A20 Line: Difference between revisions

The traditional method for A20 line enabling is to directly probe the keyboard controller. The reason for this is that Intel's 8042 keyboard controller had a spare pin which they decided to route the A20 line through. This seems foolish now given their unrelated nature, but at the time computers weren't quite so standardized. Keyboard controllers are usually derivatives of the [http://www.diakom.ru:8000/el/elfirms/datashts/Smsc/42w11.pdf 8042] chip. By programming that chip accurately, you can either enable or disable bit #20 on the address bus.