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Table Of Contents  CertiGuide to A+ (Operating Systems)
 9  Chapter 0011:  DOS
      9  V  File-Related DOS Commands

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DEFRAG

Earlier in this chapter you learned that DOS took away the messy details about how and where files are stored on a floppy or hard drive. Now it is time to get a hot tip about how DOS does this magic. The truth is, DOS is lazy. When you delete a file, it doesn’t waste time sending in digital scrubbing bubbles. If it did, Peter Norton wouldn’t have made millions with his un-erase program.

What DOS does do is go through its index system, known as the File Allocation Table (FAT), and erases the first character in the head of the index, and puts a reserved character that says it is ok to use this area again. That leaves the clusters of magnetic media available for re-use later.

[spacer]OOPS! Erased IT!

Un-erase programs from the early days depended on the fact that nothing was re-written to those areas. The program would simply ask for the first character and once supplied, ‘magically’ the deleted data was restored.


When DOS is told to write new data, is simply looks for the closest available magnetically ready media that is available. Given some deleting and re-writing, a single file can get really scattered around on a hard drive. This can slow performance greatly.

This sound like a need, and seeing a dollar to be made, defragmentation programs proliferated. Eventually, DEFRAG made its way into the GUI version of DOS, known as Windows. This made third party programmers write even better DFRAG programs.

Performance

A DEFRAG program improves performance by re-ordering programs and data files into a contiguous order.



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