INT 21 - DOS 1+ - GET ALLOCATION INFORMATION FOR DEFAULT DRIVE
        AH = 1Bh
Return: AL = sectors per cluster (allocation unit)
        CX = bytes per sector
        DX = total number of clusters
        DS:BX -> media ID byte (see below)
Note:   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
SeeAlso: AH=1Ch,AH=36h

Values for media ID byte:
 FFh    floppy, double-sided, 8 sectors per track (320K)
 FEh    floppy, single-sided, 8 sectors per track (160K)
 FDh    floppy, double-sided, 9 sectors per track (360K)
 FCh    floppy, single-sided, 9 sectors per track (180K)
 F9h    floppy, double-sided, 15 sectors per track (1.2M)
 F8h    hard disk
 F0h    other

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