INT 2F - SMARTDRV v4.00+ - GET DOUBLE-BUFFER STATUS
        AX = 4A10h
        BX = 0005h
        BP = drive # (0=A, 1=B...)
Return: AX = BABEh if double-buffered
            ES:DI -> ???
SeeAlso: AX=4A10h/BX=0000h,AX=4A10h/BX=0003h,AX=4A10h/BX=0006h

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