Modo protegido

Modo Protegido (também conhecido como PMode) é um modo operacional dos processadores x86-compativeis da série 80286 e posteriores. Protected mode possui um número de novas definições para a criação de multitasking(várias tarefas a serem executadas ao mesmo tempo) ,estabilidade do sistema, protecção de memória, sistema de paging(uso de memória virtual), e suporte de hardware para memória virtual. O outro modo operacional da série 80286 e posteriores é o modo real (RMode), sendo este modo compativel com o PMode e tendo sido o primeiro a ser criado.

Embora seja possivel utilizar multitasking em sistemas que correm em real mode, a protecção de erros de memória que o protected mode asssegura, evita que o programa provoque estragos na memória que está a ser utilizada por outra tarefa ou pelo sistema operativo. 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 death", for example, it signifies that such an exception has occurred. For instance, interrupt 0E (14 in decimal) is a general protection fault and interrupt 00 is division by zero.

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 emulation.

See also: X86, x86-assembly