Global Enable A20 (Function 03h)

        ARGS:   AH = 03h

        RETS:   AX = 0001h if the A20 line is enabled, 0000h otherwise

        ERRS:   BL = 80h if the function is not implemented
                BL = 81h if a VDISK device is detected
                BL = 82h if an A20 error occurs

This function attempts to enable the A20 line.  It should only be used
by programs which have control of the HMA. The A20 line should be turned
off via Function 04h (Global Disable A20) before a program releases
control of the system.

NOTE: On many machines, toggling the A20 line is a relatively slow operation.