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http://www.extremetech.com/article2/0,3973,1156059,00.asp
Windows XP improves performance on saving and returning from hibernation in several areas. Before the hibernation file is written to disk, unused memory pages are freed, reducing the overall amount of memory that needs to be saved. The remaining physical memory pages are compressed as well. When the compressed memory pages are written to disk, Windows XP uses IDE DMA (Direct Memory Access), a more efficient method, rather than PIO (Programmed Input/Output). With PIO access, data goes through the CPU, while DMA goes direct from memory to the drive. In addition, the compression algorithm has been optimized to work on large, 64K blocks of memory for more efficient data transfer. Lastly, Windows XP overlaps compression and disk writes so while one block is being written, another is being compressed.