The reason they are not available under WINDOWS is because MicroSoft didn't want them to be available to applications programs. WINDOWS uses them otherwise it wouldn't be able to address 32 Gigabytes of memory. (more recently 64 Gigabytes) Microsoft is on a path to destroy any control of the hardware that now remains in applications programmers' hands like it did under DOS. One example of that is "Full Screen Mode" which MicroSoft did away with an applications programmer's ability to use in Windows 7. It allows the applications programmer full control of the monitor screen. It still can be used under WINDOWS XP. Anybody with any real intelligence can see MicroSoft's aim to take over all applications programming of PCs. Back in the late 1990s, when the courts attempted to put a leash on MicroSoft's ability to monopolize the PC program market, they didn't realize that MicroSoft could stop the competition by doing away with programs' ability to control the hardware. MicroSoft don't want a program, like I've written, that uses the full screen to run on newer versions of WINDOWS. I'll be glad to send a copy of a system that uses "Full Screen Mode" to anyone who has a desktop running WINDOWS XP or an older version of WINDOWS. (I do have a WINDOWS 7 version of the system but I had to drop the mouse and graphics support of it because MicroSoft did away with that function on WINDOWS 7.)Segment registers are not accessible under any modern operating system, and that fact is quite irrelevant to application programmers. Many hardware platforms don't even have segment registers. Even more to the point, why would you need to? Learn to write modern software, and you'll be much better off. You seem to be stuck in a DOS mode of thinking in regard to programming. DOS programming (especially low level) concepts are largely irrelevant to modern programs.