High memory area

In Memory management, the high memory area (HMA) is the RAM area consisting of 65520 bytes beyond the upper memory area of an IBM AT or compatible computer.

In real mode, the segmentation architecture of the Intel 80286 and subsequent processors identifies memory locations with 16-bit segment and 16-bit offset, which is resolved into a physical address via (segment) x 16 + (offset). Although intended to address only 1 Megabyte (MB) (2 bytes) of memory, segment:offset addresses at FFFF:0010 and beyond reference memory beyond 1 MB (FFFF0 + 0010 = 100000). So this mode can actually address the first 65520 bytes of extended memory as part of the 64 KB range starting 16 bytes before the 1 MB mark—FFFF:0000 (0xFFFF0) to FFFF:FFFF (0x10FFEF). The Intel 8086 and Intel 8088 processors, unable to handle more than 1 MB of memory because they had only 20 address lines, wrapped around at the 20th bit, so that address FFFF:0010 was equivalent to 0000:0000.