Int 21H, AH=36H

2 and higher

GET FREE DISK SPACE

      AH = 36h
      DL = drive number (00h = default, 01h = A:, etc)

AX = FFFFh if invalid drive

      else
          AX = sectors per cluster
          BX = number of free clusters
          CX = bytes per sector
          DX = total clusters on drive

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' MS-DOS reference, the value in DX is
        incorrect for non-default drives after ASSIGN is run
      this function does not return proper results on CD-ROMs;
        use AX=4402h"CD-ROM" instead
      (FAT32 drive) the reported total and free space are limited to 2G-32K
        should they exceed that value
        

AH=1Bh,AH=1Ch,AX=4402h“CD-ROM”,AX=7303h

Text based on Ralf Brown Interrupt List Release 61