Microsoft KB Archive/46980

= CALL INTERRUPT Example to Get Disk Drive Capacity, Free Space =

Article ID: 46980

Article Last Modified on 11/21/2006



This article was previously published under Q46980



SUMMARY
A compiled Basic program can retrieve a disk drive's free space by calling an MS-DOS interrupt. The interrupt 21 Hex, with subfunction 36 Hex, allows a program to obtain a drive's allocation information, which provides the drive size and available space. The sizes are returned in clusters and must be converted to bytes.

This method is faster than using Basic's SHELL statement to execute the MS-DOS CHKDSK (check disk) command.



MORE INFORMATION
The code sample below shows how to use the CALL INTERRUPT routine to display a drive's used space, available space, and total capacity. The example works under MS-DOS in Microsoft QuickBasic versions 4.0, 4.0b, and 4.5, Microsoft Basic Compiler versions 6.0 and 6.0b, and Microsoft Basic PDS versions 7.0 and 7.1 for MS-DOS. For QuickBasic versions 2.0, 2.01, or 3.0, you need to modify the program to use the CALL INT86 statement instead of CALL INTERRUPT.

Below is further information about MS-DOS interrupt 21 Hex (33 decimal), with subfunction 36 Hex (54 decimal). This subfunction gets disk-drive allocation information, from which the drive's capacity and remaining free space can be calculated.

Invoke this function with the following register values: AH = 54 DL = drive code (0 = default, 1 = A, 26 = Z) If the function is successful, it returns the following:   AX = Sectors per cluster BX = Number of available clusters CX = Bytes per sector DX = Clusters per drive If the function is unsuccessful (drive invalid), it returns the following:   AX = FFFF Hex

Code Sample
You can compile this program within the QuickBasic environment as follows: QB filename /L [ QB.QLB ] For Basic PDS 7.0 and 7.1, start the QBX.EXE environment as follows: QBX filename /L [ QBX.QLB] Or, compile with BC.EXE and LINK.EXE as follows: BC filename [ options ] LINK filename,,,QB.LIB [ options ] For Basic PDS 7.0 and 7.1, use BC.EXE and LINK.EXE as follows: BC filename [ options ] LINK filename,,,QBX.LIB [ options ]

DECLARE FUNCTION unsignedlong& (num%) REM ****************************************************************** REM **                                                             ** REM ** RegType is a user-defined type used by the CALL INTERRUPT   ** REM ** routine. The file QB.BI contains the user-defined type and  ** REM ** the prototyping for the subprogram INTERRUPT. The           ** REM ** INTERRUPT routine is located in the QB.QLB and the QB.LIB. ** REM **                                                             ** REM ** For Basic PDS 7.0 use QBX.BI for the include file and the  ** REM ** library QBX.QLB or QBX.LIB for CALL INTERRUPT support. ** REM **                                                             ** REM ** Note: When not using the user-defined type provided with    ** REM **      the QuickBasic package, the length of the TYPE must    ** REM **      be 20 bytes long. Unpredictable results will occur    ** REM **      if the length is less than 20 bytes. ** REM **                                                             ** REM ******************************************************************

TYPE RegType AX   AS INTEGER BX   AS INTEGER CX   AS INTEGER DX   AS INTEGER BP   AS INTEGER SI   AS INTEGER DI   AS INTEGER FLAGS AS INTEGER DS   AS INTEGER ES   AS INTEGER END TYPE DIM REGS AS RegType CLS PRINT &quot;** Drive Capacity Utility  **&quot;: PRINT PRINT &quot;NOTE: Drive number ranges 0 through 26, where A = 1, C = 3&quot; PRINT INPUT &quot;Drive number to obtain drive capacity from : &quot;, DriveNum% IF DriveNum% < 0 OR DriveNum% > 26 THEN PRINT PRINT &quot;ERROR: Invalid Drive Number (RANGE: 0 thru 26)&quot; PRINT END END IF REGS.AX = &H3600     '** AH = 36, AL = 00 REGS.DX = DriveNum%  '** DH = ??, DL = DriveNum% CALL INTERRUPT(&H21, REGS, REGS) IF REGS.AX = -1 THEN   '** -1 = &HFFFF PRINT PRINT &quot;ERROR: Invalid Drive Specification&quot; PRINT END END IF

REM **Convert the signed register values to &quot;unsigned&quot; long integer REM  values. This insures that the amount of free disk space will be REM  a positive unsigned value.

BXLong& = unsignedlong&(REGS.BX) CXLong& = unsignedlong&(REGS.CX) DXLong& = unsignedlong&(REGS.DX)

REM ** TEMP = [ Sectors per cluster ] TIMES [ Bytes per Sector ]   ** Temp& = REGS.AX * CXLong&

REM ** CAPACITY = [ TEMP ] TIMES [ Clusters per Drive ]            ** Capacity& = Temp& * DXLong&

REM ** AVAILABLE = [ TEMP ] TIMES [ Number of Available Clusters ] ** Available& = Temp& * BXLong&

REM ** INUSE    = [ CAPACITY ] MINUS [ AVAILABLE ]                 ** InUse& = Capacity& - Available&

PRINT PRINT &quot;Drive Number = &quot;; DriveNum% IF DriveNum% = 0 THEN PRINT &quot;** Default drive number used **&quot; ELSE PRINT &quot;     Letter =  &quot;; CHR$(64 + DriveNum%) END IF PRINT PRINT USING &quot;Capacity = ###,###,###&quot;; Capacity& PRINT USING &quot;In Use   = ###,###,###&quot;; InUse& PRINT &quot;           ===========&quot; PRINT USING &quot;Available = ###,###,###&quot;; Available& END

FUNCTION unsignedlong& (num%) IF num% < 0 THEN unsignedlong& = num% + 65536 ELSE unsignedlong& = num% END IF END FUNCTION

Additional query words: QuickBas BasicCom

Keywords: KB46980

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