On 29 Sep 98 at 18:01, roman G wrote:

> I have a few questions about compression.  What exactly does it do?

  "Data compression" refers to any method of increasing information
"density".  Many common encodings, especially those used for plain
text, don't make very efficient use of the "space" they occupy, and
compression stores the same information in less -- sometimes a LOT
less -- space.
  Computing issues tend to involve trade-offs along three axes:
space, time/speed, and money.  Compressing data reduces its size
requirement, but there is a speed penalty in converting the data back
to a usable form.

> Does it achieve the same results as using Winzip?

  Aha.  You're not just asking about compression in general -- you
have some *specific* implementation of compression in mind.
  WinZip does data compression; if you use WinZip, you will get
exactly the same results as using WinZip.

  Glancing ahead, I'm guessing that you're asking about DriveSpace,
the *drive* compression offered as part of Win95/98.

  You could think of a DriveSpace "compressed volume file" as being
like a .zip file, with two important differences:

1.  .zip files grow and shrink, as needed.  DriveSpace volumes are of
fixed size, and contain a varying amount of unused space.  This makes
them subject to fragmentation.

2.  A DriveSpace compressed volume (that has been *mounted*, see
below) can be navigated in Explorer just like a regular drive.
[There are third-party products that allow you to navigate .zip files
as if they were folders, but WinZip doesn't do this.]

> What is meant by mounting a FDD and a HDD?

  Back in the Dark Ages (before about 1977 or so), "fixed" hard
drives were considered pretty exotic.  Most used removable stacks of
platters; after physically installing such a stack in a drive, the
system operator had to go through a process to power up the drive,
and then to tell the OS about it.
  In the context of DriveSpace, it refers to telling the DriveSpace
driver to make a compressed volume usable as if it were a drive.

> If I deleted drvspace.bin and doublespace.bin can I still compress
> my HD or create a second compressed HD from the free space on the
> old HD?

  Remember that speed trade-off I mentioned?  That time is spent
executing the compression/decompression code, which in this case is
encapsulated inside a driver.  [It looks like a driver for a disk
drive, but in fact it provides a compression/decompression front-end
to the driver for the "host" drive.]
  So, if you delete the driver, can you still use it?  No, that's not
how computers work.  Without drvspace.bin, a compressed volume is
just a huge file taking up space on your drive.
  You *might* consider getting rid of doublespace.bin, which is
really only needed by people with compressed volumes from DOS 6.x's
DoubleSpace facility.

David G

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