protected mode

[abcd12321]

protected mode prevents user programs from modifying the operating system by assigning a lower privelidge to it.my question is,what's there to stop someone from writing code with the highest privelidge and the modifying the OS's core?





thanks

[NoHero]

Originally Posted by abcd12321

protected mode prevents user programs from modifying the operating system by assigning a lower privelidge to it.my question is,what's there to stop someone from writing code with the highest privelidge and the modifying the OS's core?

The GDT ... The Global Descriptor Table. This table - a set of data - defines where the segments starts and ends. If they can be accessed with reading/writing, what privilige level they are etc. etc.

The lgdt instructions loads a new GDT into the memory. A tutorial on the GDT.

This is a dumm GDT which specifies that every memory is highest privileged (kernel) memory. This is dumm, but at first easier to understand:

SECTION .data
	
gdt:
; NULL descriptor
	dw 0		; limit 15:0
	dw 0		; base 15:0
	db 0		; base 23:16
	db 0		; type
	db 0		; limit 19:16, flags
	db 0		; base 31:24

; unused descriptor
	dw 0
	dw 0
	db 0
	db 0
	db 0
	db 0

LINEAR_DATA_SEL	equ	$-gdt
	dw 0FFFFh
	dw 0
	db 0
	db 92h		; present, ring 0, data, expand-up, writable
	db 0CFh		; page-granular (4 gig limit), 32-bit
	db 0

LINEAR_CODE_SEL	equ	$-gdt
	dw 0FFFFh
	dw 0
	db 0
	db 9Ah		; present,ring 0,code,non-conforming,readable
	db 0CFh		; page-granular (4 gig limit), 32-bit
	db 0
gdt_end:

gdt_ptr:
	dw gdt_end - gdt - 1
	dd gdt

And loades is this GDT by the following instruction:

lgdt [gdt_ptr]

(NASM code)