Although its initial release was an iffy affair, technical progress and strong-armed marketing have made Windows the world's dominant operating system. Let's take a look how Windows has evolved since it was released in 1985.

Windows 1

Development of a graphical user interface for the IBM PC began in 1981. The goals were: multitasking ability, a graphical user interface, and device-independent screen and printer graphics; that is, application programs would not be required to know the details of how graphics were to be transmitted to each and every supported model of printer and display adapter. The original concept placed the menus at bottom of screen, but this was abandoned for drop-down menus and dialog boxes as seen on Xerox Alto and Macintosh.

Announced in 1983, Windows version 1.01 was not actually released until November 1985, and even then it did not realize its intended potential. In version 1, windows could not be positioned so that they overlapped, but could only be placed side-by-side. Its release really served just as a placeholder, to affirm to the marketplace that Microsoft did intend to release a graphical environment along the likes of Macintosh, GEM, and VisiOn.

While limited in performance and appearance, version 1.0 did include Windows Write and Windows Paint, Notepad, and smaller applications such as Clock, Calculator, Reversi (a game), CardFile, and Terminal, a serial data communications program. It required 256KB of memory, and could run from two double-sided double-density (360KB) floppy disks, or a hard disk. A program called the MS-DOS Executive served as a sort of file manager, and served as the program's shell. It ran entirely in real mode, and could not address memory past the 640KB limit imposed by the initial C architecture. Minor versions 1.03 and 1.04 were released between August 1986 and April 1987 with additional support for national languages, MS-DOS 3.2, more fonts and printers, and additional PC models.

Windows 2

Windows 2 was released in November 1987, and addressed many of the shortcomings of version 1. This release added icons to the interface metaphor, and supported overlapped windows. Version 2 required at least 512KB of memory, but could still be run from floppy disks. Version 2.1, released later, was the first to require a hard disk. Version 2 introduced the Smartdrive disk cache and Dynamic Data Exhange (DDE) support, which allowed a level of automatic interaction between applications.

All program code still ran in "real mode," with a limit of 640KB of RAM directly addressable. However, on computers with enhanced memory, multiple applications whose total memory requirements exceeded 640KB could still be run. As Windows switched between applications it swapped program code and data modules between the lower 640KB and enhanced memory. When memory usage was pushed to the limit, program code modules were discarded from memory and reloaded from the hard disk when needed again. This caused a massive slowdown and the familiar sound of disk "thrashing," but it did work.

Windows 2.10, released in May 1988, permitted the use of mice from other manufacturers, and was the first version to require the use of a hard disk. A new version of the himem.sys driver learned the trick of using the first 64K of extended memory for system code, freeing up memory for applications.

As with Digital Research's GEM windowing system, Windows was available as a "runtime-only" version that software vendors could bundle with application software. Windows version 2.11 was the last to be made available in this bundled format.

Windows 386

Windows 386 was released in 1987 as a specialized version of Windows 2 that required the use of an Intel 80386 processor. The 386 provided hardware support for virtual memory paging, more total system memory, and used the CPU's protected mode to gain direct access to extended memory. The processor also provided a "virtual 86" CPU mode that let Windows run several DOS applications at once, with preemptive multitasking between DOS applications and Windows. Preemptive multitasking gave each DOS application time to run, in round-robin fashion. The memory management and preemptive-multitasking of DOS applications that appeared in Windows 386 were carried forward into the "386 enhanced mode" of Windows 3.0.

Windows 3

Windows version 3.0 was a significant rewrite of Windows, and was released in May 1990. At this time there was a still a significant installed base of 80286-based computers, but 80386 and 80486 processors were found in virtually all newer computers, and it was time for Windows to start to take advantage of these processors' significant memory access improvements.

The 8088 processor used in the original IBM PC and its clones was capable of addressing at most 1MB of memory, due to the processor's design. Programs specified memory locations using two components: a 16-bit address value, and a 16-bit segment register. 16-bit values alone can specify at most 65,535 distinct addresses, but the segment register increased this many fold. When a program makes reference to a memory location, the processor automatically takes the value from a segment register, multiplies it by 10 (in hexadecimal, which is 16 in decimal), and adds the 16-bit address value to get a physical memory address.

