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The hardware of the original Macintosh was elegant but hardly revolutionary. What made the Mac so different from other computers available in 1984 was the Macintosh operating system, generally referred to as the system software at the time and later known as the Mac OS.

A text-based single-user operating system such as CP/M or MS-DOS isn't very large and doesn't have an extensive application programming interface (API). As I explained in Chapter 22, mostly what's required in these text-based operating systems is a way for applications to use the file system. A graphical operating system such as the Mac OS, however, is much larger and has hundreds of API functions. Each of them is identified by a name that describes what the function does.

While a text-based operating system such as MS-DOS provides a couple of simple API functions to let application programs display text on the screen in a teletypewriter manner, a graphical operating system such as the Mac OS must provide a way for programs to display graphics on the screen. In theory, this can be accomplished by implementing a single API function that lets an application set the color of a pixel at a particular horizontal and vertical coordinate. But it turns out that this is inefficient and results in very slow graphics.

It makes more sense for the operating system to provide a complete graphics programming system, which means that the operating system includes API functions to draw lines, rectangles, and ellipses (including circles) as well as text. Lines can be either solid or composed of dashes or dots. Rectangles and ellipses can be filled with various patterns. Text can be displayed in various fonts and sizes and with effects such as boldfacing and underlining. The graphics system is responsible for determining how to render these graphical objects as a collection of dots on the display.

Programs running under a graphical operating system use the same APIs to draw graphics on both the computer's video display and the printer. A word processing application can thus display a document on the screen so that it looks very similar to the document later printed, a feature known as WYSIWYG (pronounced wizzy wig). This is an acronym for "What you see is what you get," the contribution to computer lingo of the comedian Flip Wilson in his Geraldine persona.

Part of the appeal of a graphical user interface is that different applications work roughly the same and leverage a user's experience. This means that the operating system must also support API functions that let applications implement the various components of the user interface, such as buttons and menus. Although the GUI is generally viewed as an easy environment for users, it's also just as importantly an environment for programmers. Programmers can implement a modern user interface without reinventing the wheel.

Even before the introduction of the Macintosh, several companies had begun to create a graphical operating system for the IBM PC and compatibles. In one sense, the Apple developers had an easier job because they were designing the hardware and software together. The Macintosh system software had to support only one type of diskette drive, one type of video display, and two printers. Implementing a graphical operating system for the PC, however, required supporting many different pieces of hardware.

Moreover, although the IBM PC had been introduced just a few years earlier (in 1981), many people had grown accustomed to using their favorite MS-DOS applications and weren't ready to give them up. It was considered very important for a graphical operating system for the PC to run MS-DOS applications as well as applications designed expressly for the new operating system. (The Macintosh didn't run Apple II software primarily because it used a different microprocessor.)

In 1985, Digital Research (the company behind CP/M) introduced GEM (the Graphical Environment Manager), VisiCorp (the company marketing VisiCalc) introduced VisiOn,