Windows 95 (codenamed Chicago) is a consumer-oriented operating system developed by Microsoft. It was released on August 24, 1995, and was a significant improvement over the company’s previous DOS-based Windows products.

Windows 95 merged Microsoft’s formerly separate MS-DOS and Windows products. It featured significant improvements over its predecessor, Windows 3.1, most notably in the graphical user interface (GUI) and in its simplified “plug-and-play” features. There were also major changes made to the core components of the operating system, such as moving from a mainly co-operatively multitasked 16-bit architecture to a pre-emptively multitasked 32-bit architecture.

Accompanied by an extensive marketing campaign, Windows 95 introduced numerous functions and features that were featured in later Windows versions, such as the taskbar, the “Start” button and the way the user navigates. It was also suggested that Windows 95 had an effect of driving other major players (including OS/2) out of business, something which would later be used in court against Microsoft.

Three years after its introduction, Windows 95 was succeeded by Windows 98. Microsoft ended support for Windows 95 on December 31, 2001.

Development

The initial design and planning of Windows 95 can be traced back to around March 1992, just after the release of Windows 3.1. At this time, Windows for Workgroups 3.11 and Windows NT 3.1 were still in development and Microsoft’s plan for the future was focused on Cairo. Cairo would be Microsoft’s next-generation operating system based on Windows NT and featuring a new user interface and an object-based file system, but it was not planned to be shipped before 1994. However, Cairo would partially ship in July 1996 in the form of Windows NT 4.0, but without the object-based file system, which would later evolve into WinFS.

Simultaneously with Windows 3.1’s release, IBM started shipping OS/2 2.0. Microsoft realized they were in need of an updated version of Windows that could support 32-bit applications and preemptive multitasking, but could still run on low-end hardware (Windows NT did not). So the development of Windows “Chicago” was started and, as it was planned for a late 1993 release, became known as Windows 93. Initially, the decision was made not to include a new user interface, as this was planned for Cairo, and only focus on making installation, configuration, and networking easier. Windows 93 would ship together with MS-DOS 7.0, offering a more integrated experience to the user and making it pointless for other companies to create DOS clones. MS-DOS 7.0 was in development at that time under the code name “Jaguar” and could optionally run on top of a Windows 3.1-based 32-bit protected mode kernel called “Cougar” in order to better compete with DR-DOS. The first version of Chicago’s feature specification was finished on September 30, 1992. Cougar was to become Chicago’s kernel.

Beta

Prior to Windows 95’s official release, users in the United States had an opportunity to preview it in the Windows 95 Preview Program. For US$19.95, users would receive several 3.5-inch floppy disks that would be used to install Windows 95 either as an upgrade from Windows 3.1x or as a fresh installation. Participants were also given a free preview of The Microsoft Network (MSN), the online service that Microsoft launched with Windows 95. During the preview period, Microsoft established various electronic distribution points for promotional and technical documentation on Chicago, including a detailed document for media reviewers describing the new system highlights. The preview versions expired in November 1995, after which the user would have to purchase their own copy of the final version of Windows 95.

Architecture

Windows 95 was designed to be maximally compatible with existing MS-DOS and 16-bit Windows programs and device drivers, while offering a more stable and better performing system. The Windows 95 architecture is an evolution of Windows for Workgroups’ 386 enhanced mode. The lowest level of the operating system consists of a large number of virtual device drivers (VxDs) running in 32-bit protected mode and one or more virtual DOS machines running in virtual 8086 mode. The virtual device drivers are responsible for handling physical devices (such as video and network cards), emulating virtual devices used by the virtual machines or providing various system services. The three most important virtual device drivers are:

Virtual Machine Manager (VMM32.VXD)
Responsible for memory management, event handling, interrupt handling, loading and initializing virtual device drivers, creating new virtual machines and thread scheduling.
Configuration Manager (CONFIGMG)
Responsible for implementing Plug and Play functionality; monitoring hardware configuration changes; detecting devices using bus enumerators; and allocating I/O ports, IRQs, DMA channels and memory in a conflict-free fashion.
Installable File System Manager (Input/Output Subsystem)
Coordinates access to supported file systems. Windows 95 initially shipped with support for FAT12, FAT16, the VFAT extension, ISO 9660 (CDFS) and network redirectors, with later releases supporting FAT32.

Access requests to physical media are sent to Input/Output Supervisor, a component responsible for scheduling the requests. Each physical media has its own device driver: access to the disk is performed by a port driver, while access to a SCSI device is handled by a miniport driver working atop the SCSI layer. Port and miniport drivers perform I/O operations in 32-bit protected mode, bypassing MS-DOS and BIOS, giving a significant performance improvement. In case there is no native Windows driver for a certain storage device, or if a device is forced to run in compatibility mode, the Real Mode Mapper can access it through MS-DOS.

32-bit Windows programs are assigned their own memory segments, which can be adjusted to any desired size. Memory area outside the segment cannot be accessed by a program. If a program crashes, nothing else is harmed. Before this, programs used fixed non-exclusive 64 KB segments. While the 64 KB size was a serious handicap in DOS and Windows 3.x, lack of guarantee of exclusiveness was the cause of stability issues because programs sometimes overwrote each other’s segments. A crashing Windows 3.x program could knock out surrounding processes.

The Win32 API is implemented by three modules, each consisting of a 16-bit and a 32-bit component:

Kernel
Provides high level access to memory and process management, and access to the file system. Consists of KRNL386.EXE, KERNEL32.DLL, and VWIN32.VXD.
User
Responsible for managing and drawing the various user interface components, such as windows, menus and buttons. Consists of USER.EXE and USER32.DLL.
Graphics Device Interface (GDI)
Responsible for drawing graphics in a device-independent way. Consists of GDI.EXE and GDI32.DLL.

