The Windows Environment

Windows hardly needs an introduction. Yet it's easy to forget the sea change that Windows brought to office and home desktop computing. Windows had a bumpy ride in its early years and was hardly destined to conquer the desktop market.

A History of Windows

Soon after the introduction of the IBM PC in the fall of 1981, it became evident that the predominant operating system for the PC (and compatibles) would be MS-DOS, which originally stood for Microsoft Disk Operating System. MS-DOS was a minimal operating system. For the user, MS-DOS provided a command-line interface to commands such as DIR and TYPE and loaded application programs into memory for execution. For the application programmer, MS-DOS offered little more than a set of function calls for doing file input/output (I/O). For other tasks—in particular, writing text and sometimes graphics to the video display—applications accessed the hardware of the PC directly.

Due to memory and hardware constraints, sophisticated graphical environments were slow in coming to small computers. Apple Computer offered an alternative to character-mode environments when it released its ill-fated Lisa in January 1983, and then set a standard for graphical environments with the Macintosh in January 1984. Despite the Mac's declining market share, it is still considered the standard against which other graphical environments are measured. All graphical environments, including the Macintosh and Windows, are indebted to the pioneering work done at the Xerox Palo Alto Research Center (PARC) beginning in the mid-1970s.

Windows was announced by Microsoft Corporation in November 1983 (post-Lisa but pre-Macintosh) and was released two years later in November 1985. Over the next two years, Microsoft Windows 1.0 was followed by several updates to support the international market and to provide drivers for additional video displays and printers.

Windows 2.0 was released in November 1987. This version incorporated several changes to the user interface. The most significant of these changes involved the use of overlapping windows rather than the "tiled" windows found in Windows 1.0. Windows 2.0 also included enhancements to the keyboard and mouse interface, particularly for menus and dialog boxes.

Up until this time, Windows required only an Intel 8086 or 8088 microprocessor running in "real mode" to access 1 megabyte (MB) of memory. Windows/386 (released shortly after Windows 2.0) used the "virtual 86" mode of the Intel 386 microprocessor to window and multitask many DOS programs that directly accessed hardware. For symmetry, Windows 2.1 was renamed Windows/286.

Windows 3.0 was introduced on May 22, 1990. The earlier Windows/286 and Windows/386 versions were merged into one product with this release. The big change in Windows 3.0 was the support of the 16-bit protected-mode operation of Intel's 286, 386, and 486 microprocessors. This gave Windows and Windows applications access to up to 16 megabytes of memory. The Windows "shell" programs for running programs and maintaining files were completely revamped. Windows 3.0 was the first version of Windows to gain a foothold in the home and the office.

Any history of Windows must also include a mention of OS/2, an alternative to DOS and Windows that was originally developed by Microsoft in collaboration with IBM. OS/2 1.0 (character-mode only) ran on the Intel 286 (or later) microprocessors and was released in late 1987. The graphical Presentation Manager (PM) came about with OS/2 1.1 in October 1988. PM was originally supposed to be a protected-mode version of Windows, but the graphical API was changed to such a degree that it proved difficult for software manufacturers to support both platforms.

By September 1990, conflicts between IBM and Microsoft reached a peak and required that the two companies go their separate ways. IBM took over OS/2 and Microsoft made it clear that Windows was the center of their strategy for operating systems. While OS/2 still has some fervent admirers, it has not nearly approached the popularity of Windows.

Microsoft Windows version 3.1 was released in April 1992. Several significant features included the TrueType font technology (which brought scaleable outline fonts to Windows), multimedia (sound and music), Object Linking and Embedding (OLE), and standardized common dialog boxes. Windows 3.1 ran only in protected mode and required a 286 or 386 processor with at least 1 MB of memory.

Windows NT, introduced in July 1993, was the first version of Windows to support the 32-bit mode of the Intel 386, 486, and Pentium microprocessors. Programs that run under Windows NT have access to a 32-bit flat address space and use a 32-bit instruction set. (I'll have more to say about address spaces a little later in this chapter.) Windows NT was also designed to be portable to non-Intel processors, and it runs on several RISC-based workstations.

