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 doesnt waste time sending in digital scrubbing bubbles. If it did, Peter Norton wouldnt 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.
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.
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