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Global Disable A20 (Function 04h)

        ARGS:   AH = 04h

        RETS:   AX = 0001h if the A20 line is disabled,
                     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 attempts to disable the A20 line.  It should only be used
by programs which have control of the HMA.  The A20 line should be
disabled before a program releases control of the system.

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