Editing Windows 9x (section)

===MS-DOS===

[[Windows 95]] was able to reduce the role of [[MS-DOS]] in Windows much further than had been done in [[Windows 3.1x]] and earlier. According to Microsoft developer Raymond Chen, MS-DOS served two purposes in Windows 95: as the boot loader, and as the 16-bit legacy device driver layer.<ref name="chen">{{cite web|url=https://devblogs.microsoft.com/oldnewthing/20071224-00/?p=24063|title=What was the role of MS-DOS in Windows 95?|author=Chen|first=Raymond|author-link=Raymond Chen (Microsoft)|date=December 24, 2007|website=The Old New Thing|publisher=[[Microsoft]]|access-date=October 6, 2019}}</ref>

When Windows 95 started up, MS-DOS loaded, processed [[CONFIG.SYS]], launched [[COMMAND.COM]], ran [[AUTOEXEC.BAT]] and finally ran [[WIN.COM]]. The WIN.COM program used MS-DOS to load the virtual machine manager, read SYSTEM.INI, load the virtual device drivers, and then turn off any running copies of [[EMM386]] and switch into protected mode. Once in protected mode, the virtual device drivers ([[VxD]]s) transferred all state information from MS-DOS to the 32-bit file system manager, and then shut off MS-DOS. These VxDs allow Windows 9x to interact with hardware resources directly, as providing low-level functionalities such as [[32-bit disk access]] and memory management. All future file system operations would get routed to the 32-bit file system manager.<ref name="chen" /> In [[Windows Me]], win.com was no longer executed during the startup process; instead it went directly to execute VMM32.VXD from IO.SYS.

The second role of MS-DOS (as the 16-bit legacy device driver layer) was as a backward compatibility tool for running DOS programs in Windows. Many MS-DOS programs and device drivers interacted with DOS in a low-level way, for example, by patching low-level BIOS interrupts such as [[INT 13|int 13h]], the low-level disk I/O interrupt. When a program issued an int 21h call to access MS-DOS, the call would go first to the 32-bit file system manager, which would attempt to detect this sort of patching. If it detects that the program has tried to hook into DOS, it will jump back into the 16-bit code to let the hook run. A 16-bit driver called IFSMGR.SYS would previously have been loaded by CONFIG.SYS, the job of which was to hook MS-DOS first before the other drivers and programs got a chance, then jump from 16-bit code back into 32-bit code, when the DOS program had finished, to let the 32-bit file system manager continue its work.<ref name="chen" /> According to Windows developer [[Raymond Chen (Microsoft)|Raymond Chen]], "''MS-DOS was just an extremely elaborate decoy. Any 16-bit drivers and programs would patch or hook what they thought was the real MS-DOS, but which was in reality just a decoy. If the 32-bit file system manager detected that somebody bought the decoy, it told the decoy to quack.''"<ref name="chen" />

====MS-DOS Virtualization====
Windows 9x can run MS-DOS applications within itself using a method called "Virtualization", where an application is run on a [[Virtual DOS machine]].

====MS-DOS Mode====
Windows 95 and Windows 98 also offer backwards compatibility for DOS applications in the form of being able to boot into a native "DOS Mode" (MS-DOS can be booted without booting Windows, but not putting the CPU in protected mode). Through Windows 9x's memory managers and other post-DOS improvements, the overall system performance and functionality is improved. Some old applications or games may not run properly in the virtual DOS environment within Windows and require real DOS Mode.

Having a command line mode outside of the GUI also offers the ability to fix certain system errors without entering the GUI. For example, if a virus is active in GUI mode it can often be safely removed in DOS mode, by deleting its files, which are usually locked while infected in Windows. Similarly, corrupted registry files, system files or boot files can be restored from real mode DOS. Windows 95 and Windows 98 can be started from DOS Mode by typing <code>'WIN'</code> at the command prompt and then hitting "Enter", akin to earlier versions of Windows such as Windows 3.1.