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:

aaaaaaaaaaaaabbbbcccdddd
eeeeeeeeeeeffffeedddwww

This could be written as:

13*a4*b3*c4*d
12*e4*f2*e3*d3*w

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.

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