gistfile1.asm

[BITS 16]            
[ORG 0x7c00]

;setting to protected mode
;---------------------------------------------------------------

;save the registers data
pushf
pusha
push ds
push  es

;set ds and es to zero
xor ax,ax
mov ds,ax
mov es,ax

;loading of GDT
lgdt [GDTP]          

cli

;setting to protected mode
mov eax,CR0           
or eax,01h      
mov CR0,eax    

;jumps to protected mode code segment
jmp 08h:protected

[BITS 32]           
protected:

;setting the data segments for a protected mode
mov ax,010h       
mov ds,ax
mov es,ax

;---------------------------------------------------------------
;output string in protected mode
mov bx,0B8000h
mov es,bx
mov di, protected_string
mov cx, [protected_str_len] ;loop counter
xor bx, bx
mov bx, 3040

out_protected_mod_loop:
	mov ah, byte [di]
	mov byte [es:bx], ah
	mov byte [es:bx+1], 4fh
	add bx, 2
	inc di
loop out_protected_mod_loop

mov ax,010h       
mov ds,ax
mov es,ax

;setting to real mode
;---------------------------------------------------------------
;jumps to real mode code segment
jmp 018h:real_1
[BITS 16]      
real_1:

;setting data segments for a real mode
mov ax,020h
mov ds,ax
mov es,ax

;setting to real mode
mov eax,CR0           
xor eax,01h      
mov CR0,eax    

;setting real mode code segment
jmp 0000h:real_2
real_2:

sti

;load registers from the stack
pop es
pop ds
popa
popf

;---------------------------------------------------------------
;output string in real mode
mov bx,0B800h
mov es,bx
mov di, real_string
mov cx, [real_str_len] ;loop counter
xor bx, bx
mov bx, 3200
out_real_mod_loop:
	mov ah, byte [di]
	mov byte [es:bx], ah
	mov byte [es:bx+1], 4fh
	add bx, 2
	inc di
loop out_real_mod_loop

;Global descriptors table
;---------------------------------------------------------------
GDT:  

;empty descriptor
dq 0  

;GDT (protected mode code segment)
dw 0FFFFh  
dw 00000h
db 00h   
db 010011010b 
db 011001111b
db 00h   

;LDT (protected mode data segment)
dw 0FFFFh  
dw 00000h
db 00h   
db 010010010b 
db 011001111b
db 00h  

;GDT (real mode code segment)
dw 0FFFFh  
dw 00000h
db 00h   
db 010011010b 
db 010001111b
db 00h   

;LDT (real mode data segment)
dw 0FFFFh  
dw 00000h
db 00h   
db 010010010b 
db 010001111b
db 00h  

;GDT pointer
GDTP:                           
dw GDTP-GDT-1        
dd GDT           

;-------------------------------------------------------------

real_string db 'Hi from real mode!'
real_str_len dw $-real_string
protected_string db 'Hi from protected mode!'
protected_str_len dw $-protected_string
;filling free space to 510 bytes
times 510-($-$$) db 0 ;2 bytes less now
db 0x55
db 0xAA