A20

   (Address line 20) The 80286 and higher CPUs allow addresses in
real mode to extend slightly beyond the one megabyte mark, which causes
an incompatibility with some older programs which expect such addresses
to wrap back to the beginning of the address space.  For complete
compatibility with the 8088, newer machines thus contain circuitry
which permits the twenty-first address line (A20) to be disabled.  The
CPU then effectively has only twenty address lines in real mode, just
as the 8088 does, and addresses which would extend beyond the one
megabyte mark wrap to the beginning of the address space.  See also
High Memory Area, Real Mode.