Short for high memory area, HMA is the first 64 KB (65,520 bytes) of extended memory above the first 1 MB 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. For additional information about these files, see our Autoexec.bat and Config.sys guide.
Early computers had issues with HMA and the keyboard controller that caused A20 errors.