High Memory Area
A 65520-byte area of memory from ffff:0010 to ffff:ffff.
It is available on 286-based and 386+ computers in which
the HIMEM.SYS device driver has been installed.

DOS can load itself into the HMA, freeing up nearly 64K
of conventional memory for use by application programs.
■ DOS=HIGH (in CONFIG.SYS) puts DOS in the HMA
■ If the HMA has not been taken, it may by used by
  TSRs, etc.  See XMS in the Index.
■ Part of the HMA may be free; see HMA Suballocations