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Q52093: BASIC 7.00 Example to Find All Available Disk Drives

Article: Q52093
Product(s): See article
Version(s): 7.00   | 7.00
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | SR# S891214-80 | mspl13_basic
Last Modified: 14-JAN-1990

Microsoft BASIC Professional Development System (PDS) Version 7.00 for
MS-DOS and MS OS/2 can change the current drive from within a BASIC
program with the command

   CHDRIVE <drive letter>

where <drive letter> is a string expression that is a valid drive.
This statement, coupled with error trapping, can be used to find all
available disk drives.

The example program below collects a list of all the available disk
drives and puts it into the fixed-length STRING array "Drives". This
is done by using the CHDRIVE statement to log onto each possible drive
(A through Z) and adding to the list only those drives that were
successfully logged onto.

The INTEGER variable "IsDrive%" is used to flag whether or not the
CHDRIVE statement was successful in trying to log onto the next drive.

If CHDRIVE is unsuccessful, BASIC will generate a "Device unavailable"
error. This error will be trapped by the local error-handling routine,
"DriveError", which will set "IsDrive%" to FALSE.

If CHDRIVE is successful, "IsDrive%" will remain TRUE and the drive
will be added to the list. The total number of available drives is
stored in the INTEGER variable "TotalDrives%".

Note that on a PC a legal drive name can only be a single letter
between  "A" and "Z". Also, DOS does not distinguish between uppercase
and lowercase letters. Therefore, in the following program we can use
the numbers 65 through 90 whose corresponding ASCII characters are "A"
through "Z" to circulate through all the possible drives. The counter
variable of a FOR...NEXT loop is just right for performing this task.
Our counter is "DriveLetter%".

Code Example
------------

CONST TRUE = -1
CONST FALSE = 0

DIM Drives(26) AS STRING * 1       'The list of drives.

ON LOCAL ERROR GOTO DriveError   'If CHDRIVE unsuccessful trap error.
TotalDrives% = 0

FOR DriveLetter% = 65 TO 90      'ASCII "A" through "Z"
   IsDrive% = TRUE
   CHDRIVE CHR$(DriveLetter%)   'Try to log onto drive.
   IF IsDrive% THEN
      TotalDrives% = TotalDrives% + 1   'CHDRIVE was successful.
      Drives(TotalDrives%) = CHR$(DriveLetter%)   'Add drive letter.
   END IF
NEXT DriveLetter%

END

DriveError: IsDrive% = FALSE   'CHDRIVE was unsuccessful.
            RESUME NEXT        'Resume at the IF statement.

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