Windows 95 was introduced in August 1995. Like Windows NT, Windows 95 also supported the 32-bit programming mode of the Intel 386 and later microprocessors. Although it lacked some of the features of Windows NT, such as high security and portability to RISC machines, Windows 95 had the advantage of requiring fewer hardware resources.

Windows 98 was released in June 1998 and has a number of enhancements, including performance improvements, better hardware support, and a closer integration with the Internet and the World Wide Web.

Aspects of Windows

Both Windows 98 and Windows NT are 32-bit preemptive multitasking and multithreading graphical operating systems. Windows possesses a graphical user interface (GUI), sometimes also called a "visual interface" or "graphical windowing environment." The concepts behind the GUI date from the mid-1970s with the work done at the Xerox PARC for machines such as the Alto and the Star and for environments such as SmallTalk. This work was later brought into the mainstream and popularized by Apple Computer and Microsoft. Although somewhat controversial for a while, it is now quite obvious that the GUI is (in the words of Microsoft's Charles Simonyi) the single most important "grand consensus" of the personal-computer industry.

All GUIs make use of graphics on a bitmapped video display. Graphics provides better utilization of screen real estate, a visually rich environment for conveying information, and the possibility of a WYSIWYG (what you see is what you get) video display of graphics and formatted text prepared for a printed document.

In earlier days, the video display was used solely to echo text that the user typed using the keyboard. In a graphical user interface, the video display itself becomes a source of user input. The video display shows various graphical objects in the form of icons and input devices such as buttons and scroll bars. Using the keyboard (or, more directly, a pointing device such as a mouse), the user can directly manipulate these objects on the screen. Graphics objects can be dragged, buttons can be pushed, and scroll bars can be scrolled.

The interaction between the user and a program thus becomes more intimate. Rather than the one-way cycle of information from the keyboard to the program to the video display, the user directly interacts with the objects on the display.

Users no longer expect to spend long periods of time learning how to use the computer or mastering a new program. Windows helps because all applications have the same fundamental look and feel. The program occupies a window—usually a rectangular area on the screen. Each window is identified by a caption bar. Most program functions are initiated through the program's menus. A user can view the display of information too large to fit on a single screen by using scroll bars. Some menu items invoke dialog boxes, into which the user enters additional information. One dialog box in particular, that used to open a file, can be found in almost every large Windows program. This dialog box looks the same (or nearly the same) in all of these Windows programs, and it is almost always invoked from the same menu option.

Once you know how to use one Windows program, you're in a good position to easily learn another. The menus and dialog boxes allow a user to experiment with a new program and explore its features. Most Windows programs have both a keyboard interface and a mouse interface. Although most functions of Windows programs can be controlled through the keyboard, using the mouse is often easier for many chores.

From the programmer's perspective, the consistent user interface results from using the routines built into Windows for constructing menus and dialog boxes. All menus have the same keyboard and mouse interface because Windows—rather than the application program—handles this job.

To facilitate the use of multiple programs, and the exchange of information among them, Windows supports multitasking. Several Windows programs can be displayed and running at the same time. Each program occupies a window on the screen. The user can move the windows around on the screen, change their sizes, switch between different programs, and transfer data from one program to another. Because these windows look something like papers on a desktop (in the days before the desk became dominated by the computer itself, of course), Windows is sometimes said to use a "desktop metaphor" for the display of multiple programs.

Earlier versions of Windows used a system of multitasking called "nonpreemptive." This meant that Windows did not use the system timer to slice processing time between the various programs running under the system. The programs themselves had to voluntarily give up control so that other programs could run. Under Windows NT and Windows 98, multitasking is preemptive and programs themselves can split into multiple threads of execution that seem to run concurrently.

An operating system cannot implement multitasking without doing something about memory management. As new programs are started up and old ones terminate, memory can become fragmented. The system must be able to consolidate free memory space. This requires the system to move blocks of code and data in memory.

