HMA - Definition and Overview

The High Memory Area (HMA) is the RAM area consisting of the first 64 kilobytes (KB), minus 16 bytes, of the extended memory on an IBM PC or compatible microcomputer.

By a strange design glitch the Intel 80x86 processors can actually address 17×64 KB minus 16 bytes of memory (from 0000:0000 to FFFF:FFFF, written in PC memory segment notation with hexadecimal numbers) 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 became equivalent to 0000:0000. For compatibility reasons, later processors still wrapped around by default, but this feature could be switched off.

So-called A20 handlers can control the addressing mode dynamically, thereby allowing programs to load themselves into the 1024–1088 KB region and run in real mode. Starting with version 5.0 of DR-DOS, parts of most MS-DOS versions can be loaded into HMA as well, freeing up to 46 KB of conventional memory.

See also

• Unreal mode