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=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

See also: 211C

211B - DOS 1+ - GET ALLOCATION INFORMATION FOR DEFAULT DRIVE