Source : Free On-Line Dictionary of Computing

high memory area
     
         (HMA) The first 64 {kilobytes} (minus 16 byte) of
        the {extended memory} on an {IBM PC}.  By a strange design
        glitch the {Intel 80x86} processors can actually address 17*64
        kbyte minus 16 byte of memory (from 0000:0000 to ffff:ffff) in
        real mode.  In the {Intel 8086} and {Intel 8088} processors,
        unable to handle more than 1 {megabyte} of memory, addressing
        wrapped around, that is, address ffff:0010 was equivalent to
        0000:0000.  For compatibility reasons, later processors still
        wrapped around by default, but this feature could be switched
        off.  Special programs called {A20 handlers} can control the
        addressing mode dynamically, thereby allowing programs to load
        themselves into the 1024--1088 kbyte region and run in {real
        mode}.  From version 5.0 parts of {MS-DOS} can be loaded into
        HMA as well freeing up to 46 kbytes of {conventional memory}.
     
        (1995-01-10)