Michael Maardt: PC since 1984 • on the net since 1990.

Get the most out of your PCMS-DOS 6.2

The most important reason for releasing DOS 6.2, according to Microsoft, was to improve DBLSPACE, compared to version 6.0, which is why DoubleSpace is in this section. But first, a little on the other improvements:

SCANDISK is a program that can investigate and repair hard-disk errors in a similar way to Norton?s Disk Doctor. Microsoft intends it to be a replacement for CHKDSK. The program can also test and repair errors on a drive compressed by DBLSPACE.

VERIFY=ON in AUTOEXEC.BAT can cause problems with certain hard disks (Conner). Therefore, delete this line, if it exists.

The DISKCOPY command, which makes an identical copy of a diskette, can at last carry out this operation by only needing the original disk in the drive once, followed by the copy disk; then the process is complete.

COPY, XCOPY and MOVE now warn if you are about to overwrite a file.

DEFRAG can now use extended memory and so can manage a larger number of files.

HIMEM.SYS now supports 64 MB RAM and by default checks the RAM chips in extended memory during booting.

Disk compression with DBLSPACE

In recent years, the need for more hard disk space has grown enormously. In 1993, at a time when hard disks were still relatively expensive, many firms developed a variety of programs that packed (compressed) data on the hard disk so that it took up less space than it otherwise would. And then the price of hard disks fell drastically! If you can afford to buy another hard disk, then forget all about this section on DoubleSpace. You can always use your old, slow hard disk for making backups.

Microsoft included a compression program called DBLSPACE with DOS 6.0 and 6.2.

When data is used, it is unpacked, and when a program stores data, it is compressed. The aim is to prevent a drop in program speed, so that the user doesn?t notice the unpacking process.

Use Dblspace?

DBLSPACE certainly caught the public imagination. Some people were certain that the DOS 6.0 version was buggy, which Microsoft has perhaps indirectly confirmed by releasing DOS 6.2.

I have tested this program briefly so I can comment on it, and I have not experienced any problems with it. Will DBLSPACE slow down my system? This is what Microsoft has to say in HELP:

"If you have a computer with a fast CPU and a fast hard disk, you probably won?t notice much difference in system speed after installing DBLSPACE. If you have a fast CPU and a slow hard disk, DBLSPACE might actually improve your system?s speed. If your computer has a slow CPU, you may notice a reduction in speed after compressing your drive."

Reasonable enough, but what is a fast CPU, etc? My guess is that a fast CPU was at that time at least a 386DX/40 MHz, a slow hard disk is a standard hard disk bought before 1992 (with a transfer rate of 5-700 KB per second), and a slow CPU is a 386SX. To check how correct this is, you will just have to try it out!

If you are short of space on your hard disk, then begin with the kind of installation suggested here and see how it works. Remember that you will need 38 KB of upper memory free if you want to avoid using any of your conventional memory!

If you decide to use this program, then I would suggest that before installation you:

  1. Upgrade to DOS 6.2 because this version of DBLSPACE has been improved and is more reliable.
  2. Back up all vital files.
  3. Defragment your disk. Choose the FULL option to create the maximum possible free space.
  4. If you are on a network confirm that you are logged on before you run DBLSPACE.

I assume that your hard disk consists of only one drive, the C: drive, so when I write about a drive in these instructions, this is equivalent to a disk. If you have DOS 6.2, then SCANDISK will start automatically before the actual disk compression begins.

If you want to know more, then you can read the manual or write Help Dblspace for extra information before beginning the process. There are two ways to install DBLSPACE.

C:\>Dblspace

The first screen informs you that Setup is loading the file DBLSPACE.BIN, a system file that needs about 40 KB in memory. The next screen gives a choice between the two possible options, express and custom, and explains that express is the easiest, while custom is for the more experienced user. This is correct but what Microsoft does not explain is that it is much harder to change your mind and uninstall an express installation, which is a compression of your entire drive C:.

Using express compresses a whole drive.

Custom creates a new drive on your existing drive, using the free disk space.

Most users will probably choose to compress their hard disks by using the express option, which is the simplest, but also the most difficult to change, i.e. undo. In order to gain some experience, or if you are a little unsure, I would suggest that you start with custom, which is easy to remove again.

The new drive, no matter which method you choose, must have a letter as a drive denominator. Dblspace jumps four drive letters from the last existing drive, so if you start with drive C:, your new drive will be called H. This is so that there is room for other programs (a ramdisk or netdrive) that wish to use a different drive letter from the drives already physically installed.

Your new drive H is different according to which method of compression you use, custom or express, but I will return to that.

Custom

I suggest that you start by creating a small DBLSPACE drive to gain a little experience, and especially to test whether your PC slows down. If you decide later to drop DBLSPACE, you can do it easily and return to your normal configuration. If you decide later that you wish to compress your entire hard disk, you can change your custom installation reasonably easily, and by then you will know more about DBLSPACE. So, with this in mind, choose custom.

The choices on the next screen are confusing. You are presented with two options:

  • Compress an existing drive
  • Create a new empty compressed drive

(If you only have one drive ? drive C ? then the first option is the same as express, and we have just told the program that we do not want an express installation. DBLSPACE should be able to test how many drives the machine has.)

I suggest that you use Enter to choose the Create a new? option. This results in the next screen presenting you with a figure for how much space you have available (current free space) and an estimate of how much free space a compressed drive will give you (projected size of new drive). Make a note of the first figure; you will need it later.

