To better understand the architecture of DirectX, you need to understand how it came about and why it was needed.
The Early Days
Since the early days of video game development, game developers had to rely on detailed knowledge of the hardware they were working with. This knowledge came from lots of experimentation and electronics know-how.
For example, to create some of the earliest games, such as Pong or Asteroids, game designers had to have a good understanding of what each chip on the game motherboard could give them. They needed to know how much memory they had to work with, what colors were available, and how to produce them. In some cases, they had to go to manufacturers and ask for help designing chips to achieve what they wanted to accomplish. Sound and graphics capabilities were primitive at best, but they managed. Because these early games were truly a pioneering endeavor, they also had to create new user controls to work with. These efforts resulted in a unique developers: part designer, part engineer. Because of the advanced knowledge needed to create arcade games in this environment, there were very few game designers.
The DOS Days
From the early technologies of arcade game developers and computer scientists arose the personal computer. With this invention, the need to know the chips was replaced with the need to know a programming language. As computers and programming languages evolved, more and more software people turned to game development. Some of the early computer games were very simple. They relied heavily on text-based and rudimentary graphics, much like the early arcade days. As computers developed in their capabilities, so did the games.
Early DOS games required knowledge of machine and assembly languages to access the operating system's lower levels. As time went on, more user-friendly languages were created, such as BASIC and LOGO; however, they didn't have the power of assembly language. Then came such languages as Pascal and C. With C, game developers could write advanced code routines without having to write tons of assembly code, thus making development efforts easier. C soon became the preferred language of game developers.
Through all these language changes, one problem remained. To write good games, developers still needed an in-depth knowledge of the hardware they were working with. They no longer had to know each chip's capabilities, but they had to understand things such as video cards, sound cards, and input devices. They also had to deal with memory constraints, which were becoming a hindrance to game development.
Along Came Windows
With the development of Microsoft Windows and other operating systems, creating standards in the computer industry became much easier. These standards included hardware interfaces, video memory standards, sound standards, and CPU memory specifications. However, there was still a lot of dissension in the industry, and not all hardware behaved as advertised on the Windows operating system. As a result, many games written in Windows had problems and suffered from performance hits. Microsoft realized this problem early on and focused its efforts on standardizing its environment and working to increase performance for the parts of the operating system that pertain to game and multimedia needs. These efforts were demonstrated in Windows 95, one of the first operating systems that made consistent hardware-software interaction a reality. Because all hardware manufacturers had to meet a similar standard and Microsoft controlled how its operating system communicated to the hardware, development became standard as well. In the DOS days, you had to create your own drivers for each type of video card, but with Windows, you had to develop to only one standard. Instead of spending time creating and understanding so many hardware specifications, developers relied on Microsoft to do all that for them and package it for their use in the OS.
There was one flaw with all of this. Windows was originally designed for the home and office, not for multimedia development. It lacked a few essential items that had become standard in the multimedia marketparticularly graphics speed, which could no longer be achieved in the Windows environment as it had been in DOS. It was the same hardware, but there were many more layers between the developers' code and the hardware they wanted to control. The result was that many developers abandoned the idea of Windows game development and relied on the Windows ability to shell out to DOS.
Then There Was DirectX
After Microsoft realized that game developers agreed with its concepts but weren't willing to give up the performance they had become accustomed to in DOS, Microsoft began researching ways to enable developers to get to that hardware layer. Some of the early attempts included WinG, WinToon, and OpenGL.
WinG came into the software spotlight around the time of Windows 3.1 and Windows for WorkGroups. WinG was the first attempt at a full-blown graphics API (Application Programming Interface) library to overcome the limitations of the Windows API. WinG supported high-performance graphics techniques that opened up the Windows game development world to those people who worked exclusively in DOS.
WinToon was an early API for Windows 95 that provided the capability of easy animation playback. On top of full-screen playback capabilities, this API also offered quicker performance than existing Windows 95 APIs did. This technology helped improve the multimedia playback capabilities of animation programs such as kids' games and teaching programs.
OpenGL is a powerful set of APIs that allowed developers to cut through the Windows red tape to get to some of the hardware capabilities of 2D and 3D animation. Surprisingly, OpenGL was not created by Microsoft, but by Silicon Graphics, Inc. Silicon Graphics is one of the pioneers in 3D special effects, however, so it's not that surprising. Several games today still use OpenGL as well as DirectX because many video cards support both these technologies.
These technologies were a great boost, but ultimately came up short. As the game market exploded and the need for more games on the Windows 95 platform increased, developers at Microsoft went back to the drawing board. What they came up with was the Windows 95 Game Software Developer's Kit (SDK), which contained the first version of DirectX. DirectX was initially just for the computer game industry but has evolved to include other areas, such as innovations in the arcade industry and handheld Windows CE devices. As DirectX has developed, Microsoft has added network support, force feedback support, and 3D graphics capabilities. When innovations are needed, the DirectX team has continued to increase the capabilities of this remarkable software library.
Microsoft has also made it easier for DirectX to be used by more than one language. C and C++ had become the game developer's language of choice, so DirectX and the examples included with the SDK were designed with C developers in mind. As DirectX has become more accepted, and Visual Basic (VB) taken more seriously as a programming language, DirectX and Visual Basic have finally come together. It also hasn't hurt that Microsoft has been encouraging VB users to start using DirectX 7 in their applications.