Updated: 10/02/2017 by Computer Hope

A20 is the 21st memory address line on 80286 and later processors that allow real mode software to gain access to high memory or 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 get access to the computers high memory.

Related pages

Hardware terms, Real mode