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Protected Mode

Starting with the 80286 chip in the IBM AT, a new processor mode was introduced called protected mode. This is a much more powerful mode of operation than real mode, and is used in all modern multitasking operating systems. The advantages of protected mode (compared to real mode) are:

• Full access to all of the system's memory. There is no 1 MB limit in protected mode.
• Ability to multitask, meaning having the operating system manage the execution of multiple programs simultaneously.
• Support for virtual memory, which allows the system to use the hard disk to emulate additional system memory when needed.
• Faster (32-bit) access to memory, and faster 32-bit drivers to do I/O transfers.

The name of this mode comes from its primary use, which is by multitasking operating systems. Each program that is running has its own assigned memory locations, which are protected from conflict with other programs. If a program tries to use a memory address that it isn't allowed to, a "protection fault" is generated. If you've ever used Windows 3.x, you know exactly what I am talking about. :^)

Although introduced with the 286, the operating system world back in the early 80s was stuck squarely in DOS. The use of protected mode didn't become popular until the rise in dominance of the Microsoft Windows operating system. Protected mode is now currently the way that most people use their PCs. All of the major operating systems today use protected mode, including Windows 3.x, Window 9x, Windows NT, OS/2 and Linux. Even DOS, which normally runs in real mode, can access protected mode using DPMI (DOS protected mode interface), which is often used by DOS games to break the 640 KB DOS conventional memory barrier.

All processors from the 286 on can use protected mode. 386 and later processors can switch on the fly from real to protected mode and vice-versa; the 286 can only switch from real to protected mode once (switching back requires a reboot). Protected mode is also sometimes called 386 Enhanced Mode, since it became mainstream with that family of processors.

Next: Virtual Real Mode

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