Protected mode

Protected mode

The Intel x86 architecture supports two modes of operation, called real mode and protected mode, respectively. The latter is often abbreviated pmode and first appeared in the 80286. Protected mode enables 32-bit memory addressing, allowing a programmer to access extended memory that cannot be easily accessed from real mode (16-bit addressing and physically addressed real mode segments lead to a maximum of 1 MB of accessible memory). Protected mode also supplies protection (hence the name) by not allowing one task to see the memory belonging to another task (ie, address space).

A clear advantage of protected mode is stable multitasking. While multitasking is certainly possible on systems running in real mode, the memory protection features of protected mode prevent an erroneous program from damaging the memory "owned" by another task or by the operating system kernel.

In protected mode, the first 32 interrupts are reserved for CPU exceptions. Every time Windows shows a "Blue screen of death", for example, it signifies that such an exception has occured. For instance, interrupt 0E (13 in decimal) is a general protection fault and interrupt 00 is division by zero.

For compatibility reasons, Intel compatible CPUs boot into real mode, not protected mode. A task designed for real mode can run in protected mode using virtual 8086 emulation, should it be desired.

In addition, even when protected mode is enabled, memory above 1MB is not accessible due to memory wrap-around, a feature designed to mimic (now obsolete) IBM XT systems. This limitation can be circumvented by enabling the A20 Line.

External link

http://x86.ddj.com/articles/pmbasics/tspec_a1_doc.htm