HMA
Short for High Memory Area, HMA is the first 64KB (65,520 bytes) of extended memory above the first 1MB of memory found on IBM compatible computers. The HMA is used to swap programs from conventional memory into high memory making the computer believe more conventional memory is available on the computer.
In MS-DOS, the HMA can be accessed using Microsoft's HIMEM.SYS file and specifying the DOS=HIGH command in the config.sys. Additional information about the Autoexec.bat and Config.sys can be found on our Autoexec.bat and Config.sys page.
Early computers had issues with HMA and the keyboard controller that caused A20 errors.
Also see: Memory definitions