User Interface

Windows 95 introduced a redesigned shell based around a desktop metaphor; the desktop was re-purposed to hold shortcuts to applications, files and folders. By contrast, Windows 3.1’s desktop was used to display icons of running applications. In Windows 95, they were now displayed as buttons on a taskbar across the bottom of the screen, which also contained a notification area used to display icons for background applications, a volume control and the current time. The Start menu, invoked by clicking the “Start” button on the taskbar, was introduced as an additional means of launching applications or opening documents. While maintaining the program groups used by its predecessor Program Manager, it now displayed applications within cascading sub-menus. The previous File Manager program was also replaced by Windows Explorer.

In 1994, Microsoft designers Mark Malamud and Erik Gavriluk approached Brian Eno to compose music for the Windows 95 project. The result was the six-second start-up music-sound of the Windows 95 operating system, The Microsoft Sound.

When released for Windows 95 and NT4, Internet Explorer 4 came with an optional Windows Desktop Update, which modified the shell to provide new features integrated with Internet Explorer, such as Active Desktop (which allowed Internet content to be displayed directly on the desktop) and additional updates to Windows Explorer.

Some of the user interface elements introduced in Windows 95, such as the desktop, taskbar, Start menu and Windows Explorer file manager, remained fundamentally unchanged on future versions of Windows.

Technical Improvements

Windows 95 included support for 255-character mixed-case long filenames and preemptively multitasked protected-mode 32-bit applications.

Long file names

32-bit File Access is necessary for the long file names feature introduced with Windows 95 through the use of the VFAT file system extension. It is available to both Windows programs and MS-DOS programs started from Windows (they have to be adapted slightly, since accessing long file names requires using larger pathname buffers and hence different system calls). Competing DOS-compatible operating systems released before Windows 95 cannot see these names. Using older versions of DOS utilities to manipulate files means that the long names are not visible and are lost if files are moved or renamed, as well as by the copy (but not the original), if the file is copied. During a Windows 95 automatic upgrade of an older Windows 3.1 system, DOS and third-party disk utilities which can destroy long file names are identified and made unavailable. When Windows 95 is started in DOS mode, e.g. for running DOS programs, low-level access to disks is locked out. In case the need arises to depend on disk utilities that do not recognize long file names, such as the MS-DOS 6.x’s defrag utility, a program called LFNBACK for backup and restoration of long file names is provided on the CD-ROM, specifically in its \ADMIN\APPTOOLS\LFNBACK directory.

32-bit

Windows 95 followed Windows for Workgroups 3.11 with its lack of support for older, 16-bit x86 processors, thus requiring an Intel 80386 (or compatible). While the OS kernel is 32-bit, much code (especially for the user interface) remained 16-bit for performance reasons as well as development time constraints (much of Windows 95’s UI code was recycled from Windows 3.1). This had a rather detrimental effect on system stability and led to frequent application crashes.

The introduction of 32-bit File Access in Windows for Workgroups 3.11 meant that 16-bit real mode MS-DOS is not used for managing the files while Windows is running, and the earlier introduction of the 32-bit Disk Access means that the PC BIOS is often no longer used for managing hard disks. DOS can be used for running old-style drivers for compatibility, but Microsoft discourages using them, as this prevents proper multitasking and impairs system stability. Control Panel allows a user to see which MS-DOS components are used by the system; optimal performance is achieved when they are bypassed. The Windows kernel uses MS-DOS style real-mode drivers in Safe Mode, which exists to allow a user to fix problems relating to loading native, protected-mode drivers.

System Requirements

Official system requirements were an Intel 80386DX CPU of any speed, 4 MB of system RAM and 50–55 MB of hard disk space depending on features selected. These minimal claims were made in order to maximize the available market of Windows 3.1 migrations. This configuration would rely heavily on virtual memory and was only optimal for productive use on single-tasking dedicated workstations. It was possible to run Windows 95 on a 386 SX, but this led to even less acceptable performance due to its 16-bit external data bus. To achieve optimal performance, Microsoft recommends an Intel 80486 or compatible CPU with at least 8 MB of RAM. Windows 95 may fail to boot on computers with more than approximately 480 MB of memory. In such case, reducing the file cache size or the size of video memory can help. The theoretical maximum according to Microsoft is 2 GB.

Windows 95 was superseded by Windows 98 and could still be directly upgraded by either Windows 2000 Professional or Windows ME. On December 31, 2001, Microsoft ended its support for Windows 95, making it an “obsolete” product per the Microsoft Lifecycle Policy. Even though support for Windows 95 has ended, the software has occasionally remained in use on legacy systems for various purposes. In addition, some video game enthusiasts choose to use Windows 95 for their legacy system to play old DOS games, although some other versions of Windows such as Windows 98 can also be used for this purpose.

Most copies of Windows 95 were on CD-ROM, but a floppy version could also be had for older machines. The retail floppy disk version of Windows 95 came on 13 DMF formatted floppy disks, while OSR 2.1 doubled the floppy count to 26. Both versions exclude additional software that the CD-ROM version might have featured. Microsoft Plus! for Windows 95 was also available on floppy disks. DMF was a special 21-sector format that Microsoft used to store 1.68MB on floppy disks rather than the usual 1.44MB. While the floppy version of Windows was normally on 3.5″ disks, a 5.25″ version could be specially ordered as well.

 

Source: https://en.wikipedia.org/wiki/Windows_95