Press Enter and the next screen will tell you that the new compressed drive H will be created from the free space on C. Now comes the important part. The top line in the frame specifies that a miserly 2 MB will be left on drive C after compression (note that this is free space; it has nothing to do with your files in C). If you started with 50 MB free space, then 2 MB will remain in uncompressed form. DBLSPACE will compress the other 48 MB physical free space so that it can contain about 96 MB of files.

I suggest that you alter this 2 MB to a larger figure, so that you end up with a compressed drive that can contain about 20 MB. If you had 50 MB free, then use the arrow key to move up to the 2 MB and press Enter. In the next screen, write 40 MB and press Enter again. This gives DBLSPACE (50 - 40) = 10 MB of free space to create the new drive H.

It is a disappointment that the amount of free space is not shown on each screen. Dblspace is the big attraction in DOS 6 and one would have thought that Microsoft would have made it more user-friendly!

You can cheat by writing a ridiculously high figure, like 999, and the program will then tell you what the maximum figure can be, which is the amount of disk space free. The programmers have measured the free space, but they cannot be bothered to tell the user what it is.

The last screen before the process starts tells you that DBLSPACE is ready to begin, and you are requested to press C to continue. When I tested the program, it estimated it would take 53 minutes; in fact it took 10. SCANDISK starts first and if it runs into any problems (lost clusters or crosslinked files), you will have to abort the installation and fix these (see CHKDSK, p. *). If you do this using SCANDISK, it is not necessary to perform a surface scan at the same time, as DBLSPACE will carry one out later. After fixing any problems you can start DBLSPACE again.

(A) SCANDISK now carries out a surface scan, and if this reports that all is OK, the PC is rebooted and the file C:\DBLSPACE.INI is created. DBLSPACE.BIN is copied from C:\DOS to C:\ and both files are given system, hidden and read-only attributes.

The last screen gives the new compressed drive?s size and the amount of remaining (uncompressed) space left on C:. Note that drive C: is still drive C: but is now host to (contains) the new, compressed, drive H:.

Express

If you choose this, easier, method, then nothing appears to change except that your hard drive gets bigger, and maybe slower. You can use your PC as you always did, but if problems arise or you decide it was not a good idea to compress your disk, it might help if you understand what is going on.

(A) DBLSPACE creates a drive, typically H, called the Host Drive. (It is just a coincidence that H is the initial letter of Host.)

In an Express installation, this drive H: is not compressed, and actually contains all the data from C: in a large compressed file known as a Compressed Volume File. Note that the Express installation is the reverse of the Custom installation, and here the H: drive is host to drive C:, which exists as a file called DBLSPACE.000 and is "converted" to drive C: during booting (before CONFIG.SYS is loaded). This is because DBLSPACE.BIN is part of the DOS 6 operating system and is a system file no less important than the other two. If you create several compressed drives using DBLSPACE, they will have sequential extensions, i.e. DBLSPACE.001, etc.

The other system files and boot files are placed on the H: drive, along with hidden files from other programs and your Windows swap file (see below). These files cannot function in compressed form.

Warning: Never touch/delete/move the files belonging to DBLSPACE; you risk losing all your other files at the same time.

A Windows Permanent swap file can only exist on an uncompressed drive. If, before installing DBLSPACE you had a permanent swap file, it should have been moved to H: during the installation process.

If you did not have a permanent swap file (perhaps you did not have Windows) and later you decide you need one, you might find ? although I have not personally tried this ? that you need to create more free space on drive H. You can do this by starting DBLSPACE and using the Resize option in the Drive menu to increase the free space on drive H: so there is enough for your swap file. You end up with the same result by giving the command:

C:\>DBLSPACE/SIZE/REServe=8

where reserve=8 means: reserve 8 MB on the uncompressed host drive.

Custom and express

This line is added at the end of CONFIG.SYS:

devicehigh=C:\dos\dblspace.sys/move

Move the line to make it the first devicehigh line. This ensures that DBLSPACE.BIN is read into upper memory.

(A) DBLSPACE.SYS loads DBLSPACE.BIN and if the line in CONFIG.SYS is buffers=8 or less, there will always be room in High Memory for part of DBLSPACE.BIN (see Buffers=, page *).

In general

Data compression explained simply: Imagine a document in which many identical words occur (and, if, when, then, etc.). Instead of having to store the word every time, there could be a very short code for each word. All that needs to be stored is the code plus a pointer indicating where to find the answer. The risk of error if something goes wrong in a compressed file is higher than with "normal" storage. It would be like having the same pin-code for hundreds of different credit cards ? and then forgetting the code!

DBLSPACE.BIN is both a "disk space manager" and a program that can compress and decompress files. Since DBLSPACE is an integral part of the operating system, it is loaded before CONFIG.SYS. DBLSPACE.BIN is first loaded into conventional memory, but later (via the line added in CONFIG.SYS) it is placed in upper memory.

The improved version of SMARTDRV, included with DOS 6, can hold data before DBLSPACE decompresses it, which means it can hold relatively large amounts of data.

Do not use Nortons Disk Doctor on a DBLSPACE drive - use scandisk instead.

Uninstalling

It is easy enough to remove a compressed drive but it requires enough free space on your hard disk to take the uncompressed versions of the files that were compressed. You might find that you have to move some files to disk or tape.

Start DBLSPACE and choose uncompress from the Tools menu. If you only have one compressed drive, you will be asked if you want to remove DBLSPACE completely. Reply YES to this question.

C:/DBLSPACE.INI and C:/DBLSPACE.BIN are deleted, along with the compressed drive DBLSPACE.000. The line containing DBLSPACE.SYS is deleted from CONFIG.SYS

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You are on michaelmaardt.com • © Michael Maardt 21. Jul. 2018