Windows 95

Windows 95 is a consumer-oriented graphical user interface-based operating system. It was released on August 24, 1995 by Microsoft, and was a significant progression from the company's previous Windows products. During development it was referred to as Windows 4.0 or by the internal codename Chicago.

Windows 95 was intended to integrate Microsoft's formerly separate MS-DOS and Windows products. It features significant improvements over its predecessor, Windows 3.1, most visibly in the graphical user interface (GUI). There were also major changes made at lower levels of the operating system.

In the marketplace, Windows 95 was a major success, and within a year or two of its release had become the most successful operating system ever produced. It also had the effect of driving other major players in the DOS-compatible operating system out of business, something which would later be used in court against Microsoft. Some three years after introduction, Windows 95 was succeeded by Windows 98.
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Development

The initial design and planning of Windows 95 can be traced back to around May 1992, just after the release of Windows 3.1. At this time Windows for Workgroups 3.1 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 (Cairo would eventually partially ship in July 1996 in the form Windows NT 4.0, without the object-base file system however, 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 at that time under development 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 30 September 1992. Cougar was to become Chicago's kernel.
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Architecture

Windows 95 was designed to be maximally compatible with existing MS-DOS and 16-bit Windows applications and device drivers, while under this constraint offering a more stable and better performing system. Architecturally, Windows 95 can be considered an evolution of Windows for Workgroups' 386 enhanced mode. The lowest level of the operating system is formed by 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 can be 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
The Virtual Machine Manager (VMM32.VXD) can be considered the kernel of Windows 95 and is responsible for tasks such as memory management, event and interrupt handling, loading and initialization of all other virtual device drivers, the creation of new virtual machines, and scheduling of threads.

Configuration Manager
The Configuration Manager (CONFIGMG) is responsible for implementing Plug and Play functionality. It detects all devices in the system using several bus enumerators, monitors the system for changes to the hardware configuration. It is responsible for assigning various resources (I/O ports, IRQs, DMA channels, and memory) to the devices in a conflict free fashion.

Installable File System Manager (Input/Output Subsystem)
The Installable File System Manager coordinates the access to various file systems. Windows 95 ships with support for FAT16, and in later releases FAT32, file systems (VFAT), ISO 9660 (CDFS), and several network redirectors. A dynamically sized disk buffer is provided by VCACHE. In the Block I/O Subsystem, requests are scheduled by the Input/Output Supervisor. Access to the disk is performed by a port driver, or in the case of a SCSI device, by a miniport driver working atop the SCSI layer.
Like Windows for Workgroups running with 32-bit file and disk access enabled, I/O operations can be handled entirely in 32-bit protected mode, bypassing MS-DOS and the 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 port driver can access it through MS-DOS.

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

Kernel
Kernel (KRNL386.EXE, KERNEL32.DLL, and VWIN32.VXD) provides high level access to functions such as memory and process management, and access to the file system.

User
User (USER.EXE and USER32.DLL) is responsible for managing and drawing the various user interface components, such as windows, menus, and buttons.

GDI
The Graphics Device Interface (GDI.EXE and GDI32.DLL) is responsible for drawing graphics in a device-independent way.
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Uses of MS-DOS
MS-DOS was an underlying component of Windows 95. It was possible to prevent loading the graphical user interface and boot the system into a real-mode MS-DOS environment. This sparked an intense debate[by whom?] over the question of to what extent Windows 95 was a "real" operating system, as opposed to merely being a graphical shell running on top of MS-DOS.

The GUI, when started, took over all functionality from MS-DOS, which itself was demoted to a compatibility layer for 16-bit device drivers. This contrasted with earlier versions of Windows which relied on MS-DOS to perform file and disk access. (Windows for Workgroups 3.11 could also largely bypass MS-DOS when 32-bit file access and 32-bit disk access was enabled.) Keeping MS-DOS running allowed Windows 95 to use DOS device drivers if suitable Windows drivers were unavailable. MS-DOS also still handled some requests for allocating memory and parsing file names, issued by legacy Win16 applications, as Microsoft saw little benefit in replacing these helper functions with newly written 32-bit code.[citation needed]

A negative consequence of keeping MS-DOS around was that Windows had to do some work to keep DOS's internal data structures synchronized with those of Windows itself. When starting an application, even a native 32-bit Windows application, MS-DOS would momentarily execute to create a data structure (the program segment prefix) and it was even theoretically possible for MS-DOS to run out of conventional memory while doing so, preventing the application from launching.
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system requirements
Official system requirements were an Intel 80386 DX CPU of any speed, 4 MB of system RAM, and 120 MB of hard drive space. These minimal claims were made in order to maximize the available market of Windows 3.1 converts. This configuration was distinctly suboptimal for any productive use on anything but single tasking dedicated workstations due to the heavy reliance on virtual memory. Also, in some cases, if any networking or similar components were installed the system would refuse to boot with 4 megabytes of RAM. 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 microprocessor with at least 8 MB of RAM.

Windows 95 was superseded by Windows 98 and could still be directly upgraded by both Windows 2000 and Windows Me. On 31 December 2001, Microsoft ended its support for Windows 95, making it an "obsolete" product according to the Microsoft Lifecycle Policy. Even though support for Windows 95 has ended, the software still remains in use on some home and school computers because of budget issues, a lack of knowledge or lack of desire to upgrade to newer editions of Windows. 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.

Windows 95 has been released on both floppy disks and on CD-ROM, as some computer systems at the time did not include a CD-ROM drive. 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 CD-ROM might have featured. Microsoft Plus! for Windows 95 was also available on floppy disks.