The DOS API and Windows
In Windows 9x, DOS was generally used as a bootloader which loaded the protected-mode operating system and graphical shell. DOS was usually accessed from a virtual DOS machine (VDM) but it was also possible to boot directly to real mode MS-DOS 7.0 without loading Windows. The DOS API was extended with enhanced internationalization support and long filename support though the long filename support was only available in a VDM. With Windows 95 OSR2, DOS was updated to 7.1 which added FAT32 support and functions were added to the DOS API to support this. Windows 98 and Windows ME also implement the MS-DOS 7.1 API though Windows ME reports itself as MS-DOS 8.0.
Windows NT and the systems based on it (e.g. Windows XP and Windows Vista) are not based on MS-DOS, but use a virtual machine, NTVDM, to handle the DOS API. NTVDM works by running a DOS program in virtual 8086 mode (an emulation of real mode within protected mode available on 80386 and higher processors). NTVDM supports the DOS 5.0 API. DOSEMU for Linux uses a similar approach.
Read more about this topic: MS-DOS API
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