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Local Disable A20 (Function 06h)

        ARGS:   AH = 06h

        RETS:   AX = 0001h if the function succeeds,
                     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
                BL = 94h if the A20 line is still enabled

This function cancels a previous call to Function 05h (Local Enable
A20).  It should only be used by programs which need direct access to
extended memory.  Previous calls to Function 05h must be canceled before
releasing control of the system.

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