The uppermost end of memory.
In PCs, it may refer to any of three areas of memory: (1) the upper memory area (UMA) between 640K and 1M, (2) extended memory above 1M or (3) the High Memory Area (HMA), a 64K region between 1024K and 1088K. In the early days of PCs, these memory areas were very precious. See PC memory
and extended memory