KnowledgeBase Archive

An Archive of Early Microsoft KnowledgeBase Articles

View on GitHub

Q62458: How to Find All Available Logical Drives in BASIC

Article: Q62458
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S900521-38 B_BasicCom | mspl13_basic
Last Modified: 29-JAN-1991

A BASIC program can call interrupts to find out which logical drives
are available. The interrupts used are 21Hex, function 0EHex (Select
disk) and 21Hex, function 19Hex (Get current disk). The method
involves circulating through each possible logical drive (A-Z) and
calling the interrupt to select that drive. If this is successful,
subsequently calling the interrupt to get the current drive should
return that same drive. If it isn't the same, the drive wasn't
successfully selected and therefore isn't available.

This information applies to Microsoft QuickBASIC Compiler versions
4.00, 4.00b, and 4.50 for MS-DOS, Microsoft BASIC Compiler versions
6.00 and 6.00b for MS-DOS, and Microsoft BASIC Professional
Development System (PDS) versions 7.00 and 7.10 for MS-DOS.

Function 0EH of interrupt 21H (Select disk) requires input from the
following registers:

   AH (upper byte of AX) = 0EH
   DL (lower byte of DX) = drive code (0=A, 1=B, etc.)

When called, it makes the drive passed in DL current, if possible. If
that drive is not available, the interrupt does not return an error.
That is why function 19H of interrupt 21H (Get current disk) must be
called to see if function 0EH was successful. Function 19H requires
the following registers for input:

   AH (upper byte of AX) = 19H

It returns the following:

   AL (lower byte of AX) = current drive code (0=A, 1=B, etc.)

For more information on interrupt 21H, functions 0EH and 19H, see
Pages 355 and 367 (respectively) of "Advanced MS-DOS Programming,
Second Edition," by Ray Duncan (Microsoft Press, 1988).

The following program displays all the available logical drives:

'Remember to invoke QB.EXE (or QBX.EXE) with the /L switch) because
'support for CALL Interrupt is in QB.QLB (or QBX.QLB).

'$INCLUDE: 'qb.bi'                    'Use 'qbx.bi' for QBX.EXE.
DIM Regs AS RegType                   'Holds register values.

PRINT "The following logical drives are available:";

FOR Drive% = 0 TO 25                  'Circulate through drives A-Z.

   Regs.ax = &HE00                    'Function 0EH (AH = 0EH).
   Regs.dx = Drive%
   CALL Interrupt(&H21, Regs, Regs)   'Select a drive, if it's there.

   Regs.ax = &H1900                   'Function 19H (AH = 19H).
   CALL Interrupt(&H21, Regs, Regs)   'Get currently selected drive.

   IF (Regs.ax AND &HFF) = Drive% THEN  '(Regs.ax AND &HFF) = AL.
      PRINT " "; CHR$(Drive% + 65);   'If current drive = last drive
   END IF                             'selected, drive is available.

NEXT Drive%

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.