In DOS 6.0 and later systems, you can defragment a disk with the DEFRAG command. You can also buy software utilities, called disk optimizers or defragmenters, that defragment a disk.

See "Learn How to Defrag Your Hard Drive" in the Did You Know...? section of Webopedia.

(2) Fragmentation can also refer to RAM that has small, unused holes scattered throughout it. This is called external fragmentation. With modern operating systems that use a paging scheme, a more common type of RAM fragmentation is internal fragmentation. This occurs when memory is allocated in frames and the frame size is larger than the amount of memory requested.

Defrag Guide
How to considerably speed up your defrags.

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