INT 21 - DOS 1+ - GET ALLOCATION INFORMATION FOR SPECIFIC DRIVE
        AH = 1Ch
        DL = drive (00h = default, 01h = A:, etc)
Return: AL = sectors per cluster (allocation unit), or FFh if invalid drive
        CX = bytes per sector
        DX = total number of clusters
        DS:BX -> media ID byte (see AH=1Bh)
Notes:  under DOS 1.x, DS:BX points at an actual copy of the FAT; later
          versions return a pointer to a copy of the FAT's ID byte
        on a DBLSPACE drive, the total number of clusters is based on the
          estimated compression ratio
SeeAlso: AH=1Bh,AH=36h

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