Microsoft KB Archive/101884

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Article ID: 101884

Article Last Modified on 11/19/1999


  • Microsoft MS-DOS 6.0 Standard Edition
  • Microsoft MS-DOS 6.2 Standard Edition
  • Microsoft MS-DOS 6.22 Standard Edition

This article was previously published under Q101884

This information applies to both Microsoft DoubleSpace and Microsoft DriveSpace. For MS-DOS 6.22, use DRVSPACE in place of DBLSPACE for commands and filenames.


MS-DOS batch files cannot distinguish between compressed drives and uncompressed drives. However, you can use the MS-DOS Debug utility to create a program that sets an errorlevel depending on whether a drive is compressed with DoubleSpace. An MS-DOS batch file can then test this errorlevel to determine whether the drive is a DoubleSpace- compressed drive.

NOTE: Microsoft Product Support Services does not provide support for writing the Debug script or batch file discussed in this article, or troubleshoot problems related to compiling, running or using this program or batch file.


This following is an example of how to use the MS-DOS Debug utility to create a program file called ISDBL.COM. ISDBL.COM determines the following:

  • Whether DoubleSpace is loaded in memory -and-

  • Whether a specified drive is a DoubleSpace-compressed drive

ISDBL.COM returns the following errorlevels:

   Results                         Errorlevel

   DoubleSpace compressed drive         1
   DBLSPACE.BIN is not loaded           0
   Invalid drive                        0
   Other errors                         0

NOTE: ISDBL.COM tests the current drive if no drive is specified on the command line.

Creating ISDBL.COM

ISDBL.COM is created using a Debug script file, which contains all the commands you would enter from within DEBUG to create ISDBL.COM.

To create ISDBL.COM, follow these steps exactly:

  1. Create a text file called C:\ISDBL.SRC using a text editor (such as Windows Notepad or MS-DOS Editor). This is the Debug script file. C:\ISDBL.SRC should contain the following lines exactly as they appear.

    NOTE: Press the ENTER key at the end of each line below.

          A 100
          MOV AX,2901
          MOV SI,81
          MOV DI,012D
          INT 21
          MOV DL,[012D]
          DEC DL
          JGE 0119
          MOV AH,19
          INT 21
          MOV DL,AL
          MOV AX,4A11
          MOV BX,1
          INT 2F
          MOV AX,4C00
          AND BL,80
          JZ 012B
          INC AL
          INT 21
          N ISDBL.COM

    NOTE: You MUST press the ENTER key twice after the last "INT 21" line and once after the "Q" at the end of the Debug script.

  2. From the MS-DOS command prompt on drive C, enter the following command to create the ISDBL.COM program:

          DEBUG < ISDBL.SRC

    Debug reads and follows the instructions in the ISDBL.SRC file. It is recommended that you do not use this command from within Windows or when other programs are running. If ISDBL.SRC contains an error, Debug probably is not told when to quit; therefore, you will have to restart your system. Debug may even be instructed to cause your system to stop responding (hang). If this happens, restart your system and examine ISDBL.SRC for accuracy.

    If there are no errors in ISDBL.SCR, a few screens of information are displayed, ending with

    <nnnn>:012B INT 21
    CX 0000

    Writing 000AE bytes

    where <nnnn> is a combination of letters and numbers, the exact combination of which varies. The remaining information should be exactly as above if ISDBL.COM was compiled correctly.

    Debug places the ISDBL.COM file in the current directory, which in the example above is C:\. It should be 174 bytes in size.


You can now run the ISDBL.COM program from within a batch file to get information about a specific drive. Below is an example batch file:


   REM This batch file uses ISDB.COM to test a drive to see if it
   REM was compressed with DoubleSpace.
   REM DBLTEST.BAT assumes that ISDBL.COM in in the current
   REM directory or a directory on the MS-DOS Path.
   REM The syntax for running this batch file is:
   REM    DBLTEST <drive>:
   REM where <drive>: is the name of the drive to test, for example C:
   REM If no drive is specified, the current drive is tested.
   ISDBL %1
   ECHO The drive %1 is a DoubleSpace compressed drive.
   ECHO The drive %1 is not a DoubleSpace compressed drive.

How ISDBL Works

In determining whether the drive is compressed, ISDBL assumes that DoubleSpace is running.

ISDBL checks the specified drive by using Interrupt 2F, function 4A11. This function is the DSGetDriveMapping application programming interface (API) call for DoubleSpace. If the drive is compressed, then Bit 7 of BL will be set. Otherwise, it is zero. This result is simply returned at the end of ISDBL.


Additional query words: 6.00 6.20

Keywords: KB101884