This item is from the PC Hardware FAQ, by Willie Lim and Ralph Valentino with numerous contributions by others. (v1.25).
The memory between 640k and 1Meg is used for the BIOS, the video aperture, and a number of other things. With the proper memory manager, DOS can take advantage of it. Many systems, however, won't identify its existence on boot. This does not mean it isn't there.
 
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