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While software-mediated 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. Protected mode also has hardware support for interrupting a running program and shifting execution context to another, enabling preemptive multitasking.

Most CPUs which support protected mode also feature 32 bit registers (e.g. any chip of the 80386 series or later); leading to the confusion of protected mode as such with the idea of 32 bit processing. The 80286 chips, however, supported protected mode but still had only 16 bit registers. The protected mode enhancements in Windows 2.0 and later were called 386 enhanced mode because they required 32 bit registers in addition to protected mode, and would not run on a 286 (even though 286es support protected mode).

Even when protected mode is enabled on a 32 bit chip, 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.

In protected mode, the first 32 interrupts are reserved for CPU exceptions. Every time Windows shows a " Blue screen of deathThe Blue Screen of Death also abbreviated as BSoD refers to the screen displayed by Microsoft's Windows operating system when it cannot (or is in danger of being unable to) recover from a system error. There are two Windows error screens that are both ref", for example, it signifies that such an exception has occurred. For instance, interrupt 0E (14 in decimal) is a general protection faultThis article is about the error. For the web comic, see General Protection Fault (comic). A general protection fault in the Intel x86 architecture is a fault which can encompass several cases, where protection mechanisms within the architecture are violat and interrupt 00 is division by zeroIn mathematics, a division is called a division by zero if the divisor is zero. Such a division can be formally expressed as , where a is the dividend. Whether or not this expression can be assigned a meaningful ( well-defined) value depends upon how the.

As a primary design specification for x86 chips is that they be backwards compatible with all previous x86 chips, x86 CPUs boot into real mode at power on, not protected mode. Most modern operating systems switch the CPU into protected mode immediately at startup. It is also possible to run a task designed for real mode in protected mode using virtual 8086 emulationIn the x86 CPU architecture Virtual 8086 mode also called virtual-real mode, allows the emulation of real mode in protected mode. It is used to execute DOS programs in Microsoft Windows..

See also: X86, x86-assembly

External links

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

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