Описание тега protected-mode

X 86 protected mode allows system software to support virtual memory, paging and preemptive multi-tasking.

• Protected mode was first added to the x86 architecture in 1982, with the release of Intel's 80286 processor, and later extended with the release of the 80386 processor in 1985.
• To maintain backward compatibility, x86 processors begins executing instructions in real mode.

To enter protected mode:

• The Global Descriptor Table (GDT) must first be created with a minimum of three entries: a null descriptor, a code segment descriptor and data segment descriptor.
• In an IBM-compatible machine, the A20 line (21st address line) also must be enabled to allow the use of all the address lines so that the CPU can access beyond 1 megabyte of memory.
• Then the PE bit must be set in the CR0 register and a far jump must be made to clear the prefetch input queue:

; set PE bit
mov eax, cr0
or eax, 1
mov cr0, eax; far jump (cs = selector of code segment)
jmp cs:@pm

@pm:; Now we are in PM.