The following has been adapted from the Microsoft Windows Knowledge Base. Any settings not already found in your MSDOS.SYS file can be manually inserted. This file has been altered from its original state, with the inclusion of HTML formatting and additional undocumented entries.

Note: MSDOS.SYS is a hidden file, located in the root directory of your boot drive (usually C:\). If you don't see it, you'll have to configure Windows to show hidden files.

PSS ID Number: Q118579

The Windows Setup program creates a file called Msdos.sys in the root directory and sets the file's Read-Only System, and Hidden attributes. Unlike the Msdos.sys file in MS-DOS, this file is a text file. It contains a [Paths] section that lists the locations for other Windows files (such as the registry file) and an [Options] section that you can use to personalize the boot process.

Documented Entries (by Microsoft):

The [Paths] section can contain the following settings:

HostWinBootDrv={Root of Boot Drive}
Default: C
Purpose: Specifies the location for the root of the boot drive.

WinBootDir={Windows Directory}
Default: Directory specified during Setup (for example, C:\WINDOWS)
Purpose: Lists the location of the necessary files for booting.

WinDir={Windows Directory}
Default: Directory specified during Setup (for examples C:\WINDOWS)
Purpose: Lists the location of the Windows directory specified during Setup.

The [Options] section can contain the following settings:

BootDelay={Seconds}
Default: 2
Purpose: Sets the amount of time the "Starting Windows" message remains on the screen before Windows continues to boot.

BootFailSafe={Boolean}
Default: 0
Purpose: A setting of 1 forces your computer to boot in safe mode.

BootGUI={Boolean}
Default: 1
Purpose: A setting of 1 forces the loading of the GUI interface. A setting of 0 disables the loading of the GUI interface.

BootKeys={Boolean}
Default: 1
Purpose: A setting of 1 enables the use of the function key boot options (that is, F4, F5, F6, and F8). A setting of 0 disables the use of these function keys during the boot process
NOTE: A setting of BootKeys=0 overrides the use of BootDelay=n.

BootMenu={Boolean}
Default: 0
Purpose: A setting of 1 enables the startup menu. If this setting is 0, then you must press the F8 key when "Starting Windows" appears to invoke the startup menu.

BootMenuDefault={Number}
Default: 1 if the system is running correctly, 4 if the system hung in the previous instance
Purpose: Use this setting to set the default menu item for startup.

BootMenuDelay={Number}
Default: 30
Purpose: This setting is used to set the number of seconds your system will pause on the startup menu. If the number of seconds counts down to 0 without intervention, the BootMenuDefault is activated.

BootMulti={Boolean}
Default: 0
Purpose: A setting of 0 disables the multi-boot option. (For example, with a setting of 0 you cannot boot your previous operating system.) A setting of 1 enables the F4 and F8 keys to boot your previous operating system.
NOTE: This setting is set to 0 by default to avoid the corruption of data by allowing you to inadvertently boot MS-DOS and run a disk utility that does not recognize long filenames.

BootWarn={Boolean}
Default: 1
Purpose: A setting of 0 disables the safe mode boot warning message and the startup menu.

BootWin={Boolean}
Default: 1
Purpose: A setting of 1 forces Windows to load at startup. A setting of 0 disables Windows as your default operating system (this is useful only if you have MS-DOS version 5.x or 6.x on the computer).
NOTE: Pressing F4 inverts the default only if BootMulti=1. (For example, pressing the F4 key with a setting of 0 forces Windows to load.)

DoubleBuffer={Boolean}
Default: 0
Purpose: A setting of 1 is a conditional setting that enables double-buffering for controllers that need it (for example, SCSI controllers). A setting of 2 is an unconditional setting that enables double-buffering regardless of whether the controller needs it or not.

DBLSpace={Boolean}
Default: 1
Purpose: A setting of 1 allows the automatic loading of the DBLSPACE.BIN file. A setting of 0 prevents the automatic loading of this file.

DRVSpace={Boolean}
Default: 1
Purpose: A setting of 1 allows the automatic loading of the DRVSPACE.BIN file. A setting of 0 prevents the automatic loading of this file.

LoadTop={Boolean}
Default: 1
Purpose: A setting of 0 does not let Windows load COMMAND.COM or DRVSPACE.BIN/DBLSPACE.BIN at the top of 640K. If you are having compatibility problems with software that makes assumptions about the available memory try setting this to 0.

Logo={Boolean}
Default: 1
Purpose: A setting of 1 forces the default Windows logo to appear. A setting of 0 prevents the animated logo from being displayed. A setting of 0 also avoids hooking a variety of interrupts that can create incompatibilities with certain third-party memory managers.

Network={Boolean}
Default: 0
Purpose: A setting of 1 means the network was installed and adds "Start Windows, bypassing startup files, with network support" as an option on the Windows startup menu.

Undocumented Entries (not by Microsoft, anyway):

The [Options] section can contain the following settings:

DisableLog={Boolean}
Default: 0
Purpose: disables creation of bootlog.txt during startup.

The Msdos.sys file also contains a section that contains seemingly useless information. This information is necessary to support programs that expect the Msdos.sys file to be at least 1024 bytes in length. For example, if an anti-virus program detects that the Msdos.sys file is less than 1024 bytes, it may assume that the Msdos.sys file is infected with a virus. If you delete the Msdos.sys file your computer will not start.

The following statement, followed by a series of "X"s, appears in the Msdos.sys file:

;The following lines are required for compatibility with other programs. ;Do not remove them (Msdos.sys needs to be }1024 bytes).

Since each line begins with a semicolon (;), the lines are not read by the system.

How to Edit the Msdos.sys File

If you want to change any of the values in the Msdos.sys file, follow these steps to edit the file:

1. Click the Start button, point to Find, then click Files Or Folders.

2. In the Named box, type "msdos.sys" (without quotation marks). In the Look In box, click your boot drive (usually drive C). Click the Find Now button.

3. Use the right mouse button to click the Msdos.sys file and then click Properties on the menu that appears.

4. Click the Read-Only and Hidden check boxes to remove these attributes from the Msdos.sys file and then click OK.

5. Use the right mouse button to click the Msdos.sys file and then click Open With on the menu that appears.

6. In the "Choose the program you want to use" box, click WORDPAD and then click OK.

7. Make the changes you want to the Msdos.sys file. When you are done, save the file and then quit WordPad.

8. Use the right mouse button to click the Msdos.sys file and then click Properties on the menu that appears.

9. Click the Read-Only and Hidden check boxes to set these attributes for the file and then click OK. Close the Find window.

10. Quit and then restart Windows.