Unlike later versions, Windows 1.0 offered limited multitasking of existing MS-DOS applications; instead, attention was paid to creating an interaction paradigm, an execution model and stable APIs that would be used in the future for native applications.
Windows 1.0 is often mistakenly considered a true "MS-DOS interface", which has also been applied to later versions. In fact, Windows 1.0 was launched from MS-DOS, could in turn invoke MS-DOS functions, and interface programs ran from .exe files just like MS-DOS programs. However, the Windows.exe files had a "new executable" format of their own, which only Windows could process and which, for example, allowed you to load bits of code and data on demand. Applications could manage memory only through Windows' own memory management system, which implemented a software virtual memory scheme and allowed the execution of programs larger than available RAM. Considering Windows 1.0 a "DOS interface" does not take into account the fact that it was designed only to be a graphical environment used by applications, and not a complete operating system.
Windows 1.0 included original drivers for video cards, mice, keyboards, printers, and serial ports, and applications could only invoke APIs based on these drivers. Taking into account that at the time the graphics capabilities of MS-DOS were extremely limited and given the limited usefulness of the other services it offered, MS-DOS applications had to directly access the hardware (or sometimes take advantage of the BIOS) to do their job. Therefore, rather than serving as an interface to MS-DOS, Windows 1.0 complemented it. The extension gradually replaced DOS only with later versions.