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b. The Mac OS

47. The inability of Apple to compete effectively with Windows provides another example of the applications barrier to entry in operation. Although Apple''s Mac OS supports more than 12,000 applications, even an inventory of that magnitude is not sufficient to enable Apple to present a significant percentage of users with a viable substitute for Windows. The absence of a large installed base, in turn, reinforces the disparity between the applications made available for the Mac OS and those made available for Windows, further inhibiting Apple''s sales. The applications barrier thus prevents the Mac OS from hindering Microsoft''s ability to control price, regardless of whether the Mac OS is regarded as being in the relevant market or not.

c. Fringe Operating Systems

48. The applications barrier to entry does not prevent non?Microsoft, Intel?compatible PC operating systems from attracting enough consumer demand and ISV support to survive. It does not even prevent vendors of those products from making a profit. The barrier does, however, prevent the products from drawing a significant percentage of consumers away from Windows.

49. As discussed above, Be markets an Intel?compatible PC operating system, called BeOS, that is specially suited to support multimedia functions. The operating system survives on a relatively minuscule number of applications (approximately 1,000) and a user base which, at around 750,000, is trivial compared to the number of Windows users. One of the reasons the BeOS can even attract that many users despite its small base of applications is that it advertises itself as a complement to, rather than as a substitute for, Windows. Although the BeOS could run an Intel?compatible PC system without Windows, it is almost always loaded on a system along with Windows. What is more, when these dual?loaded PC systems are turned on, Windows automatically boots; the user must then take affirmative steps to invoke the BeOS. While this scheme allows the BeOS to occupy a niche in the market, it does not place the product on a trajectory to replace Windows on a significant number of PCs. The special multimedia support provided by the BeOS may, for a small number of users, outweigh the disadvantages of maintaining two large, complex operating systems on one PC. Of that group, however, it is likely that only a tiny number of users will find that support so attractive that they would be willing to forego Windows, and its huge base of compatible applications, altogether.

50. The experience of the Linux operating system, a version of which runs on Intel? compatible PCs, similarly fails to refute the existence of an applications barrier to entry. Linux is an ``open source'''' operating system that was created, and is continuously updated, by a global network of software developers who contribute their labor for free. Although Linux has between ten and fifteen million users, the majority of them use the operating system to run servers, not PCs. Several ISVs have announced their development of (or plans to develop) Linux versions of their applications. To date, though, legions of ISVs have not followed the lead of these first movers. Similarly, consumers have by and large shown little inclination to abandon Windows, with its reliable developer support, in favor of an operating system whose future in the PC realm is unclear. By itself, Linux''s open?source development model shows no signs of liberating that operating system from the cycle of consumer preferences and developer incentives that, when fueled by Windows'' enormous reservoir of applications, prevents non?Microsoft operating systems from competing.

3. Open?Source Applications Development

51. Since application developers working under an open?source model are not looking to recoup their investment and make a profit by selling copies of their finished products, they are free from the imperative that compels proprietary developers to concentrate their efforts on Windows. In theory, then, open?source developers are at least as likely to develop applications for a non?Microsoft operating system as they are to write Windows?compatible applications. In fact, they may be disposed ideologically to focus their efforts on open?source platforms like Linux. Fortunately for Microsoft, however, there are only so many developers in the world willing to devote their talents to writing, testing, and debugging software pro bono publico. A small corps may be willing to concentrate its efforts on popular applications, such as browsers and office productivity applications, that are of value to most users. It is unlikely, though, that a sufficient number of open?source developers will commit to developing and continually updating the large variety of applications that an operating system would need to attract in order to present a significant number of users with a viable alternative to Windows. In practice, then, the open? source model of applications development may increase the base of applications that run on non? Microsoft PC operating systems, but it cannot dissolve the barrier that prevents such operating systems from challenging Windows.