Windows 9x

Windows 9x refers to a series of Microsoft Windows operating systems released between 1995 and 2000. It was introduced with Windows 95, a major update to the MS-DOS-based Windows line that extended the 386 Enhanced Mode of previous versions to add support for 32-bit user applications, as well as greatly overhauled the user experience with the introduction of the Windows Explorer. It was succeeded by Windows 98 and then by Windows Me in 2000, after which the series was discontinued and ultimately superseded by Windows XP in 2001.

The series was envisioned as a stopgap to ship a 32-bit consumer version of Windows, as Windows NT at the time was too heavy to run on an average home computer. One of the design constraints of Windows 95 was the ability to run on an average Windows 3.1 machine with a 386 processor and 4 MB of RAM. However, Windows NT required at least three times as much memory and could not be reasonably cut down without major changes to the architectural design. Once home systems became powerful enough to run Windows NT, it was decided to ultimately discontinue the Windows 9x series.

Architecture
The Windows 9x architecture is based on the 386 Enhanced Mode of Windows 3.0 and Windows 3.1x, the key component of which is the 32-bit virtual machine monitor (VMM32) that utilizes the Virtual 8086 mode of the i386 processor, enabling the execution of 16-bit real mode code in the otherwise 32-bit protected mode. However, the key difference between 16-bit Windows and Windows 9x is the ability to run 32-bit user applications, although simpler applications could already run under Windows 3.1 using the separately available Win32s supplement.

MS-DOS
Main article: MS-DOS in Windows 9x The Windows 9x line uses a modified version of MS-DOS that includes major changes to improve cooperation with the Windows environment. It is used to bootstrap Windows on boot, as well as to provide the environment for virtual DOS machines.

The MS-DOS subsystem originally identified as MS-DOS 7.0 in Windows 95, and was followed by two major update throughout the series. MS-DOS 7.1, introduced as a part of the Windows 95 OEM Service Release 2, notably added support for the FAT32 filesystem. MS-DOS 8.0, shipping with Windows Me, then considerably cut down access to real mode MS-DOS to improve Windows boot times. Neither version was ever available as a standalone retail product, although unofficial repacks of the DOS subsystem of the respective Windows versions exist and tend to be mislabeled as actual standalone MS-DOS versions.

The integration of MS-DOS into Windows was a subject of a major lawsuit by Caldera, the developer of the competing DR-DOS operating system, who viewed this as an anticompetitive measure. The company has later demonstrated Windows 9x running on top of a custom version of DR-DOS that included the functionality necessary to boot the environment. In 2000, Microsoft settled out of court for an undisclosed sum, which was later revealed to be $280 million.