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'''Protected mode''' is the 32 bit 'native' operating mode of Intel processors (and clones) since the 80386. It allows the developer to work with several virtual address spaces, each of which has 4GB of addressable memory and allows the system to enforce strict memory protection as well as restricting the available instruction set (so that your application cannot control the hard disk directly while the kernel can)
'''Protected mode''' is the main operating mode of modern Intel processors (and clones) since the 80286 (16 bit). On 80386s and later, the 32 bit Protected Mode allows working with several virtual address spaces, each of which has a maximum of 4GB of addressable memory; and enables the system to enforce strict memory and hardware I/O protection as well as restricting the available instruction set via [[Security#Rings|Rings]].


Protected mode unleashes the real power of your CPU, so you better get informed about it if you are considering writing an OS. However, it will prevent you from using virtually any of the BIOS interrupts (unless you have a V86 monitor).
A CPU that is initialized by the [[BIOS]] starts in [[Real Mode]]. Enabling Protected Mode unleashes the real power of your CPU. However, it will prevent you from using most of the BIOS interrupts, since these work in Real Mode (unless you have also written a [[Virtual 8086 Mode|V86]] monitor).


==Entering Protected Mode==
⚫
Whether the CPU is in [[Real Mode]] or in protected mode is defined by the lowest bit of the CR0 register, so basically


Before switching to protected mode, you must:
;; make sure interrupts are disabled, etc.
*Disable interrupts, including [[Non Maskable Interrupt|NMI]] (as suggested by Intel Developers Manual).
⚫
mov eax, cr0
*Enable the [[A20 Line]].
or al,1
*Load the [[Global Descriptor Table]] with segment descriptors suitable for code, data, and stack.
⚫
mov cr0,eax


⚫
Whether the CPU is in Real Mode or in Protected Mode is defined by the lowest bit of the CR0 or MSW register.
takes you to protected mode ... however you'll discover that there are many other things to be done before and after that operation to switch gracefully to pmode rather than resetting the CPU...

This example loads a descriptor table into the processor's GDTR register, and then sets the lowest bit of CR0:
<syntaxhighlight lang="asm">
cli ; disable interrupts
lgdt [gdtr] ; load GDT register with start address of Global Descriptor Table
⚫
mov eax, cr0
or al, 1 ; set PE (Protection Enable) bit in CR0 (Control Register 0)
⚫
mov cr0, eax

; Perform far jump to selector 08h (offset into GDT, pointing at a 32bit PM code segment descriptor)
; to load CS with proper PM32 descriptor)
jmp 08h:PModeMain

PModeMain:
; load DS, ES, FS, GS, SS, ESP
</syntaxhighlight>


==See Also==
==See Also==
===Articles===
*[[Real Mode]]
*[[Virtual 8086 Mode]]

===External Links===
===External Links===
*http://www.osdever.net/tutorials/view/the-world-of-protected-mode - very good tutorial on how to enter protected mode
*[http://www.nondot.org/sabre/os/articles/ProtectedMode/ OSRC: protected mode]
*[http://www.nondot.org/sabre/os/articles/ProtectedMode/ OSRC: protected mode]
*http://home.swipnet.se/smaffy/asm/info/embedded_pmode.pdf - pragmatic tutorial on pmode
*http://home.swipnet.se/smaffy/asm/info/embedded_pmode.pdf - pragmatic tutorial on protected mode ([http://web.archive.org/web/20030604185154/http://home.swipnet.se/smaffy/asm/info/embedded_pmode.pdf Cached copy])
*http://www.osdever.net/tutorials.php?cat=4&sort=1
*http://www.brokenthorn.com/Resources/OSDev8.html
*[[Wikipedia:Protected_mode|Protected mode Wikipedia page]]
*http://members.tripod.com/protected_mode/alexfru/pmtuts.html - PMode tutorials in C & Asm


[[Category:Protected Mode]]
[[Category:X86 CPU]]
[[Category:Operating Modes]]

Latest revision as of 04:52, 9 June 2024

Protected mode is the main operating mode of modern Intel processors (and clones) since the 80286 (16 bit). On 80386s and later, the 32 bit Protected Mode allows working with several virtual address spaces, each of which has a maximum of 4GB of addressable memory; and enables the system to enforce strict memory and hardware I/O protection as well as restricting the available instruction set via Rings.

A CPU that is initialized by the BIOS starts in Real Mode. Enabling Protected Mode unleashes the real power of your CPU. However, it will prevent you from using most of the BIOS interrupts, since these work in Real Mode (unless you have also written a V86 monitor).

Entering Protected Mode

Before switching to protected mode, you must:

  • Disable interrupts, including NMI (as suggested by Intel Developers Manual).
  • Enable the A20 Line.
  • Load the Global Descriptor Table with segment descriptors suitable for code, data, and stack.

Whether the CPU is in Real Mode or in Protected Mode is defined by the lowest bit of the CR0 or MSW register.

This example loads a descriptor table into the processor's GDTR register, and then sets the lowest bit of CR0: 

cli            ; disable interrupts
lgdt [gdtr]    ; load GDT register with start address of Global Descriptor Table
mov eax, cr0 
or al, 1       ; set PE (Protection Enable) bit in CR0 (Control Register 0)
mov cr0, eax

; Perform far jump to selector 08h (offset into GDT, pointing at a 32bit PM code segment descriptor) 
; to load CS with proper PM32 descriptor)
jmp 08h:PModeMain

PModeMain:
; load DS, ES, FS, GS, SS, ESP

See Also

Articles

External Links

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