Protected mode

Intel architecture supports real mode and protected mode, in short pmode first in 80286. Protected mode enables 32bit memory addressing, allowing to reach extended memory which cannot be easily done from real mode (16bit addressing & rmode segments = max 1Mb accessible). Protected mode also supplies *protection*, by not allowing one task to see another task's memory (address space). Another advantage is multitasking, which is much stabler than simillar multitasking in rmode.

In protected mode, first 32 interrupts are reserved for CPU exceptions. Every time Windows shows a "Blue screen of death", such an exception has happened. For instance, 0E is General Protection Fault and 00 Division by Zero.

For compatibility reasons, Intel compatible CPUs boot with real mode. A real mode task can be run under protected mode in Virtual 8086 mode (however v86 is a mode only in name).

In addition, even when protected mode in enabled, memory above 1MB is not accessible due to memory wrap-around designed to mimic 80286s. This is fixed by enabling the A20 Line

See also: X86, x86-assembly

External link

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