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Local Enable A20 (Function 05h)

        ARGS:   AH = 05h

        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 need direct access to extended memory. Programs which
use this function should call Function 06h (Local Disable A20) before
releasing control of the system.

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