386 1.0 (June 1987). However, the architecture of these multiuser multitasking protected mode operating systems was not DOS-based by themselves. Concurrent DOS 386 was later developed to become Multiuser DOS (since 1991) and REAL/32 (since 1995). FlexOS 386 later became 4690 OS in 1993. DOS-based VDMs In contrast to these protected mode operating systems, DOS, by default, is a real-mode operating system, switching to protected mode and virtual 86 mode only on behalf of memory managers and DOS extenders in order to provide access to extended memory or map in memory into the first megabyte, which is accessible to normal DOS programs. DOS-based VDMs appeared with Microsoft's Windows/386 2.01 in September 1987. DOS-based virtual DOS machines were also present in Windows 3.0, 3.1x and Windows for Workgroups 3.1x running in 386 Enhanced Mode as well as in Windows 95, 98, 98 SE and ME. One of the characteristics of these solutions running on top of DOS is that the memory layout shown inside virtual DOS machines are virtual instances of the DOS system and DOS driver configuration run before the multitasker is loaded, and that requests which cannot be handled in protected mode are passed down into the system domain to be executed by the underlying DOS system. Similar to Windows 3.x 386 Enhanced Mode in architecture, EMM386 3.xx of Novell DOS 7, Caldera OpenDOS 7.01, DR-DOS 7.02 (and later) also uses DOS-based VDMs to support pre-emptive multitasking of multiple DOS applications, when the EMM386 /MULTI option is used. This component has been under development at Digital Research / Novell since 1991 under the codename "Vladivar" (originally a separate device driver KRNL386.SYS instead of a module of EMM386). While primarily developed for the next major version of DR DOS, released as Novell DOS 7 in 1994, it was also used in the never released DR DOS "Panther" and "Star Trek" project in 1992/1993. OS/2 MVDM VDMs called MVDM (Multiple Virtual DOS Machine) are used in OS/2 2.0 and later since 1992. OS/2 MVDMs are considerably more powerful than NTVDM. For example, block devices are supported, and various DOS versions can be booted into an OS/2 MVDM. While the OS/2 1.x DOS box was based on DOS 3.0, OS/2 2.x MVDMs emulate DOS 5.0. Seamless integration of Windows 3.1 and later Win32s applications in OS/2 is a concept looking similar on surface to the seamless integration of XP Mode based on