When IBM used the 80286 in their IBM Personal Computer AT, they solved this problem by including a software-settable gate to enable or disable (force to zero) the A20 address line, between the A20 pin on the 80286 and the system bus; this is known as Gate-A20 (the A20 gate), and it is still implemented in PC chipsets to this day.
Компьютеры IBM PC/AT построены на процессоре Intel 80286, но, из соображений совместимости с IBM PC и IBM PC/XT, построенных на Intel 808x, в них был введён логический элемент (вентиль), управляющий работой 21-го адресного провода (A20).