Even Windows 1.0, running on an 8088 microprocessor, was able to perform this type of memory management. Under real-mode restrictions, this ability can only be regarded as an astonishing feat of software engineering. In Windows 1.0, the 640-kilobyte (KB) memory limit of the PC's architecture was effectively stretched without requiring any additional memory. But Microsoft didn't stop there: Windows 2.0 gave the Windows applications access to expanded memory (EMS), and Windows 3.0 ran in protected mode to give Windows applications access to up to 16 MB of extended memory. Windows NT and Windows 98 blow away these old limits by being full-fledged 32-bit operating systems with flat memory space.

Programs running in Windows can share routines that are located in other files called "dynamic-link libraries." Windows includes a mechanism to link the program with the routines in the dynamic-link libraries at run time. Windows itself is basically a set of dynamic-link libraries.

Windows is a graphical interface, and Windows programs can make full use of graphics and formatted text on both the video display and the printer. A graphical interface not only is more attractive in appearance but also can impart a high level of information to the user.

Programs written for Windows do not directly access the hardware of graphics display devices such as the screen and printer. Instead, Windows includes a graphics programming language (called the Graphics Device Interface, or GDI) that allows the easy display of graphics and formatted text. Windows virtualizes display hardware. A program written for Windows will run with any video board or any printer for which a Windows device driver is available. The program does not need to determine what type of device is attached to the system.

Putting a device-independent graphics interface on the IBM PC was not an easy job for the developers of Windows. The PC design was based on the principle of open architecture. Third-party hardware manufacturers were encouraged to develop peripherals for the PC and have done so in great number. Although several standards have emerged, conventional MS-DOS programs for the PC had to individually support many different hardware configurations. It was fairly common for an MS-DOS word-processing program to be sold with one or two disks of small files, each one supporting a particular printer. Windows programs do not require these drivers because the support is part of Windows.

Dynamic Linking

Central to the workings of Windows is a concept known as "dynamic linking." Windows provides a wealth of function calls that an application can take advantage of, mostly to implement its user interface and display text and graphics on the video display. These functions are implemented in dynamic-link libraries, or DLLs. These are files with the extension .DLL or sometimes .EXE, and they are mostly located in the \WINDOWS\SYSTEM subdirectory under Windows 98 and the \WINNT\SYSTEM and \WINNT\SYSTEM32 subdirectories under Windows NT.

In the early days, the great bulk of Windows was implemented in just three dynamic-link libraries. These represented the three main subsystems of Windows, which were referred to as Kernel, User, and GDI. While the number of subsystems has proliferated in recent versions of Windows, most function calls that a typical Windows program makes will still fall in one of these three modules. Kernel (which is currently implemented by the 16-bit KRNL386.EXE and the 32-bit KERNEL32.DLL) handles all the stuff that an operating system kernel traditionally handles—memory management, file I/O, and tasking. User (implemented in the 16-bit USER.EXE and the 32-bit USER32.DLL) refers to the user interface, and implements all the windowing logic. GDI (implemented in the 16-bit GDI.EXE and the 32-bit GDI32.DLL) is the Graphics Device Interface, which allows a program to display text and graphics on the screen and printer.

Windows 98 supports several thousand function calls that applications can use. Each function has a descriptive name, such as CreateWindow. This function (as you might guess) creates a window for your program. All the Windows functions that an application may use are declared in header files.

In your Windows program, you use the Windows function calls in generally the same way you use C library functions such as strlen. The primary difference is that the machine code for C library functions is linked into your program code, whereas the code for Windows functions is located outside of your program in the DLLs.

When you run a Windows program, it interfaces to Windows through a process called "dynamic linking." A Windows .EXE file contains references to the various dynamic-link libraries it uses and the functions therein. When a Windows program is loaded into memory, the calls in the program are resolved to point to the entries of the DLL functions, which are also loaded into memory if not already there.

When you link a Windows program to produce an executable file, you must link with special "import libraries" provided with your programming environment. These import libraries contain the dynamic-link library names and reference information for all the Windows function calls. The linker uses this information to construct the table in the .EXE file that Windows uses to resolve calls to Windows functions when loading the program.