When you launch a DOS or Windows 3.x 16-bit software from a DOS window, the virtual real mode is used. Windows generates a virtual DOS computer in which a DOS application may operate when you launch it.

It's worth noting that all Intel and Intel-compatible CPUs (such as AMD and VIA/Cyrix) start up in real mode. When you install a 32-bit operating system, it instantly changes the CPU to 32-bit mode and takes over control.

It's also worth noting that some 16-bit (DOS and Windows 3.x) software act incorrectly in a 32-bit environment, doing things that virtual real mode doesn't allow. This is exemplified by diagnostic software. Under Windows, such a program does not execute correctly in a real mode (virtual real) window. In such a situation, you may still use a self-booting CD or DVD that has the diagnostic software to boot to DOS or Windows 9x/Me and use your modern system in its original no-frills true mode.

Although the real mode is used by 16-bit DOS and "normal" DOS applications, special programs exist that "extend" DOS and give access to more memory (over 1MB). These are also known as DOS extenders, and they are normally provided with any DOS or Windows 3.x program that makes use of them. DOS protected mode interface is the protocol that explains how to make DOS function in protected mode (DPMI).

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