Updated: 10/02/2017 by Computer Hope
Black box with characters A20 in yellow lettering

A20 is the 21st memory address line on 80286 and later processors that allow real mode software to access high memory or an area of memory beyond the first megabyte. With MS-DOS and early versions of Windows, this technique of accessing the high memory area was done using an A20 handler. For example, you could load the himem.sys in the config.sys file and use the DOS=HIGH,UMB line to access the computers high memory.

Hardware terms, Real mode