Notes on Disk Compression

(Historical note: this page is imported from the old DOS reference for completeness. Wikipedia has a much better explanation. Whole-disk compression is less important these days because disks are much cheaper. In the early 1990s it was rare for even a top-end machine to have more than a few hundred MB of storage. By the mid 1990s 1 GB disks were available but were seriously expensive, and the eight GB hard disk I bought in the late 90s was over £100)

DoubleSpace and DriveSpace

As DoubleSpace and DriveSpace are quite complicated, I’m giving them a
special article to explain what they are and how they are used.

What are DoubleSpace and DriveSpace?

DoubleSpace and DriveSpace are disk compression utilities. They claim to
give extra disk space by removing duplicated sequences of text from files
and replacing them with tokens. Take the following sentence:

the cat sat on the mat

After compression it could read:

the cat s#on %m#

The word “the ” is replaced by “%” and “at ” is replaced by “#” (including
the space). The second sentence is 16 characters while the first is 22
characters long. This means the second sentence is 73% as long as the first.
Another type of compression is called Run Length Encoding and is often
used in image files. Take the following text:



This could be written as:



DoubleSpace & DriveSpace place all files on the disk into a large
file called a compressed volume file (or CVF) . When DOS is booted, a virtual
drive is set up which looks like a normal drive, but is actually the CVF.
The drive containing the CFV is given another letter to keep it out of
the way.

What are the advantages and disadvantages of DoubleSpace
and DriveSpace?

The main advantage is that you will gain more disk space. This is claimed
to be double, but in reality can be much less and in a worst case can be
nothing extra at all (if you store lots of PKZip files on the disk for
example). There are many disadvantages. DoubleSpace and DriveSpace can
slow things down a lot, especially if you have a slow computer. The CVF
can be vulnerable. If a single part of it is damaged, you will lose all
files on the disk.