Q47484: Determining the Number of Valid Drives in DOS

Article: Q47484
Product(s): See article
Version(s): 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | S_QuickC S_QuickASM H_MASM O_MSDOS | mspl13_c
Last Modified: 9-AUG-1989

Question:

I have an application that needs to determine the number of logical
drives installed on a given computer. I have tried using the DOS
function "SelectDisk" (0x0E), but it seems to return the number 5 on
all machines, no matter how many drives are installed. Is there a way
to get this information from DOS?

Response:

There are no DOS calls for determining the number of logical drives
installed on a computer. The DOS call SelectDisk returns only the
value stored in the DOS variable LASTDRIVE. This variable normally
contains 5 (maximum drive letter is "E") unless LASTDRIVE is
specifically set in the CONFIG.SYS file.

The LASTDRIVE value is useful, however, when determining which drives
are valid and which are not. Since LASTDRIVE reflects the highest
available drive letter, we can use DOS calls to determine which of the
available drives are valid. The DOS calls SelectDisk (0x0E) and
GetDisk (0x19) can be used to provide this information.

Note: Any network drives to which you're attached are also considered
valid. If you need to distinguish between network and non-network
drives, modify the program below to use INT 21h function 44h (IOCTL),
subfunction 09h ("Check if block device is remote"). For more
information, see a good DOS reference such as "Advanced MS-DOS
Programming" (by Duncan, published by Microsoft Press), or the "MS-DOS
Encyclopedia" (also published by Microsoft Press).

If you try, one at a time, to change the current drive to each of the
available drive letters, you will see which ones are valid. Since
SelectDisk does not return an error code in the event that it fails to
select a new disk, call GetDisk afterwards to determine if it was
successful.

The following C source code illustrates this method:

/*

Will not run under OS/2.  For DOS only.

The following program displays all of the logical drives installed on
a DOS system. This program uses DOS function calls to obtain the
current drive and change it to a new drive.

*/

#include <stdio.h>
#include <dos.h>

/* simple types */

#define BYTE unsigned char
#define BOOLEAN int

/* function prototypes */

BYTE DosGetDisk(void);
int DosSelectDisk(BYTE);
BOOLEAN ValidDrive(BYTE);

void main(void)
{
BYTE CurDrive;
int  NumDrives;
int  x;

CurDrive  = DosGetDisk();                // currently active drive
NumDrives = DosSelectDisk(CurDrive);     // returns number set in
                                         // LASTDRIVE

for (x = 0; x < NumDrives; x++)   // go through all possible drives
  if (ValidDrive((BYTE)x))        // print message if drive x valid
    printf("Drive %c is valid\n",(char)x+'A');

DosSelectDisk(CurDrive);          // set default drive back

}  /* end of main */

BOOLEAN ValidDrive(BYTE drive)  // returns true if drive valid,
{                               // false otherwise
  DosSelectDisk(drive);         // attempt to make drive current

  return (DosGetDisk() == drive);  // did it succeed?
}

BYTE DosGetDisk(void)   // returns drive number of current drive
{
  union REGS reg;

  reg.h.ah = 0x19;           // call DOS to find
  int86(0x21,&reg,&reg);

  return (reg.h.al);
}

int DosSelectDisk(BYTE drive)      // sets current drive
{
  union REGS reg;

  reg.h.ah = 0x0e;                 // set by calling DOS
  reg.h.dl = drive;
  int86(0x21,&reg,&reg);

  return ((int) reg.h.al);
}

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