Msdos.sys was one of the three files (along with io.sys and command.com) that DOS 6 and earlier versions was composed of. It was a binary file that created a disk buffer and file control block for service routines, and performed hardware initialization. Msdos.sys was is a critical file in the boot sequence of early Windows operating systems.
Beginning with Windows 95, msdos.sys was combined into io.sys and the new msdos.sys became a text editable configuration file. Windows Vista, Windows 2008, and Windows 7 have a different boot sequence that doesn't use msdos.sys, it uses bootmgr instead. But you'll still need msdos.sys if you want to dual-boot with an earlier operating system.
Msdos.sys is a hidden system file located in the root of your boot drive. To view hidden files, in Explorer's View menu select Folder Options. In the "Folder Options dialog box click on the View tab and under Hidden Files check Show all files. You may want to make a backup copy of msdos.sys named msdos.sys.bak before you make any changes to the file.
There are three sections to msdos.sys. The [Paths] section tells Windows where to find the necessary startup files. The WinDir= entry contains the path to the folder containing the operating system. The WinBootDir= entry contains the path to the boot folder. The WinBootDrv= entry contains the letter of the boot drive. You shouldn't make any changes to this section.
The [Options] section is used to configure startup settings. The last section of the file is filler to make sure the file is at least 1,024 characters long. Before you can edit the msdos.sys file you need to right-click on the file name and in the Properties dialog box that appears, uncheck the Read-only attributes check box.
The [Options] section can contain up to 16 entries. The default msdos.sys usually contains only five entries. You can edit these settings in Notepad for purposes of debugging or just to make it start faster. Below is a list of some useful entries.
AutoScan - Controls whether Windows runs ScanDisk after an improper shutdown. If you have a large hard disk, ScanDisk can take a long time. You may want to prevent it from running automatically. Setting AutoScan=0 prevents ScanDisk from running automatically.
BootGUI - Controls whether your system starts in Windows mode or DOS 7 mode. Setting BootGUI=0 causes it to start in DOS mode. (Note: Windows Me does not support booting in DOS mode, so this will have no effect.)
BootKeys - Controls whether Windows will recognize keys pressed during startup. You might want to press F8 or Ctrl during startup to display the Startup Menu. However, if you want to keep unauthorized people from bypassing your logon, you should set BootKeys=0 to ignore keys presses during startup.
BootMenu - If you want the Startup Menu to appear each time that you start Windows, set BootMenu=1.
BootMenuDefault - Specifies the Startup Menu command that is highlighted and selected by default when the Startup Menu appears. Set this to one of the menu numbers shown below.
1=Normal
2=Logged
3=Safe Mode
4=Step-by-step
5=Command prompt only
6=Safe Mode command prompt only
BootMenuDelay - Sets the delay in seconds that the Startup Menu is displayed before the default option is automatically executed.
BootMulti - If you had DOS on your computer when Windows was installed, BootMulti=1 puts the option to boot from this previous version of DOS in the Startup Menu. (Windows Me does not support booting in DOS mode.)
BootSafe - For the thousands of Windows users whose system stopped working properly years ago, set BootSafe=1 to start in Safe Mode automatically.
BootWarn - Again, for the thousands of Windows users whose system stopped working properly years ago. Set BootWarn=0 to start normally even if the previous boot failed. But because you haven't fixed what's preventing Windows from starting properly in the first place, this may cause your system to freeze up with the "blue screen of death".
BootWin - If you had DOS on your computer when Windows was installed, setting BootWin=0 causes your system to boot to previous version of DOS rather than Windows. (Not Windows Me).
Logo - Setting Logo=0 will prevent the Windows logo from being displayed during startup. This is useful if you want to see system messages rather than Microsoft's logo during startup.
There are several other msdos.sys [Option] settings that are of limited usefulness. Also, don't be surprised if any of these options don't work the way Microsoft claims they do in the Windows Resource Kit. But you can try using these settings to change the way Window starts for purposes of debugging or to make it start faster.
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