Intel 80286 and later processors could use a more flexible memory access scheme called protected mode. In protected mode, the value of segment register was not used directly, but was instead used to select an entry from a table that contained 24-bit (80286 processors) or 32-bit (80386 or better) base address values. With the ability to add larger segment values, the protected mode scheme lets 16-bit programs access up to 4GB of physical memory.

Because 16-bit versions of Windows rely upon DOS and the computer's BIOS code to perform input/output operations, and DOS and the BIOS are real-mode programs, Windows needs to be able to rapidly switch the processor back and forth between protected and real mode. The 80286 could not be easily returned to real mode from protected mode. This is why Windows 1 and 2 were forced to rely solely on real mode.

The 80386 and later processors, however, could instantly be switched back and forth between real and protected mode, so with the advent of these processors, Windows 3 was completely redesigned to take advantage of protected mode's vastly larger potential memory space.

For compatibility with older computers, Windows 3.0 could be run in any of three modes:

• Real mode, with access to at most 1 MB of memory
• Standard mode, which put the CPU into protected mode, and increased the maximum directly usable memory to 16 MB
• 386 Enhanced mode, which let the CPU use the 386 processor's hardware support for running multiple DOS programs at once. This mode had been developed and tested in Windows 386. The DOS multitasking came at a slight decrease in performance, so both Standard and 386 Enhanced modes were made available as options.

Windows 3.0 required 640K of main memory plus at least 256K of extended memory, and required MS-DOS 3.1 or better.

The Windows user interface was also significantly revamped. The MS-DOS Executive was replaced with Program Manager, which used icons to represent program groups, and File Manager, which was a graphical file system browser comparable to today's My Computer. VGA graphics adapters were much more common by this point, and Windows graphical code was enhanced to support the use of more than the 16 primitive colors available in Windows 1 and 2. This made it possible for Windows to display photographic images for the first time.

While Windows 3's access to more memory was a huge improvement over Windows 1 and 2, protected mode introduced a new and soon to become annoying occurrence: the General Protection Fault. In real mode, buggy programs (or hardware drivers, or Windows components) that read from or wrote to incorrect memory addresses would often proceed as if nothing had happened, and only later might the system freeze up due to overwritten program code. Protected mode gave Windows the ability to detect when a program tried to access memory it wasn't supposed to (hence the word protected). Because there was nothing that Windows could really do to remedy the situation, it would just display the protection fault warning and terminate the program.

Unfortunately, when Windows 3.0 was first released, most application programs and drivers contained bugs that had never before been detected. Many were benign, but the protection mechanism couldn't know that. And, with much of Windows freshly rewritten, Windows itself contained countless bugs, too. Windows 3.0 quickly gained a reputation as being a phenomenally improved, but barely usable, operating system.

Windows 3.1

Microsoft released Windows 3.1 in April 1992, and for the first time, really got 16-bit Windows right. Most of the bugs were fixed, and developers had gotten their applications cleaned up. Even the General Protection Fault mechanism itself was improved, and was less often encountered. Some technical advances were made: Real mode support was dropped (for Windows itself; DOS applications were run in Real or Virtual 86 mode). Truetype scalable font support was added, as were extensions for multimedia support. Object Linking and Embedding (OLE) technology was released, which made it possible for applications to cooperate without knowing details about each other's internal operations. Windows 3.1 also included client-side networking support as a standard feature.

By this time, the market was way past ready for a reliable multitasking operating system, and Windows 3.1 sold one million copies in two months. Development of character-mode DOS applications virtually ceased, with WordPerfect Corporation being a notablyand fatallylate exception. The software industry focused almost entirely on Windows, and to a lesser extent on Macintosh.

Windows for Workgroups

Windows for Workgroups (WFWG) was a product line based on Windows 3.1 that included file- and printer- sharing support built in at a small additional cost. This made it possible to build peer-to-peer networks with no additional software, and directly targeted Novell's NetWare and Artisoft's LanTastic products, which up until this point had the small office network market to themselves.

WFWG version 3.1 (based on Windows 3.1) was released in October 1992, and version 3.11 in November, 1993.

Windows 3.11

Windows 3.11 was a bug-fix version, distributed as a free upgrade and installed on new computers, released in December 1993. This was the last issue in the 16-bit Windows product line.

For more details on each of the 16-bit Windows versions, see support.microsoft.com/kb/q32905.