drive. By no means should you modify this file. If this file is damaged or deleted, the entire compressed drive along with all of the programs and data on it will be lost.

If you use startup options to route virtual files, temporary files, or printer files to the non-compressed drive, you should make sure that the non-compressed drive is large enough to accommodate them.

If the non-compressed drive is large enough, routing temporary and virtual files may speed up your programs, since these files won't then be continuously compressed and uncompressed. However, the speed increase may not be dramatic, and this option is not a "cure-all" for speed issues.

MS-DOS 6.0 DoubleSpace Compression (Microsoft)
DoubleSpace is installed after installing MS DOS 6.0 by typing DBLSPACE at the DOS prompt and pressing ENTER. If the setup program finds Windows on the hard drive and a permanent swap file, DoubleSpace leaves room on the uncompressed drive for the permanent swap file. If Windows is set up to use a temporary swap file, it is put on the compressed drive. In this case, or if Windows is not present, DoubleSpace leaves about 2M uncompressed which it uses for drivers.

DoubleSpace swaps the drive letters just as Stacker and SuperStor do, but it normally uses C: and H:, C: being the compressed drive and H: being the uncompressed drive, or "host" drive as they call it. The actual process is that it skips four drive letters. If you have a C: and D: drive to start with and you compress D:, it will create an I: drive. DoubleSpace creates three hidden, system, Read-Only files: DBLSPACE.BIN, DBLSPACE.INI, and DBLSPACE.000 which is the actual compressed file. There is no separate device driver for swapping the drive letters as in the other programs. The only line added to the CONFIG.SYS file is
      DEVICE=C:\DOS\DBLSPACE.SYS /MOVE

When you boot your computer, MS-DOS automatically loads DBLSPACE.BIN (one of the hidden files) into memory. DBLSPACE.BIN is the actual driver that provides access to the compressed drive. It is loaded before the CONFIG.SYS file is read. It is always loaded at the top of conventional memory.

When MS-DOS carries out the commands in your CONFIG.SYS file, it loads the DBLSPACE.SYS device driver, which moves DBLSPACE.BIN from the top of conventional memory to another location in memory. This device driver does not provide access to the compressed drive, it simply determines the best location for DBLSPACE.BIN. Some programs require access to the top of conventional memory. If this is the case, DBLSPACE.BIN is moved to the bottom of conventional memory. If you have a 386 or higher computer and have run MEMMAKER, the line in the CONFIG.SYS file will be
      DEVICEHIGH=C:\DOS\DBLSPACE.SYS /MOVE
which will then move DBLSPACE.BIN to upper memory in order to free up more conventional memory.

Troubleshooting DoubleSpace Drives
Removing or REMming out the line DEVICE=C:\DOS\DBLSPACE.SYS /MOVE will not prevent you from accessing the compressed drive as in other compression programs. It can cause other problems, however, because this driver moves the actual driver (DBLSPACE.BIN) to the best location in memory. Without this line, DBLSPACE.BIN will always load at the top of conventional memory which may conflict with some programs.

Here are some troubleshooting steps which will change the way DoubleSpace loads, but shouldn't cause new problems:

1.      Change DEVICEHIGH=C:\DOS\DBLSPACE.SYS /MOVE to DEVICE=...
      This will load DBLSPACE.BIN in conventional memory instead of upper memory.

2.      Change DEVICE=C:\DOS\DBLSPACE.SYS /MOVE to DEVICEHIGH=...
      This will load DBLSPACE.BIN in upper memory instead of conventional memory. This can only be done on a 386 or higher computer and HIMEM.SYS must be loaded first.

3.      Run MEMMAKER (if the customer is not already using a memory manager). This will optimize the computer's memory and may allow DBLSPACE.BIN to load into upper memory if it won't currently fit. (This applies to 386 and higher computers only.) DO NOT TELL THE CUSTOMER TO RUN MEMMAKER IF QEMM IS ALREADY INSTALLED. While it shouldn't cause any damage, it certainly won't help.

Common Disk Compression Terms:
algorithm: A series of mathematical instructions detailing a solution to a problem.

compression: The process of reducing the space required to store data. As the data is stored, recurring byte sequences are replaced with shorter "tokens."

compression ratio: Comparison of file size before and after compression takes place. A file reduced from 100 bytes to 50 bytes would be said to have undergone a 2:1 compression. The more redundant the data is, the more it can be compressed. Graphics and database files compress best.

Lempel-Ziv: A compression algorithm, published in 1977, that searches data for repetitive, variable-length byte strings and converts them to smaller fixed-length tokens representative of the entire contents. These tokens are used to restore the data to its original string of characters.

lossless compression: A method of file-size reduction that employs a compression algorithm (like Lempel-Ziv) to ensure that data is not changed or lost in the process. Decompression produces an exact replica of the original file.

logical disk: A logical disk may consume all or part of a physical disk. Application programs are only concerned with logical disks. A logical disk may be fixed or mountable.

lossy compression: A method of reducing the file size that use a compression algorithm to eliminate or discard information specified as superfluous content in decompressed form.

on-the-fly compression/decompression: The continuous, real-time process of reducing file sizes and restoring data to its original state during read and write requests to a system's hard disk. It occurs in the background of normal computer operations in a way that is transparent to users. Real-time compression and decompression may be achieved by software or by interaction with an add-in coprocessor board. By contrast, archiving software shrinks and restores selected files through users' manual execution of commands.

partition: The physical division of the maximum amount of disk space determined by the limits of DOS, which may be assigned to a drive letter. For DOS 3.3, the partition limit is 32M, while DOS 5.0's limit is 256M. A single disk may have several partitions, so a 40M drive could have one 10M partitions and two 15M partitions.

Stacker drive: A logical drive created by Stacker to hold compressed data.

STACVOL file: A DOS file created by Stacker containing all data stored in a Stacker drive. STACVOL files have their system, hidden, and Read-Only attributes set and are stored in the host drive. They are visible to you and your applications only as new drive letters (i.e., you will not see a filename during a DOS DIR command).

SuperStor Disks: A logical disk managed by SuperStor is a SuperStor disk. SuperStor disks are always logical disks.

Answer:

Details:


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