Source : Free On-Line Dictionary of Computing

base memory
     
         The lowest 640 {kilobytes} of memory in an
        {IBM PC}-compatible computer running {MS-DOS}.  Other PC
        {operating systems} can usually compensate and "ignore" the
        fact that there is a 640K limit to base memory.  This was put
        in place because the original {CPU} - the {Intel 8088} - could
        only access one {megabyte} of memory, and {IBM} wanted to
        reserve the upper 384KB for {device drivers}.  The {high
        memory area} (HMA) lies above 640KB and can be accessed on
        MS-DOS computers that have an {A20 handler}.
     
        (1997-05-30)