INT 2F - DBLSPACE.BIN - "GetExtraInfo" - DETERMINE NUMBER OF DISK_UNIT STRUCTS
        AX = 4A11h
        BX = 0009h
        DL = compressed drive number (0=A:)
Return: AX = status (see also AX=4A11h/BX=0001h)
            0000h successful
                CL = number of DISK_UNIT structures allocated
                        (see AX=4A11h/BX=0005h)
SeeAlso: AX=4A11h/BX=0000h,AX=4A11h/BX=0008h

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