INT 21 - DOS 2+ - GET FREE DISK SPACE
AH = 36h
DL = drive number (00h = default, 01h = A:, etc)
Return: AX = FFFFh if invalid drive
else
AX = sectors per cluster
BX = number of free clusters
CX = bytes per sector
DX = total clusters on drive
Notes:  free space on drive in bytes is AX * BX * CX
total space on drive in bytes is AX * CX * DX
"lost clusters" are considered to be in use
according to Dave Williams' MSDOS reference, the value in DX is
incorrect for non-default drives after ASSIGN is run
SeeAlso: AH=1Ch

See also: 211B

2136 - DOS 2+ - GET FREE DISK SPACE